,illustration3 ,satellite ,communications ,physics ,satellite ,communications ,physics ,,by ,,members ,,of ,,the ,,staff ,,of ,,bell ,,telephone ,,laboratories ,,editor3 ,,ronald ,m4 ,,foster1 ,jr4 ,how scientists and engineers use basic physical principles to solve some of the problems in communicating by means of man-made satellites #aifc ,bell ,telephone ,laboratories1 ,incorporated ,all rights reserved4 ,permission to reproduce any material contained in this book must be obtained1 in writing1 from the publisher4 ,library of ,congress ,catalog ,card ,number #fc-baffg ,printed in ,u4,s4,a4 ,foreword ,john ,r4 ,pierce ,executive ,director1 ,research1 ,communications ,principles and ,communications ,systems ,bell ,telephone ,laboratories ,when ,i first talked about the possibilities and advantages of communications satellites to the ,princeton ,section of the ,institute of ,radio ,engineers on the evening of ,october #ad1 #aied1 ,i was diligent in my analysis and enthusiastic in my presentation but1 ,i must confess1 a little skeptical as to whether or not anything would come of the idea4 ,still1 others and ,i at ,bell ,laboratories remained interested1 and1 after the launching of ,sputnik ,i on ,november #c1 #aieg1 and of ,explorer ,i on ,january #ca1 #aieh1 we worked actively toward satellite communications experiments4 ,this led to our work with ,echo ,i 7launched ,august #ab1 #aifj7 and finally to the launching on ,july #aj1 #aifb1 of ,telstar ,i--that satellite which became1 in the words of ,queen ,elizabeth1 8the invisible focus of a million eyes40 ,this work on communications satellites has been a grand exploration and opening up of a hitherto dark continent of science and technology4 ,my courageous friends at ,bell ,laboratories encountered therein surprising difficulties and perplexing problems which ,i had never dreamed of1 and these intrepid and indefatigable adventurers grappled with them and mastered them all4 ,now you1 who have in your own homes seen pictures transmitted across the ocean by satellite1 can learn first hand from the men who worked on hard and varied technical problems just what these problems were and how they were solved4 ,and1 by reading you can find out what sort of knowledge1 training1 and habits you yourself will need if you wish some day to adventure into those undiscovered or unexplored fields of technology which will be new and exciting when ,telstar has become old hat4 ,june #e1 #aifc ,high ,school ,science ,teaching ,aids ,prepared by ,bell ,telephone ,laboratories ,classroom units including books1 motion pictures1 and demonstration devices3 ,similarities in ,wave ,behavior ,ferromagnetic ,domains ,the ,speech ,chain ,bell ,system ,science ,experiments for the advanced student3 ,from ,sun to ,sound ,solar ,energy ,experiment ,speech ,synthesis ,information about these and other materials is available to teachers from local ,telephone ,company business offices ,contents ,introduction #h part #a3 ,satellite ,communications #aj ,ronald ,m4 ,foster1 ,jr41 ,editor part #b3 ,satellite ,communications ,case ,histories #ch #a4 ,franz ,t4 ,geyling ,how ,do ,we ,calculate a ,satellite's ,orbit8 #da #b4 ,peter ,hrycak ,what ,color ,should a ,satellite ,be8 #dh #c4 ,jeofry ,s4 ,courtney-pratt ,how ,can ,we ,make ,optical ,measurements on a ,satellite8 #ec #d4 ,kenneth ,d4 ,smith ,how ,do ,we ,keep ,solar ,cell ,power ,plants ,working in ,space8 #fb #e4 ,peter ,d4 ,bricker ,would ,time ,delay ,be a ,problem in ,using a ,synchronous ,satellite8 #gj #f4 ,e4 ,jared ,reid ,how ,can ,we ,repair an ,orbiting ,satellite8 #gh ,suggested ,reading #hg ,introduction ,despite the title1 this is not a physics textbook1 and it will tell you only part of the fascinating story of satellite communications4 ,however1 we have tried to tell this story in a rather special way4 ,part ,i explains why we are so interested in communicating by means of man-made satellites1 describes the important events in the progress of satellite communications 7with special emphasis on ,project ,telstar71 and points out some of the very knotty problems that had to be solved4 ,then1 in ,part ,,ii1 we pick out six typical satellite communications problems and go into them more deeply4 ,these case histories are examples of the things scientists and engineers are constantly faced with4 ,to narrate them we have called on six experts--,bell ,telephone ,laboratories engineers and scientists who actually have been working on the problems4 ,the second half of our book is taken up by their accounts of their own personal experiences4 ,we hope that reading them will give you an insight into what it is really like to be a scientist or engineer working in a laboratory on an important new venture into the future4 ,we hope you will see that what they do is not all excitement and glamour4 ,it involves hard work1 ingenious thinking1 and plugging away at tough problems4 ,but this is what scientists and engineers enjoy--along with the excitement and glamour1 of course4 ,only a part of what our authors talk about can strictly be called 8physics0--it is also engineering1 chemistry1 mathematics1 and even psychology4 ,but almost all their work is based--when you get down to fundamentals--on basic principles of classical physics taught in high school4 ,now1 a word of caution4 ,in ,part ,i1 in talking about satellite communications in general1 we have kept things at a level that should be understandable to almost everyone--even those who have never taken a high school science course4 ,but we warn you this isn't true of ,part ,,ii4 ,our authors have tried to tell their stories as carefully and as logically as possible1 but some readers may have trouble in following all they say4 ,this we expect4 ,we haven't tried to sugar-coat or gloss over any essential details of the problems or their solutions4 ,we don't want you to think that solving them was any easier than it actually was4 ,and1 since this is not intended as a textbook1 we may sometimes omit elementary material and go right to the heart of the matter4 ,when this book was written satellite communications was still a technological infant4 ,it is growing and changing so swiftly that much of what we say may soon be out of date4 ,that can't be helped1 of course1 and we ask you--who may be reading it long afterward--to be tolerant4 ,our problems may well be forgotten when new1 more sophisticated ones appear4 ,but we are dealing here in methods1 not in history2 the ways in which these problems were attacked are just as lasting and important as the problems themselves4 part #a ,satellite ,communications ,illustration3 ,title image 8,our intensive research and development in the field of communications satellites have brought us to the point where we are now certain of the technical feasibility of transmitting messages to any part of the world by directing them to satellites4444 ,the actual operation of such a system would provide a dramatic demonstration of our leadership in this area of space activity4444 ,the direct benefits--economic1 educational1 and political--of this improved world-wide communication will be invaluable40 --,,john ,f4 ,,kennedy ,why ,do ,we ,bother ,with ,satellite ,communications8 ,that's a good question to begin with4 ,why should we get involved in a vast1 complicated program such as communication by means of man-made satellites8 ,is the end result really worth all the trouble that is involved8 ,as you go further1 you will see that nothing to do with satellite communications is as simple as it first seems4 ,even some of the easier questions have been answered only after long hours of perceptive thinking1 ingenious experimenting1 and shrewd deduction4 ,they have required a lot of hard work1 led to many frustrating difficulties1 and cost quite a bit of money4 ,but the answer still is yes4 ,despite all the difficulties1 it is clear that the creation of a successful satellite communicating system is worth it4 ,there is a double reason for this4 ,on the one hand1 it is a technological target that is now clearly within our range4 ,we must either reach it or let progress pass us by4 ,satellite communications is one field in which1 as far as we know1 ,american engineering and science have been well in the lead--so we have an even greater incentive to press on in this direction as hard and as fast as we can4 ,but perhaps more important than the prestige it would give our country is a second reason for our great interest in satellite communications3 ,we need it4 ,the world today is going through one of its great periods of change4 ,this has caused many complications1 and one of the most important is the need for much better communications between nations and peoples4 ,by 8communications0 we mean all the various ways of sending information from one place to another3 mail1 telephone calls1 business data1 radio1 television4 ,the demand for these services--especially when we look ahead to the #aigj's and #aihj's--will be tremendous4 ,our international communications channels will be completely swamped unless some major improvements are made4 ,fortunately1 modern technology--given a boost by the world's interest in rockets1 missiles1 and the exploration of space--has shown us one answer to this problem3 the communications satellite4 ,the conventional pathways for long-distance communication have led along the earth's surface1 under the oceans1 and through the lower atmosphere4 ,no one of these routes has yet provided all the capacity1 speed1 or quality we need4 ,present underseas cables have a limited capacity2 surface travel by ship is too slow for anything but routine mail2 short-wave radio is subject to distortion and noise1 and the available frequencies are rapidly being used up4 ,although jet planes can span the oceans in a few hours with mail and such things as taped television shows1 the big need will be to send information instantaneously4 ,and the communications satellite offers us a very promising way to do this4 ,what a ,communications ,satellite ,can ,do ,one of the attractive things about using a satellite is that it doesn't require a revolutionary breakthrough in technical knowledge4 ,it can employ a satisfactory means of communicating that is already available3 the microwave radio relay4 ,today1 this kind of transmission is used on a routine basis to send thousands of telephone calls and television programs across long distances4 ,it gives high-quality performance and has a large message capacity4 ,but there has always been one difficulty keeping us from using it for overseas communications3 ,extremely high frequency waves can travel almost unlimited distances1 but they can go only in straight lines4 ,this means that the curvature of the earth limits a microwave's line-of-sight path to about #cj miles2 so we must build a series of transmission relay towers spaced every #cj miles or so4 ,obviously1 this isn't possible when you send messages across an ocean4 ,but1 if we could find a way to send a signal high up into the sky and then bounce it from there back again to a far-off spot1 we could send microwave messages great distances4 ,illustration3 ,curvature of the earth requires microwave towers to be about #cj miles apart ,illustration3 ,microwaves sent via an orbiting satellite can travel vast distances ,as long ago as #aide1 ,arthur ,c4 ,clarke1 an ,english writer and scientist1 proposed that a man-made satellite orbiting in space might be used to relay signals in this way4 ,in #aide1 of course1 the very idea of getting a satellite out into space seemed utterly fantastic1 and satellite communications could only be classified as science fiction4 ,ten years later1 although ,sputnik ,i had not yet been launched1 artificial satellites were close to reality4 ,at that time1 ,john ,r4 ,pierce of ,bell ,telephone ,laboratories made the first serious study of what would have to be done to build a working satellite communication system--assuming it ever became possible to put satellites into orbit4 ,and ,bell ,laboratories has been interested in satellite communications ever since4 ,the ,road to ,successful ,satellite ,communications ,with the first launching of a satellite into orbit by the ,soviet ,union in #aieg1 the real development work on satellite communications began4 ,by #aifj ,project ,echo had proved that signals could be reflected off a man-made satellite and received several thousand miles away4 ,and1 in #aifb1 ,project ,telstar demonstrated to the whole world that an active repeater satellite could send telephone calls1 data1 and television across the ocean4 ,bringing satellite communications almost to reality has required more than putting a man-made satellite into orbit around the earth4 ,just as important have been the invention and development of many remarkable new devices3 the transistor1 the solar cell1 the traveling wave tube1 the horn-reflector antenna1 the waveguide1 the solid-state maser1 and the electronic computer--to mention only some of the more important4 ,without them it would still be impossible to find a tiny speeding object miles out in space1 send signals to it1 amplify them billions of times1 and then return them to distant points on the earth4 ,some of the new devices that help make satellite communications possible ,illustration3 horn-reflector antenna ,illustration3 traveling-wave tube ,illustration3 transistor ,illustration3 solar cell ,illustration3 solid-state maser ,when you look back at it1 we have seen remarkable progress in satellite communications--and work is still continuing at a fast pace4 ,some of the milestones have been these3 ,,october #aide ,arthur ,c4 ,clarke publishes 8,extra-,terrestrial ,relays--,can ,rocket ,stations ,give ,world-,wide ,radio ,coverage80 in ,wireless ,world1 suggesting the use of satellites for communications4 ,,january #aa1 #aidf ,project ,diana of the ,u4 ,s4 ,army ,signal ,corps bounces microwave radar signals off the moon and back to the earth1 proving that relatively low power can transmit signals over very long distances4 ,,april #aiee ,john ,r4 ,pierce publishes 8,orbital ,radio ,relays0 in ,jet ,propulsion1 pointing out the requirements for a satellite communications system4 ,,july #bi1 #aieh ,congress passes the ,national ,aeronautics and ,space ,act1 setting up the ,national ,aeronautics and ,space ,administration 7,,nasa71 with satellite communications experimentation as one of its interests4 ,,december #ah1 #aieh ,score1 the first communications satellite1 is launched by the ,u4 ,s4 ,air ,force4 ,it is equipped with tape recorder units that transmit prerecorded messages back to the earth upon receipt of signals4 ,on ,december #ai a ,christmas greeting to the world recorded by ,president ,eisenhower--the first message from a satellite to the earth--is transmitted4 ,score continues to transmit for #ab days before its batteries become too weak for further use4 ,illustration3 uncaptioned ,,november #bc1 #aiei ,live voice transmission is accomplished from ,bell ,telephone ,laboratories in ,holmdel1 ,new ,jersey1 via the moon to ,jet ,propulsion ,laboratories in ,goldstone1 ,california4 ,this is the first of #ag tests in ,project ,moonbounce1 all using the moon as a reflector4 ,,july #h1 #aifj ,the ,bell ,system proposes to the ,federal ,communications ,commission a detailed plan for a world-wide communications system using active repeater satellites to provide telephone circuits and facilities for transmitting television to various parts of the world4 ,,august #ab1 #aifj ,echo ,i is launched into orbit by ,,nasa4 ,project ,echo carries on a large number of communications experiments and1 most important1 proves that it is practical to use a man-made satellite to reflect two-way telephone conversations across the ,united ,states4 ,echo also dramatizes the possibilities of satellites for communications4 ,since it is a #ajj-;foot inflated balloon made from aluminum-coated ,mylar1 it is large enough to be seen by the naked eye4 ,people throughout the world see ,echo ,i sail on schedule across the sky in its #ajjj-mile-high circular orbit4 ,three years later1 although it is now wrinkled and deflated1 the balloon is still in orbit4 ,illustration3 uncaptioned ,project ,echo provided valuable data for future work in satellite communications4 ,it demonstrated that a passive satellite--that is1 one that simply reflects the microwave signals it receives from an earth station back to another point--would work4 ,two-way conversations of good quality were sent between the ,bell ,laboratories ,holmdel station and ,jet ,propulsion ,laboratories in ,goldstone1 and successful transmission was made to other points in the ,united ,states and ,europe4 ,a scaled-up horn-reflector antenna proved itself4 ,a method of receiving microwave signals that had been little used until then1 known as frequency modulation with feedback 7,,fmfb71 performed very well4 ,new types of low-noise amplifiers using solid-state masers gave excellent results4 ,and tracking of the satellite by electronic computers1 by radar1 and by telescope proved to be extremely reliable4 ,illustration3 uncaptioned ,,october #d1 #aifj ,courier ,i-,b is launched by the ,army ,signal ,corps into a #ejj- to #fej-mile-high orbit4 ,a sphere weighing #ejj pounds and measuring #ea inches in diameter1 the ,courier satellite is powered by #bj1jjj solar cells and contains four receivers1 four transmitters1 and five tape recorders4 ,it is designed to demonstrate the possibility of using active repeaters for delayed transmission of messages4 ,signals are received1 stored on the tapes1 and then retransmitted back to earth when the satellite has moved on4 ,after #ah days in orbit1 technical difficulties ended ,courier's ability to send signals1 but it received and retransmitted #aah million words during its active life4 ,illustration3 uncaptioned ,,january #ai1 #aifa ,the ,american ,telephone and ,telegraph ,company is authorized by the ,federal ,communications ,commission to establish an experimental satellite communications link across the ,atlantic4 ,two #agj-pound satellites are to be launched by ,,nasa but will be designed1 built1 and paid for by ,,at,t4 ,this project is later given the name 8,telstar40 ,,may #ah1 #aifa ,,nasa selects the ,radio ,corporation of ,america to design and build the ,relay satellite1 which will be used to test the feasibility of transoceanic telephone1 telegraph1 and television communications4 ,,august #aa1 #aifa ,,nasa awards the ,hughes ,aircraft ,corporation a contract to build ,syncom1 an experimental active satellite to be placed into a #bb1cjj-mile-high orbit that will be synchronous with the rotation of the earth4 7,see page #cg for definitions of various kinds of satellite orbits47 ,,december #bj1 #aifa ,the ,united ,nations adopts a resolution on the peaceful uses of outer space that includes a request for world cooperation in developing a system of communications satellites4 ,both the ,united ,states and the ,soviet ,union sign the resolution4 ,,february #g1 #aifb ,president ,kennedy asked ,congress to pass a bill setting up a corporation to operate a satellite communications system4 ,the proposed corporation would be owned jointly by the public at large and the country's communications common carriers4 ,,july #aj1 #aifb ,project ,telstar is successful4 ,for the first time1 voice communications and live television are transmitted across the ,atlantic via a man-made satellite that picks up signals sent from one continent1 amplifies them1 and retransmits them to another continent4 7,on pages #ba to #cc we talk at further length about ,project ,telstar47 ,illustration3 uncaptioned ,,august #ca1 #aifb ,president ,kennedy signs the ,communications ,satellite ,act1 establishing a private corporation under government regulation--the ,communications ,satellite ,corporation--which will plan1 own1 and operate a commercial satellite communications system4 ,illustration3 uncaptioned ,,december #ac1 #aifb ,relay ,i is launched by ,,nasa4 ,similar in many ways to the ,telstar satellite1 it is an active repeater device that picks up telephone1 television1 and other electronic signals and retransmits them to a distant point4 ,relay also provides the first satellite communications link between ,north and ,south ,america4 ,the satellite is a tapered cylinder #cc inches long weighing #agb pounds4 ,a mast-like antenna at one end is used to receive and transmit a single television broadcast or #ab simultaneous two-way telephone conversations4 ,four whip antennas at the other end of the cylinder handle control1 tracking1 and telemetry--turning experiments on and off and sending information on the behavior of its components and on the amount of radiation it encounters in space4 ,relay is powered by nickel-cadmium storage batteries that are charged by more than #h1jjj solar cells mounted on its eight sides4 ,it contains two identical receiving1 amplifying1 and transmitting systems called transponders1 each with an output of #aj watts4 ,relay ,i is traveling in an orbit that ranges from #hbj to #d1fab miles high1 and circles the earth about every #ahe minutes4 ,soon after it is launched1 ,relay's telemetry reports trouble in the voltage regulator of one of the transponders1 which causes excessive power drain4 ,on ,january #c1 #aifc1 the alternate transponder is switched on1 and a successful series of tests--including live television broadcasts between the ,united ,states and ,europe--begins4 ,,january #d1 #aifc ,the ,telstar ,i satellite1 which for almost two months could not be turned on to transmit communications signals1 is reactivated by ,bell ,laboratories engineers4 7,the story of this ingenious electronic detective work is told in detail on pages #gh to #he47 ,,february #ad1 #aifc ,the first ,syncom satellite is launched by ,,nasa1 but its communications systems do not operate4 ,it is the first satellite to try for a synchronous path1 revolving around the earth once every #bd hours and thus appearing to hover continuously over the same longitude4 ,syncom is a short cylinder #bh inches in diameter and #ae inches long1 and weighs #hf pounds4 ,like ,telstar and ,relay1 it is powered by a combination of solar cells and nickel-cadmium batteries1 but it is designed to handle only one two-way telephone conversation and cannot transmit television4 ,illustration3 uncaptioned ,,may #g1aifc ,the ,telstar ,,ii satellite is launched for the ,bell ,system by ,,nasa4 7,see page #ca47 ,what ,about the ,future8 ,as this is written 7,june #aifc71 second ,relay and ,syncom launchings are in the offing4 ,and there are plans for more experimentation with passive satellites1 including a new1 more nearly rigid ,echo balloon4 ,further in the future1 studies are going on of a proposed ,intermediate ,altitude ,communications ,satellite for military use in the #f1jjj- to #aj1jjj-mile-high range 7beyond that of ,telstar and ,relay7 and ,advanced ,syncom1 a synchronous satellite of increased capacity4 ,work is also continuing to acquire new technical knowledge that will be needed in the future--such as various methods of keeping satellites stabilized in space and new ways of supplying power1 including improved solar cells and the use of radioisotopes4 ,the ultimate goal1 of course1 is a working commercial communications satellite system4 ,exactly when this will be a reality--and what form it will take--are questions whose answers still lie ahead of us4 ,illustration3 ,the orbits of four communications satellites vary in size and shape ,echo ,i #ajjj miles #ajjj miles ,relay ,i #dfab ,miles #hbj miles ,telstar ,i #ceca miles #eib miles ,telstar ,,ii #ffig miles #fjd miles ,project ,telstar ,in this section we will go into some detail about ,project ,telstar4 ,we do this because much of what we learned from this project applies to the general field of satellite communications4 ,the problems that were faced and solved are typical of the challenges that working engineers and scientists must meet today4 ,and there is1 of course1 another reason to put this much emphasis on ,telstar3 ,the six case histories in ,part ,,ii of our book were written by men who were involved with that project4 ,before reading their accounts it will be helpful for you to have some background information about it4 ,what ,project ,telstar ,was ,designed ,to ,do ,even its most enthusiastic planners at ,bell ,telephone ,laboratories never expected the sensation that ,telstar caused4 ,although it was a deadly serious venture--one of the steps along the way toward putting together a workable satellite communications system--its success made it the inspiration1 among other things1 of cartoons1 jokes1 and a couple of popular songs4 8,telstar0 soon became a name recognized around the entire globe4 ,stories about ,project ,telstar appeared in newspapers in almost every language1 in children's books1 in women's fashion magazines4 ,illustration3 ,on ,july #aj and #aa1 #aifb1 people on two continents saw these scenes on television at the same time1 with the aid of the ,telstar ,i satellite ,what caused all this stir in the summer and fall of #aifb8 ,the answer--now that we look back on it--seems rather clear3 ,for the first time1 the whole world discovered that satellite communications was really possible--that peoples separated by oceans could now be united by live television4 ,space had become an adventure1 not just for lonely astronauts1 but for everyone right in his living room4 ,project ,telstar1 of course1 had more serious objectives3 --to prove that a broadband communications satellite could transmit telephone messages1 data1 and television2 --to test1 under the stresses of an actual launch and the hazards of space1 some of the electronic equipment that had been developed for satellite communications2 --to measure the radiation that a satellite would meet in space2 --to find out the best ways to track a moving satellite accurately2 --to provide a real-life test for the special satellite communications antennas and other ground station equipment4 ,to do its principal job--communications--the ,telstar ,i satellite had to receive a signal from a ground station1 amplify it1 and then retransmit it on a different frequency back to other points on the earth4 ,this signal had to be strong enough and good enough to be received and understood on the ground4 ,to do its secondary job--measure radiation and other conditions in space--the satellite had to be equipped with special testing devices and had to have a means of reporting facts about the environment it encountered in space and the effects of radiation on solar cells and transistors4 ,to let us know how well its equipment was working1 the satellite had to record and transmit a large number of measurements--including such things as the temperature and pressure inside the satellite1 its orientation with respect to the sun1 the current and voltage in various parts of its electronic circuitry4 ,sending these measurements back to a ground station is called telemetry4 ,to help with tracking1 the satellite had to have a continuous radio beacon signal that could be easily picked up on earth4 ,finally1 the satellite had to be able to control its equipment by means of signals from the ground4 ,to keep the solar power plant from being overloaded1 there had to be some way of 8commanding0 the satellite to turn itself on or off4 ,as you will read later1 this was the one part of the satellite that caused us the most headaches once ,telstar ,i got into orbit4 ,the ,telstar ,i ,satellite--,outside ,telstar's outer appearance is very familiar by now3 a #cd-;inch sphere with #gb flat facets1 a double row of rectangular openings circling its center1 and a short1 oddly twisted antenna on one end4 ,of the #gb facets1 #fj are used for the solar cells that are the satellite's main power source4 ,when ,telstar is in sunlight1 these cells convert solar energy into electrical power2 at full capacity the #cfjj solar cells will supply about #ae watts4 ,as time goes by1 this power slowly diminishes as the cells are gradually damaged by such hazards of space as radiation particles and micrometeorites4 ,to reduce this damage1 the satellite's cells are covered with a thin layer of man-made sapphire4 ,two bands of rectangular openings go around the center of the satellite4 ,the smaller cavities1 of which there are #gb1 are receiving antennas2 the #dh larger ones are transmitting antennas4 ,this arrangement allows the antennas to transmit and receive equally well in all directions--except directly along the satellite's poles4 ,at one end of the satellite is an entirely separate receiving and transmitting antenna that takes care of all the signals needed for ,telstar's command1 tracking and telemetry4 ,the antenna is composed of four metal loops joined in the shape of a helix4 ,it receives the important command signals from the ground that give orders to the satellite's equipment4 ,it sends reports back to the ground from the special radiation measuring devices and other sensors aboard the satellite1 and it also transmits the continuous #acf-megacycle radio beacon that can be picked up by ground equipment searching for ,telstar4 ,six of the satellite's flat facets are used for special measuring devices4 ,two different radiation studies are made3 finding out how much damage will be done to solar cells and transistors1 and determining how many actual energetic particles--protons and electrons--are present in the part of space that ,telstar passes through4 ,these different jobs are done by special devices on various facets4 ,one1 for example1 consists of seven identical silicon transistors1 six having different thicknesses of shielding and one being left unshielded--the amount of damage done to each is recorded and reported back to earth4 ,devices on another facet measure the radiation damage to solar cells protected by various thicknesses of sapphire4 ,for the second radiation experiment--particle counting--four different types of silicon diodes are used as particle detectors4 ,these measure the energy deposited both by protons of three energy levels and by electrons as the satellite passes through belts of natural and man-produced radiation in space4 ,illustration3 ,the ,telstar ,i satellite--outside telemetry1 command and beacon antenna solar cells for power supply solar cells to measure radiation damage receiving antenna transmitting antenna transistors used to measure radiation damage solar aspect cells mirror ,illustration3 ,the ,telstar ,i satellite--inside 7looking at the electronics canister from the top down7 beacon transmitter traveling-wave tube amplifier radiation measurement equipment nickel-cadmium cells 7foamed7 ,measuring devices mounted on the surface of the ,telstar ,i satellite ,illustration3 particle detectors used to count protons ,illustration3 transistors used to measure radiation damage ,illustration3 solar cells used to measure radiation damage and as solar aspect indicators ,illustration3 particle detector used to count electrons ,there are two other special devices3 ,six single solar cells are spaced at regular intervals around the satellite2 these 8solar aspect0 indicators report the quantity of sunlight hitting them--and thus tell the direction in which the satellite is pointing4 ,three highly polished metal mirrors are also placed on ,telstar2 flashes of sunlight reflected from them can be seen in a telescope4 ,to give a precise indication of the satellite's position1 the data obtained from both the solar aspect cells and from the flashes off the mirrors are combined4 ,the ,telstar ,i ,satellite--,inside ,within the white aluminum-oxide outer shell of the satellite is crammed a complicated array of electronic equipment4 ,surprisingly1 most of this gear has to do not with ,telstar's prime function--communications--but with its command and telemetry systems4 ,the reason is that the satellite is an experimental device1 not just a spectacular way to relay television programs4 ,altogether1 the satellite's various electronic circuits contain more than a thousand transistors and almost #aejj semiconductor diodes1 plus a single electron tube--a traveling-wave tube used in the communications amplifier4 ,the satellite itself has a magnesium frame that is covered with aluminum panels4 ,all its electronic components are inside a aluminum canister1 #bj inches in diameter1 attached to the interior frame by special nylon lacings that reduced vibration inside the canister during launch4 ,when all the components and subassemblies had been carefully put in place and thoroughly tested1 the canister was filled with a liquid foam called polyurethane4 ,this material hardens into a very light and rigid solid1 completely enveloping the equipment and protecting it from damage and vibration4 ,after the canister was solidly foamed1 metal domes were welded onto the ends1 and it was enclosed in a many-layered blanket of aluminum-coated ,mylar 7the same material used in the ,echo balloon74 ,to keep its temperature properly controlled1 shutters on the canister's two ends are operated by bellows4 ,the satellite power system includes more than just solar cells4 ,when operating at full capacity1 the satellite's equipment needs more energy than the #cfjj solar cells can provide at one time4 ,so ,telstar also uses a storage battery made up of #ai rechargeable nickel-cadmium cells designed for this special purpose4 ,these ensure that the satellite has a continuous and sufficient supply of power1 even when all equipment is in operation or when the satellite is passing through the earth's shadow4 ,illustration3 ,after all electrical tests had been made on the satellite's components1 the electronics canister was filled with liquid polyurethane foam1 using this specially developed foam machine ,illustration3 ,the giant horn antenna at ,andover1 ,maine ,ground ,stations for ,satellite ,communications ,project ,telstar is actually an extension into space of microwave communications methods that have been thoroughly proved on the ground4 ,for ,project ,echo and other early experiments in satellite communications1 ,bell ,laboratories built a large antenna of the type known as a horn-reflector in ,holmdel1 ,new ,jersey4 ,for ,project ,telstar1 a similar but much larger antenna was designed4 ,it was located in a relatively isolated spot at ,andover1 in the western part of ,maine1 where it would be close to ,europe4 ,the site is nicely protected by a surrounding ring of low hills--high enough to keep out interfering radio signals1 but low enough not to block the satellite when it is near the horizon4 ,the giant ,andover horn is a steel and aluminum structure #agg feet long and #id feet high that weighs #chj tons4 ,at one end is a giant opening of #cfjj square feet2 from there the horn tapers down to a cab in which the very sensitive receiver and powerful transmitting equipment is located4 ,the entire antenna--horn1 cab1 and supporting framework--moves smoothly on tracks that allow it to rotate in a #cfj-;degree circle around its vertical axis 7changing azimuth74 ,it also can swing about its horizontal axis from the horizon up to the zenith 7changing elevation74 ,despite its size1 the antenna can revolve steadily and precisely in a complete circle in just four minutes4 ,signals are beamed to the satellite on a frequency of #fcij megacycles1 using modified ,bell ,system microwave equipment and a special traveling-wave tube with an output of #b kilowatts4 ,signals come back on a #dagj-megacycle frequency at a much lower power level--as small as a trillionth of a watt4 ,they are amplified by a ruby crystal maser that operates at the temperature of liquid helium--just a few degrees above absolute zero4 ,the whole antenna structure and its associated equipment are enclosed in a huge 8radome0--a bubble made from ,dacron and synthetic rubber only a sixteenth of an inch thick but measuring #baj feet in diameter and #afj feet high4 ,it is one of the largest air-supported structures ever erected4 ,the ,andover ground station includes a lot more equipment--most of it having to do with tracking the satellite1 computing its orbits1 sending and receiving command and telemetry signals1 and interconnecting the satellite with regular telephone and television land links4 ,most of this is located in a control building about a quarter mile from the giant radome4 ,illustration3 ,the ,french radome looms over the ,brittany countryside ,a ground station very similar to the ,andover installation has been built by the ,french ,national ,center of ,telecommunications ,studies at ,pleumeur-,bodou in ,brittany4 ,the ,british ,general ,post ,office has established a station at ,goonhilly ,downs in ,cornwall1 ,england1 which uses a large1 deep parabolic dish rather than a horn-reflector antenna4 ,both ,british and ,french stations participated in the first ,telstar experiments4 ,satellite communications ground stations also have been set up in ,fucino1 ,italy1 and near ,rio de ,janeiro1 ,brazil1 and others are under construction in ,west ,germany and ,japan4 ,the ,satellite ,goes ,into ,orbit ,at #d3ce a4m4 7,eastern ,daylight ,time7 on ,july #aj1 #aifb1 a ,thor-,delta rocket launched ,telstar ,i into its orbit1 almost exactly according to plan1 from the ,national ,aeronautics and ,space ,administration's ,cape ,canaveral base4 ,on ,telstar's sixth orbit around the earth--at #g3bf p4m4--the first transmission to and from the satellite took place4 ,during this pass telephone calls1 television1 and photos were transmitted between ,andover and ,holmdel4 ,some of these signals were also picked up in ,europe4 ,on the next day1 a taped television program was sent from ,france to the ,united ,states1 and a live program came from ,england via ,telstar4 ,during the next four months1 more than #djj transmissions were handled by ,telstar--including #ej television demonstrations 7both black-and-white and color71 the sending of telephone calls and data in both directions1 and the relaying of facsimile and telephotos4 ,in addition1 the satellite performed more than #cjj valuable technical tests4 ,almost all of them showed remarkably successful results4 ,radio transmission was as good as was expected4 ,telstar's communications equipment worked exactly as it should1 with no damage from the shock and vibration of the launch4 ,temperatures inside the satellite were kept under good control4 ,the satellite was successfully stabilized--prevented from tumbling over and over--by being spun around its polar axis1 with the spin rate gradually decreasing1 as predicted1 from its rate of #agg4g revolutions per minute just after launch4 ,the solar cells worked almost exactly as expected4 ,much extremely valuable data about radiation in space was reported4 ,the ground stations accurately traced the fast-moving satellite in almost routine fashion4 ,but it would be asking too much to have everything perfect4 ,telstar ,i unexpectedly met radiation in space estimated to be #ajj times more potent than had been predicted4 ,as a result1 difficulties arose during ,november #aifb in some of the transistors in its command circuit--and on pages #gh to #he we tell you what these problems were1 how they were discovered1 and what steps were taken to overcome them4 ,some time later the satellite again failed to respond to commands from the ground1 and on ,february #ba1 #aifc1 it went silent4 ,illustration3 ,new gold-domed device on the ,telstar ,,ii satellite can measure electrons in an energy range from #gej thousand to #b million electron volts4 ,the ,second ,telstar ,satellite ,on ,may #g1 #aifc1 the ,telstar ,,ii satellite was launched into an elliptical orbit almost twice as large as that of ,telstar ,i1 ranging from an apogee of #ffig miles to a perigee of #fjd miles4 ,the new satellite circles the earth once every #bbe minutes4 ,the higher altitude provides ,telstar ,,ii with longer periods when it is visible at both ,andover and ground stations in ,europe1 and keeps it out of the high-radiation regions of space for a greater part of the time4 ,the satellite itself is much the same as ,telstar ,i1 except for a few minor changes that make its weight #age rather than #agj pounds4 ,its radiation measuring devices have a greater range of sensitivity1 and there are six new measurements to be reported back to earth4 ,telemetry can now be sent on both the microwave beacon and1 as before1 on the #acf-megacycle beacon4 ,to help prevent the kind of damage that occurred in the transistors of ,telstar ,i's command decoders1 ,telstar ,,ii uses a different type of transistor1 in which the gases have been removed from the cap enclosures that surround the transistor elements4 ,a simplified method of operation for the giant ,andover horn antenna is now in operation1 with the autotrack alone being used for precise tracking and pointing4 ,telstar ,,ii's first successful television transmission took place on ,may #g1 and a new series of technical tests1 radiation measurements1 and experiments in transoceanic communications has begun4 ,how the ,telstar ,satellite ,works ,a lot of facts and figures sometimes lead only to confusion1 but these pages may help make things clearer4 ,here you can see--step by step--exactly what happens during a typical pass of the ,telstar satellite over the ,andover ground station3 ,illustration3 ,telstar satellite at work #a ,the satellite comes over the horizon4 #b ,the command tracker1 knowing from computer data the satellite's approximate location1 begins to search for its continuous #acf-megacycle beacon4 ,a quad-helix antenna 7four long spirals7 tracks the satellite to an accuracy of one degree4 #c ,when the satellite is located1 the command transmitter turns on the satellite's transistor circuits and telemetry4 ,the ground station then checks on the satellite's operating condition1 as reported by telemetry4 #d ,the command transmitter then turns on the satellite's traveling-wave tube1 which starts the transmission of a #djhj-megacycle beacon signal4 #e ,the precision tracker--an eight-foot parabolic dish 7known as a ,cassegrainian antenna7 mounted on a pylon--locates this beacon and tracks it to within one-fiftieth of a degree4 #f ,the horn antenna's autotrack mechanism1 which is pointed by both the precision tracker and data from magnetic tapes1 locates the satellite's beacon signal4 #g ,now the horn antenna locks onto the satellite1 with the autotrack continuing to make fine adjustments in pointing the horn4 #h ,the equipment is now ready for communications signals to be sent from the two-kilowatt ground transmitter to the satellite4 #i ,the satellite receives the signals and converts them down to a frequency of #ij megacycles2 they are amplified in transistor circuits and converted up to a new frequency of #dagj megacycles4 #aj ,the signals are amplified again by the traveling-wave tube--for a total amplification of as much as ten billion times--to get a radiated power of #b watts4 #aa ,the #dagj-megacycle signals are now transmitted in all directions by the satellite's equatorial antenna4 #ab ,these signals can be picked up at ,andover or at any other ground station equipped with a suitable antenna that is within line of sight of the satellite4 #ac ,at ,andover1 the received signals are amplified by means of a solid-state maser and a frequency-modulation-with-feedback circuit4 #ad ,they can now be relayed via regular land lines to their destination4 #ae ,near the end of a pass1 the command tracker turns off the communications circuits and telemetry in the satellite4 #af ,the satellite drops below the horizon4 ,some ,big ,problems in ,satellite ,communications ,we hope the last few pages haven't given you a wrong impression of satellite communications4 ,it is easy to assume1 when we list the orderly1 step-by-step progress from purely theoretical ideas to a working satellite such as ,telstar1 that everything has gone like clockwork4 ,that isn't the case at all--and in the rest of this book we are going to show you why it isn't4 ,many problems had to be solved2 many scientific and technological advances had to be made4 ,we touched on a number of the problems of satellite communications in our detailed account of ,project ,telstar4 ,most of them are not confined to that project--they are the sorts of questions that any complex advance in satellite communications will run into4 ,we will list some of the more important ones here4 ,then1 in ,part ,,ii1 we will talk about some general methods of solving scientific and technological problems4 ,all this is a rather roundabout--but necessary--way of leading up to our main interest3 the accounts by six ,bell ,laboratories engineers and scientists of their work to solve some typical problems in satellite communications4 ,the many complications of satellite communications can be divided into several groups4 ,first of all1 there are the problems involved in fitting satellite communications into an already established world communications system4 ,there are1 next1 many problems1 both small and large1 in designing the right kind of satellite4 ,there are the problems of launching a satellite and getting it into the proper orbit4 ,there are the problems in making sure it stays in the right orbit once it gets there4 ,and1 finally there are the problems in seeing that it continues to do its job reliably4 ,in these five categories there are a lot of specific questions that must be answered to plan a working satellite communications system4 ,a list of some of them follows4 ,we haven't attempted to cover everything1 but these should give you some idea of the tasks and questions involved in planning an immense project like this4 ,general ,problems of a ,satellite ,communications ,system ,what jobs could a communications satellite do best8 ,should it be used for television8 ,should it carry telephone messages8 ,how many8 ,would it be more valuable for data transmission8 ,facsimile8 ,what parts of the world should be covered8 ,can all the problems of international cooperation be solved8 ,would a satellite that could broadcast directly to home receivers be possible8 ,what military uses could a communications satellite system serve8 ,would a passive satellite--one that reflects signals without amplifying them--be worth developing8 ,or should the emphasis be on active repeaters1 which can receive1 amplify1 and retransmit signals8 ,what kind of technical standards should be set as the minimums8 ,how detrimental is time delay in sending communications to a satellite and back8 ,what kind of ground transmitters and receivers would be needed8 ,how powerful or sensitive should they be8 ,where should they be located8 ,how many satellites would be needed8 ,how much would all this cost8 ,satellite ,design ,problems ,how big should a satellite be8 ,what should it be made of8 ,what color should it be8 ,what kind of power supply should it use8 ,how powerful should its electronic equipment be8 ,what should be its message-handling capacity8 ,what are the best receiving and transmitting antennas to use8 ,what frequencies ought to be employed8 ,what kind of modulation should be used8 ,how should signals be amplified8 ,what kind of telemetry equipment will be needed8 ,how can radiation in space be measured8 ,launching and ,tracking ,problems ,how big a rocket booster would be needed8 ,from what part of the world should a satellite be launched8 ,what kind of orbit should it go into8 7,see table below7 ,how far up should the satellite go8 ,how can a satellite be tracked once it is in orbit8 ,how do we predict future orbits of a satellite8 ,orientation and ,control ,problems ,how can orders be given to a satellite while it is in orbit8 ,if it is to stay in a fixed attitude1 how can it be kept there8 ,what can be done to keep a satellite properly stabilized8 ,can optical measurements be made on a satellite8 ,what will be the effects of sunlight and gravity on its position in space8 ,problems of ,reliability ,how long will the satellite remain active8 ,what factors will affect its service life8 ,should its equipment be made redundant8 ,what kinds of components will be most reliable in space8 ,what is the best way to test its equipment before the satellite is launched8 ,what effects will radiation in space have on the satellite8 ,how can its components be protected from these radiation effects8 ,how can it be insulated from extremes of temperature8 ,what can be done to protect the satellite from the shock and vibration of launching8 ,will it be possible to repair an orbiting satellite8 ,could a satellite be brought back from orbit to be repaired or salvaged8 ,types of ,satellite ,orbits ,circular ,orbit--an orbit whose altitude from the earth remains constant2 it makes a circle that has the center of the earth as a center4 ,elliptical ,orbit--an orbit whose altitude from the earth varies from one extreme to another2 it makes an ellipse with the center of the earth as one focus4 ,the orbit's lowest altitude is called the perigee1 its highest altitude is called the apogee4 ,equatorial ,orbit--an orbit in the plane of the earth's equator4 ,polar ,orbit--an orbit in a plane formed by the ,north and ,south ,poles4 ,synchronous ,orbit--an orbit whose period is #bd hours1 the same as that of the earth revolving on its axis--so that the satellite's and the earth's angular velocities are the same4 ,although there are many possible kinds of synchronous orbits1 each must have an average altitude above the earth's surface of approximately #bb1cjj statute miles4 ,stationary ,orbit--an orbit that is circular1 equatorial1 and synchronous--so that the satellite will appear stationary from any point on the earth4 ,inclined ,synchronous ,orbit--an orbit that is synchronous but not stationary1 since it does not follow the plane of the equator4 ,from a point on earth1 it will appear to follow a figure eight pattern about a line of constant longitude4 part #b ,satellite ,communications ,case ,histories ,illustration3 ,title image ,six ,typical ,problems ,the questions we listed in ,part ,i cover a very broad area of science and technology4 ,their answers involve1 more than anything else1 physics1 electrical engineering1 and mechanical engineering4 ,some1 however1 also require that the men who work on them know chemistry1 metallurgy1 mathematics1 and occasionally even biology1 psychology1 geography1 and economics4 ,we obviously can't show you how all the problems in ,part ,i can be solved4 ,rather1 we have picked six of them as examples4 ,they are not necessarily the most important ones1 but they seem to us to be typical of what engineers and scientists working in the satellite communications program actually have to do4 ,these are the six problems we will be talking about at length3 --,how do we calculate a satellite's orbit8 --,what color should a satellite be8 --,how can we make optical measurements on a satellite8 --,how do we keep solar cell power plants working in space8 --,would time delay be a problem in using a synchronous satellite8 --,how can we repair an orbiting satellite8 ,as you can see1 we have picked problems that offer a good deal of variety4 ,some of them have been satisfactorily solved2 for others the solutions are not yet complete4 ,some deal with basic scientific research2 others are much more concerned with the engineering applications of technical knowledge4 ,some were solved by careful1 logical thinking2 others were solved almost by accident4 ,some deal with a particular immediate task 7in this case1 ,project ,telstar72 others are more concerned with general planning for satellite communications4 ,at the ,foundation3 ,basic ,physical ,principles ,despite these many important differences1 there is one common thread running through the solving of all the problems we have chosen4 ,the men who have been working on them had to know some basic principles of classical physics--principles that most of them first learned in their high school physics classes4 ,you can't1 for example1 calculate a satellite's orbit without knowing ,newton's ,laws of ,motion4 ,you can't make optical measurements on a satellite without knowing the law of reflection of light4 ,you can't decide what color a satellite should be without knowing the law of heat exchange4 ,to emphasize the importance of a solid grounding in basic physical principles1 we have tried to have our problems touch on most of the general areas of physics3 mechanics1 heat1 sound1 light1 electricity and magnetism1 electronics1 the properties of matter1 atomic physics1 physics of the solid state4 ,but most of them1 of course1 are not limited to just one of these--they cross the lines of a number of areas4 ,for instance1 the problem of keeping solar cell power plants working in space involves laws of heat1 mechanics1 and atomic physics1 as well as physics of the solid state4 ,and1 in studying the perception of time delay1 we even branch out into experimental psychology4 ,problem-,solving ,techniques ,when you start to solve a problem in science or engineering you can go about it in several ways4 ,in some cases you have no choice3 ,there may be only one practical method of doing the job4 ,other times1 there may be several ways to attack the problem4 ,you may try one1 find it to be unfruitful1 and then work on another approach4 ,you will see both these methods of attack in the case histories we present in the next chapter4 ,here are some of the techniques of scientific problem solving that we will be discussing3 --,applying basic principles directly4 ,in answering the questions 8,how do we calculate a satellite's orbit80 and 8,what color should a satellite be80 the successful procedure was to begin with basic known concepts and use them in a new field4 --,adapting known devices4 ,to answer the question 8,how can we make optical measurements on a satellite80 the story was somewhat different3 ,devices that already had been developed--mirrors1 telescopes1 and cathode ray tubes--were utilized in a new and different way4 --,developing entirely new equipment4 ,another question--8,how do we keep solar cell power plants working in space80--deals with an entirely new area of investigation2 it could only be answered by perfecting some entirely new techniques4 --,experimentation4 ,sometimes there is no definite answer to a problem4 ,in the case of 8,would time delay be a problem in using a synchronous satellite80 investigation is still going on4 ,our report tells of one set of experiments that helped add to our information about this problem--the final answer1 if there is one1 must come later4 ,but1 as with many problems1 experimentation continues4 --8,detective work40 ,our sixth problem is really a unique one--and1 for want of a better way to describe it1 we use this title4 ,it tells how the problem of 8,how can we repair an orbiting satellite80--something never even attempted before--was ingeniously solved by means of scientific deduction hundreds of miles away from the problem itself4 ,,case ,,history ,,no4 #a ,how ,do ,we ,calculate a ,satellite's ,orbit8 ,franz ,t4 ,geyling ,mechanics ,specialist--,head1 ,analytic and ,aerospace ,mechanics ,department ,,the ,,problem ,before you can do anything with a communications satellite you have to know where it is at every instant of its motion around the earth4 ,in other words1 you need to know its orbit quite accurately4 ,when you know this1 you can predict when a particular station on the ground will be able to see the satellite and communicate with it4 ,you also can tell when two or more stations can see the satellite simultaneously and communicate with each other4 ,and you can estimate how many satellites will be needed to provide a group of ground stations with enough working time to maintain a communications service4 ,this last1 after all1 is the ultimate goal of all our efforts in the communications satellite field4 ,in determining a satellite's orbit1 we find that we must do three things3 --,we must obtain information on the position--and perhaps also the velocity--of the satellite whenever possible by observing it with scientific instruments4 --,we must use this information to determine the satellite's orbit at the time of the observations4 --,we must be able to tell how this orbit changes between the times when we observe it1 because a satellite's orbit does not remain constant with the passage of time4 ,in the case of the ,echo ,i satellite 7see page #af71 we engaged in the first and third of these activities4 ,we had many chances to follow the satellite with our radars1 and we could speculate how its orbit was changing through the months4 ,in the case of the ,telstar ,i satellite1 we engaged in all three kinds of activity4 ,we shall take a look at these problems in the sequence in which we came across them1 for both the ,echo and ,telstar satellites4 ,how ,we ,track ,satellites ,we collect on the ground most of the information to calculate a satellite's orbit1 using optical instruments or radar equipment4 ,following a satellite through the sky is called tracking2 in the early days after the first ,sputniks1 some of this tracking was done with the naked eye or with very simple telescopes by the ,moonwatch teams4 ,many of you may have observed ,echo ,i on a clear night without any kind of instrument4 ,illustration3 ,figure #a satellite horizon elevation azimuth ,north ,east ,if we use a telescope1 we note the time of the observation and we usually take a photograph of the satellite4 ,we locate the satellite in terms of the two angles shown in ,figure #a4 ,one of these is the elevation angle--the number of degrees a telescope must be tilted above the horizon to see the satellite4 ,the second is the azimuth angle--the number of degrees between the plane in which we measure the elevation angle and the north direction4 ,of course1 we can also point a radar antenna at the satellite in the same manner4 ,the radar can receive a signal transmitted by the satellite1 or else it can send a signal to the satellite and watch for the reflected waves that eventually return4 ,in the latter case1 the satellite must have sufficient surface area to produce an adequate reflected signal4 ,these two kinds of precision tracking were both possible with ,echo ,i4 ,radar can also do something that optical equipment usually can't do3 measure the distance out to the satellite4 ,the ,basic ,physics of ,satellite ,motion ,illustration3 ,figure #b earth north pole satellite n t ,the ,echo ,i satellite was launched into a circular orbit inclined at an angle to the plane of the earth's equator4 ,in ,figure #b this equatorial plane intersects the plane of the satellite orbit along the line ,,opm4 ,the point ,o represents the center of the earth1 the point ,m is on the satellite orbit1 and the ,point ,p is on the equator4 ,at any instant1 the satellite may be located in its orbit by the angle 1 which is measured between the line ,,om and the line ,,oq1 where the point ,q is the satellite's location4 ,if the satellite moves in a circular orbit1 as in this case1 the angle is proportional to time4 ,that is1 we can write nt4 ,we call n the angular speed of the satellite2 one way of measuring this is in degrees per second4 ,thus1 the satellite is whirling at a constant speed about the earth like a stone tied to a string4 ,let us examine the physics of this situation a little more closely with the help of ,figure #c4 ,if the satellite is moving with the velocity v1 then we know that the centrifugal force acting on it is 7mv7_/7r71 where m is the mass of the satellite and r is its distance from the center of the earth4 ,obviously1 no string ties the satellite to the earth1 but the force of gravitational attraction between the earth and the satellite has the same effect4 ,newton's law of mutual attraction tells us that this force is proportional to the product of the two masses divided by the square of the distance between their centers1 or 7km7_/7r71 where k is a constant that essentially represents the mass of the earth4#a ,newton's law also tells us that this force will be pointing toward the center of the earth if the earth is spherical4 ,when the satellite is in circular motion1 the centrifugal force and the gravitational force must balance each other4 ,hence we have 7km7_/7r7 7mv7_/7r7 and from this we can solve to find that the velocity of the satellite must be equal to v 77k7_/7r774 ,in the case of the ,echo ,i satellite1 which was designed to have a radial distance of r #ejjj miles1 this velocity amounts to about #d4d miles per second4 ,the time for one revolution in orbit is obtained with the formula ,t 7#br7_/7v74 ,for the ,echo satellite this time1 ,t1 turns out to be just about two hours4 ,illustration3 ,figure #c ,calculating the ,orbit of ,echo ,i ,these basic physical principles of satellite motion can give us many useful answers4 ,they tell us how fast we must move a precision tracker to follow the satellite through the sky1 how much time a satellite will spend above the horizon1 and how long will be the time from one chance of seeing it to the next4 ,however1 in the ,echo project we were not merely concerned with planning our experiments from hour to hour2 we also needed to know how the satellite would move for weeks and perhaps months in advance4 ,when you study the motion of a satellite over such a length of time1 you discover that its circular orbit will not remain the same as it was at launch4 ,this fact had been observed on other satellites and was to be expected also with ,echo4 ,in everything we have said so far it was assumed that the earth was a perfect sphere1 which is the way a geographer's globe presents it to us4 ,in reality1 the earth is somewhat flattened1 with its diameter from the north pole to the south pole being somewhat shorter than its diameter at the equator4 ,one way of looking at this is to visualize the earth as a sphere with some material added in the equatorial zone1 which we may call equatorial bulge4 ,this bulge causes ,echo's orbit to have a slow 8wobble0 about the earth's polar axis1 somewhat like that of a spinning top4 ,another force that makes the satellite's orbit shift slightly is the faint pressure caused by the light from the sun4 ,although this pressure is much too small for us to perceive without the help of very delicate instruments1 it is enough to affect a satellite1 which has nothing to support it in space and is exposed to solar pressure for a very long time4 ,since the ,echo balloon is a plastic sphere1 #ajj feet in diameter1 that weighs only a little more than #ajj pounds1 the light rays striking its surface are enough to cause a second 8wobble0 effect4 ,this wobble centers about the line from the earth to the sun4 ,light pressure also forces the orbit to go slightly out of round from a perfect circle1 and other gradual effects on the satellite's orbit are caused by the gravitational attraction of the moon and the sun4 ,all these disturbances are ever-present and act simultaneously1 and a satellite's total response to them is very complicated4 ,fortunately1 however1 most of the changes take place at a very slow and uniform rate1 and we can predict them fairly accurately4 ,calculating the ,orbit of ,telstar ,i ,illustration3 ,figure #d ,in ,project ,telstar we had to calculate the satellite's orbit from observations made by our precision trackers4 ,this introduced a few problems in addition to the ones we encountered with ,project ,echo4 ,in the first place1 the orbit of the ,telstar satellite is a elongated ellipse1 as indicated in ,figure #d1 rather than being almost circular1 as in the case of ,echo ,i4 ,we mentioned earlier that a precision tracker can furnish data on a satellite's elevation angle1 ,e1 and azimuth1 ,a 7see ,figure #a74 ,it can also give us a reading for 1 the distance from the tracker to the satellite 7,figure #d74 ,if we know the position of the tracker on the earth1 we can reduce the quantities ,a1 ,e1 and to the angle and the distance r 7measured from the center of the earth to the satellite74 ,these two quantities locate the satellite in the plane of its orbit1 but in order to describe its position completely we must also specify this orbital plane4 ,in ,figure #e the orbital plane is shown as a shaded surface1 with and r being the same as before4 ,you will recall that the line ,,om represents the intersection between this plane and the equatorial plane2 we call the angle i between the two planes the inclination of the orbit4 ,finally1 we have the angle between the line ,,om and some line ,,oa to the point ,a1 which we can choose as any convenient spot in the equatorial plane4 ,now we have specified the orbital plane completely4 ,the point ,a can be found from day to day by fixing its position relative to a certain star in the sky4 ,illustration3 ,figure #e ,figures #d and #e tell us something about the geometry of the satellite's position in space1 but for the complete story we must also give the time at which it can be found there4 ,for this purpose1 there are some astronomical laws that relate position on an elliptic orbit to time4 ,two of these are illustrated in ,figure #f2 in looking at this figure1 you should imagine that you are standing off to one side of the orbital plane to get a good view of the entire orbit4 ,the longest dimension of the ellipse1 #b;a1 is called the major axis2 this dimension is related to the satellite's period--the time it takes to go once around the ellipse4 ,more than three hundred years ago the astronomer ,johannes ,kepler observed that the period ,t1 of an ellipse is ,t #b77;a7_/7k771 where k again was 7using ,newton's work7 essentially the mass of the earth4 ,illustration3 ,figure #f ,instead of a complete revolution1 we may only be concerned with part of one orbit4 ,let's say that this part lies between the two positions ,p and ,p that the satellite occupies at the two times t and t 7see ,figure #f74 ,then another of ,kepler's laws says that the ratio between the time difference t - t and the period ,t equals the ratio between the sector of the ellipse ,,op,p and the area of the entire ellipse4 ,now let us see how we can use the quantities r1 1 i1 and as well as ,kepler's two time laws to determine the motion of the satellite in space4 ,suppose that we have made observations of the ,telstar at two times t and t and that we have measured its distance along lines and in ,figure #g4 ,in other words1 we know that at these two times the satellite was at the points ,p and ,p4 ,since three points determine a plane1 we know in this case that ,p1 ,p1 and ,o define the satellite orbital plane4 ,knowing this1 we can now calculate the angles and 1 the distances r and r1 and the angles i and 4 7,the detailed formulas for this are derived from analytic geometry47 ,illustration3 ,figure #g ,however1 we still do not know the length and the width of the particular ellipse the satellite is following and how this orbit is oriented within the orbital plane4 ,let us imagine again that we can stand off to one side of the orbit and take a good look at it2 ,figure #h shows us what we would see4 ,there are the two points ,p and ,p at which we have observed the satellite4 ,we know the positions of these points relative to each other and in relation to the center of the earth1 because we have already calculated r1 r1 1 and 1 ,but any number of ellipses could be made to pass through these two points4 ,some might be very large1 others might be so narrow that they would intersect the earth and thus be impossible4 ,however1 only one of these ellipses will satisfy the time difference that we observed between ,p and ,p4 ,in other words1 the shape and period of this particular ellipse must be such that it will cause the satellite to pass through ,p and ,p in exactly the time interval t - t4 ,if we work out our time formulas1 we will convince ourselves that there is only one such ellipse4 ,when we have found it1 we have determined the orbit of the ,telstar satellite from the two observed positions and times4 ,illustration3 ,figure #h ,illustration3 ,figure #i ,in principle then1 we could keep track of the ,telstar satellite by making a pair of observations ,p and ,p1 and then predicting ahead a short segment of an orbit that is the ellipse we have computed4 ,after a while we must verify this ellipse with two more observations ,p and ,p1 predict ahead over another segment1 and verify again with ,p and ,p 7see ,figure #i71 ,the reason we have to keep taking new measurements is that the elliptic orbit does not remain the same4 ,as we discussed in connection with ,echo ,i1 the orbital plane will 8wobble0 about the earth because of the equatorial bulge4 ,we also know that the orbit's major axis will revolve within the orbital plane4 ,as we have seen before1 these effects are small and can be represented by appropriate mathematical formulas4 ,if we calculate them1 we will see the connection between one pair of observations and a later one1 and eventually we can increase the time interval between successive pairs of observations4 ,there are also mathematical formulas that we can use to predict the position of the satellite for many revolutions in its elliptic orbit4 ,in order to predict orbits successfully1 we must also realize that the measurements we obtain from a precision tracker1 such as the angles ,a and ,e and the distance 1 are always subject to small inaccuracies4 ,thus it is not really possible to take just two measurements like ,p and ,p and determine a satisfactory orbit from them4 ,in reality1 our tracker takes many readings1 and these are averaged to give adequate information about the orbit4 ,therefore1 the picture we have in mind is not quite like ,figure #g1 but rather like ,figure #aj4 ,here the trackers have established a series of points that are somewhat scattered1 and by taking averages we can calculate an orbit that passes through them in a smooth fashion4 ,the trackers we have mentioned so far have given us azimuth and elevation angles and also the distance to the satellite at every instant4 ,sometimes we must use simpler instruments that do not yield all this information4 ,they might1 for instance1 only give us the two angles4 ,the mathematics of calculating an orbit from such measurements is somewhat different1 but the process is fundamentally the same as we have discussed here4 ,when you do these calculations for the ,telstar satellite from one day to the next--and especially if you have more than one satellite to keep track of--the amount of work will become quite large4 ,nowadays our calculations are done for us on electronic computers1 which both receive information from the tracking instruments automatically through ,teletype or ,data,phone channels and send back information concerning future positions of the satellite to the ground stations4 ,there are still quite a few problems to be solved1 and we are presently working on ways of making all this equipment perform the orbit predictions for the ,telstar satellites automatically and efficiently4 ,illustration3 ,figure #aj ,franz ,t4 ,geyling was born in ,tientsin1 ,china1 and received a ,b4,s4 in #aiej1 an ,m4,s4 in #aiea1 and a ,ph4,d4 in #aied from ,stanford ,university4 ,he joined ,bell ,telephone ,laboratories in #aied1 and has been engaged in celestial mechanics studies of rockets and satellites1 as well as stress analysis of submarine cables4 ,,case ,,history ,,no4 #b ,what ,color ,should a ,satellite ,be8 ,peter ,hrycak ,mechanical ,engineer--,member of ,staff1 ,electron ,device ,laboratory ,,the ,,problem ,it is important for a satellite to stay at the proper temperature while it is orbiting in space4 ,the instruments aboard it must continue to operate properly1 and one way of insuring this is to keep them from being exposed to extreme heat or cold4 ,we can1 of course1 regulate a satellite's temperature somewhat with various kinds of devices1 and we can see that one of its ends does not point towards the sun for too long4 ,but in designing the ,telstar satellite we also wanted to control temperature in an easier way3 by covering the satellite's external surface with material with the best properties--including the right color--for maintaining its over-all temperature at the right level4 ,the ,radiation of ,heat ,a satellite's temperature is determined by the balance between the heat that enters the satellite and the heat that leaves it4 ,this means that we must be concerned with how heat is transferred4 ,heat can be transferred in three ways3 by conduction1 when two bodies are in direct contact and their molecules collide2 by convection1 which utilizes the movement of warm currents in a fluid2 and by radiation1 in which heat energy travels as electromagnetic waves at the speed of light4 ,with a satellite1 we are concerned only with the last of these1 since the only way energy can be gained or lost in space is by radiation4 ,in the transfer of heat by radiation1 the surface of the heated body--such as a satellite--is very important4 ,all energy gained must be absorbed at the surface2 all energy leaving must be emitted at the surface4 ,so the physical properties of this surface control how energy is absorbed and how it is emitted4 ,the origin of the radiant energy is vitally important2 most surfaces1 for instance1 will behave differently when exposed to solar radiation from the sun's temperature of #aj1jjj ,fahrenheit than when exposed to radiation from nearby objects at room temperature4 ,absorptivity and ,emissivity ,the physical property of a material that controls the way it absorbs radiant energy is called its absorptivity1 and the property that controls its emission of energy is its emissivity4 ,for absorptivity we use the symbol 2 for emissivity we use the symbol 4 ,when radiant energy reaches a surface1 only a certain part of it is absorbed2 the rest is either reflected1 just as light rays are reflected1 or else passes right through it4 ,the absorptivity1 1 of a substance tells us what percentage of radiant energy it will absorb4 ,a perfect absorber1 or black body1 would absorb all the radiant energy that reached it4 ,if such an ideal substance existed 7which it doesn't7 we would say it had an of #a4 ,the actual absorptivities of real substances are indicated by numbers between #j and #a3 ,the of black velvet cloth1 for example1 is about #j4ig2 that of a polished silver mirror is about #j4jh for solar radiation 7absorptivity for most polished metals for room temperature radiation is even lower74 ,we measure emissivity1 1 in very much the same way4 ,a hypothetical black body would emit all the energy it possibly could and have an of #a2 the emissivities of real substances are indicated by numbers between #j and #a4 ,for any given frequency 7or color7 of light1 a substance's absorptivity and emissivity are equal2 however1 the total spectrum of frequencies of the energy absorbed is usually different from that of the energy emitted4 ,the ratio between emissivity and absorptivity1 _/1 is very important1 as we shall see later4 ,if this ratio is greater than #a1 it means that a substance absorbs heat faster than it emits it1 and thus tends to become warmer4 ,if the ratio is less than #a1 the reverse is true--the surface emits radiant energy at a faster rate than it absorbs it1 and tends to become cooler4 ,how ,we ,measure the ,radiation of ,heat ,this is one of the fundamental relationships of modern physics3 ,qbody ,a,t4 ,it was discovered experimentally by ,josef ,stefan in #ahgi1 and verified theoretically by ,ludwig ,boltzmann2 it is known as the ,stefan-,boltzmann ,law4 ,this formula tells us the amount of radiant energy1 ,qbody1 that will be emitted by a body having the surface area ,a when it is at the temperature ,t4 ,temperature1 here1 is measured in degrees ,rankine 7,r71 or ,fahrenheit temperature above absolute zero 7to calculate degrees ,rankine1 add #dfj to the temperature in degrees ,fahrenheit74 ,the expression ,a is used to show that only a certain fraction of the energy that would leave a perfect black body of area ,a will actually leave a real body of the same size2 the size of this fraction is determined by the body's emissivity4 ,the symbol is a quantity we call the ,stefan-,boltzmann constant4 ,we can also calculate the heat from the sun that will be absorbed by a body4 ,if we let ,s be the total amount of solar energy that would be absorbed by a perfect black body1 ,s will be the amount that is actually absorbed by a body with an absorptivity of for solar radiation4 ,if our body is a spherical satellite1 the sun's rays will only strike it from a single direction4 ,thus only an area equivalent to the sphere's cross-section 7largest inscribed circle7 will receive energy at any one time4 ,since1 as shown in the sketch1 this area 7a r7 is one-fourth that of the sphere's total surface area 7,a #dr71 we know that the radiant energy from the sun that is absorbed will be ,qsun 7,a7_/7#d7,s4 ,a man-made satellite's position relative to the earth is very like that of the earth in relation to the sun2 the earth1 after all1 is itself a satellite of the sun4 ,and during most of its useful life a satellite will be in thermal equilibrium--it will be losing just as much heat energy by its own radiation into space as it will be gaining from other sources1 primarily the sun4 ,since the total amount of energy it absorbs is equal to the amount of energy it emits1 ,qbody is equal to ,qsun4 ,this means that we have the equality ,a,t 7,a7_/7#d7,s4 ,now1 if we solve this for temperature1 we will get ,t 777_/77 7,s7_/7#d77774 ,illustration3 ,although the total surface area1 ,a1 of a sphere is #dr1 light rays from the sun only strike half the surface at any one time4 ,this area1 a1 is effectively equal to the sphere's cross-section1 r4 ,this equation is well known in astronomy1 and has been used for more than #hj years to calculate the temperatures of various objects in the sky4 ,today1 we still find it useful for measuring the surface temperatures of man-made satellites such as ,telstar4 ,since both ,s and are known constants 7in this case1 we use the quantities ,s #dde and #j4agc #aj71 you can see that temperature is dependent on the _/ ratio4 ,finding the ,right ,surface for ,telstar ,illustration3 ,cutaway view of the inside of the ,telstar ,i satellite1 showing the electronics canister covered with its protective blanket of many layers of ,mylar4 ,to control temperature1 shutters automatically open all the way if the canister gets hotter than #hj;,f1 close completely if it goes down to #ej;,f4 shutter 7closed7 electrical heat transfer insulation blanket heat transfer by radiation shutter 7open7 solar cells electronic chassis heat transfer by circulation thermal control mechanism nylon lacing microwave antennas frame ,in designing the ,telstar satellite1 both its internal and external temperatures had to be controlled4 ,the electronics canister inside the satellite operates best if it stays at approximately room temperature of #fe to #ge;,f4 ,this much heat is supplied in the canister by dissipation of electrical energy from the solar cells4 ,the container is well insulated to keep its temperature relatively stable1 and it has shutters that open automatically if it begins to get overheated 7see above74 ,the operating characteristics of the solar cells on ,telstar's surface also had to be considered2 they work better at rather cool temperatures4 ,so we decided to keep the satellite's skin at an average temperature of about #j;,f1 although temperatures actually will range quite a bit above and below the average as the satellite moves from sun to shadow4 ,now1 using this average temperature of #j;,f 7converted to #dfj,r7 as ,t in our formula1 we can solve for _/4 ,we find that this gives us a ratio of approximately #j4g for the satellite's surface4 ,however1 this presents a problem4 ,almost #dj per cent of ,telstar's surface is taken up by its power plant of #cfjj sapphire-covered solar cells4 ,these cells1 unfortunately1 have a relatively high _/ ratio--their is #j4h and their is #j4ed1 for an _/ of #a4e4 ,this means that the portion of the surface not used by either solar cells or antenna openings must1 in order to give us an over-all average of about #j4g1 have a very low _/ ratio--less than #j4c4 ,to get this sort of ratio1 we had to select carefully the material for the outer surface of the ,telstar satellite4 ,there were many kinds of surfaces that might have been used2 they could have been metal or non-metal1 rough or smooth1 shiny or dull4 ,and they could have been any color from black to white4 ,however1 to get a #j4c ratio we needed something with a relatively high emissivity for the low-frequency electromagnetic radiation that the satellite emits and a rather low absorptivity for the high-frequency radiation coming from the sun4 ,high emissivity meant that we should use a nonmetal surface rather than polished metal1 since the emissivity of nonmetals is quite high at the temperatures in which we were interested1 while that of polished metals is relatively low4 ,and1 to get low absorptivity1 we decided that the color of these surface areas should be very close to a pure white4 ,illustration3 ,partially molten aluminum oxide particles being sprayed onto aluminum outer surface panels4 ,there were several substances that met our requirements4 ,after testing a number of them1 we decided to use aluminum panels coated with a thin layer of aluminum oxide 7,al,o74 ,this coating is very pure1 hard1 and stable1 and we left it rough to minimize changes due to meteoroid abrasion4 ,its _/ ratio is #j4bd4 ,the aluminum oxide coating can be applied by means of the plasma jet process--particles of aluminum oxide are heated to a partially molten state1 mixed with gases1 and then sprayed onto the cleaned1 pre-coated aluminum panels 7see illustration above74 ,using this carefully selected outer surface has helped solve the temperature-control problem4 ,since ,telstar has been in orbit its internal and skin temperatures have kept well within the ranges we wanted them to4 ,thus you can see how some basic formulas from classical physics helped us choose the right material for the satellite's surface--and even what color it should be4 ,the blue-and-white checkered appearance that ,telstar ,i finally took on was no accident--it was the result of carefully combining various colors and materials in just the right amounts to obtain the temperature balance we needed4 ,peter ,hrycak was born in ,przemysl1 ,western ,ukraine1 and received a ,b4,s4 in #aied1 an ,m4,s4 in #aiee1 and a ,ph4,d4 in #aifj from the ,university of ,minnesota4 ,he joined ,bell ,telephone ,laboratories in #aifj1 and has worked on low temperature refrigeration problems and thermal design and thermal testing of the ,telstar satellite4 ,,case ,,history ,,no4 #c ,how ,can ,we ,make ,optical ,measurements on a ,satellite8 ,jeofry ,s4 ,courtney-,pratt ,physicist--,head1 ,mechanics ,research ,department ,,the ,,problem ,since the first ,telstar satellite went into orbit1 we have tried to trace its path through space precisely4 ,but we also have had to keep a constant check on the position1 or attitude1 that the satellite takes as it travels4 ,we are particularly interested in the direction of the spin axis about which it revolves1 and we also want to know its spin rate1 which is the number revolutions the satellite makes each minute4 ,although these might seem relatively simple jobs1 they actually turned out to be rather complicated4 ,and only at virtually the last minute1 just before the satellite's design was finally set1 did we think of a new way of using reflected flashes of sunlight to report on its spin axis and spin rate4 ,why ,we ,want to ,know ,about the ,spin ,axis ,when the satellite was injected into its orbit1 it was spin-stabilized to keep it from tumbling over and over1 much as a rifle bullet is stabilized by being spun about its longitudinal axis4 ,the ,telstar satellite is roughly spherical1 and it was designed to spin with the helical antenna end as its north pole and the antenna bands as its equator4 ,on ,july #aj1 #aifb1 the satellite was given an initial spin of #agg4g revolutions a minute4 ,as we expected1 this rate is decreasing gradually2 after two years it will only be spinning one tenth as fast4 ,the most important reason for keeping a close watch on the satellite's spin axis is to make sure that microwave signals are sent and received steadily4 ,it isn't possible to build an antenna that radiates at exactly the same power in all directions4 ,telstar's antennas work very well1 but they operate better in the direction of the satellite's equator than they do towards its poles4 ,this means that if the spin axis is constantly changing transmission will fade in and out--even at times passing through 8null0 where no transmission at all is possible4 ,no single fixed orientation is perfect for the spin axis1 but we decided that the best average position would be to keep it always perpendicular to the plane of the earth's orbit4 ,we tried to make sure that the spin axis would not vary by more than five degrees from this direction at any time--although it probably could depart as much as #ae or #bj degrees without doing serious harm4 ,a second reason for being careful about the satellite's spin axis is the problem of heat balance4 ,if one end of the satellite points constantly at the sun and the other end does not1 the end near the sun will get much too hot and the other will get much too cold4 ,therefore1 we tried to fix the spin axis so that it stayed perpendicular to a line drawn from the satellite to the sun4 ,we also wanted to get a continuing report on the effects of the magnetic field of the earth at high altitudes4 ,we knew these would cause the spin axis to change with time1 or precess1 but we couldn't be exactly sure what these changes would be4 ,since the orientation of ,telstar's spin axis was so important we installed a torque coil in the satellite4 ,this is a coil of wire in which1 upon a signal from the ground1 an electric current can be made to flow4 ,the current produces a magnetic field that interacts with the earth's magnetic field to change the position of the satellite's spin axis4 ,however1 we could not be sure that this device would work properly--and this is another reason why we wanted to keep track of the exact position of the spin axis4 ,ways of ,measuring the ,spin ,axis ,one of the devices built into ,telstar is a set of six solar aspect cells spaced at regular intervals around the satellite4 ,these give a fairly accurate indication of the angle between the spin axis of the satellite and a line joining the satellite and the sun4 ,when sunlight strikes these solar cells1 they produce electric currents1 and the value of the current from each cell is sent back to the ground via telemetry4 ,three of the cells are in the satellite's northern hemisphere2 three are in the southern hemisphere4 ,if ,telstar's north pole were pointing to the sun1 for example1 the three northern cells would record large1 equal currents2 those in the southern hemisphere would show zero current4 ,but if the spin axis were perpendicular to the satellite-sun line 7as we want it to be7 all six cells would report equal1 average-sized currents1 which would fluctuate as the satellite spun around4 ,the solar cells were carefully calibrated before ,telstar was launched1 and we estimate that they can tell us the angle between the satellite's spin axis and the satellite-sun line to within one or two degrees4 ,however1 this one angle is not enough to locate the spin axis exactly4 ,as you can see in ,diagram #a1 there are many possible positions for the spin axis ,,op that have the same angle with the satellite-sun line ,,os4 ,these positions all would lie on the surface of an imaginary cone ,,opp that has ,,os as its axis and #b as its vertex angle4 ,we need to have a second measurement to find a single position for the spin axis4 ,as late as ,november #aifa we had not found a satisfactory way to make such a second measurement4 ,then ,donald ,gibble of ,bell ,telephone ,laboratories suggested that we observe the reflections of sunlight from mirrors fitted onto the satellite#b4 ,only when a satellite is in the right position can you see the reflection of sunlight from a plane surface on its body4 ,diagram #b shows how flashes of reflected light are observed4 ,the light of the sun1 ,s1 is reflected from a plane surface1 ,r1 on the satellite to our observing station1 ,t1 on the earth4 ,if we imagine the line ,,orb drawn perpendicular to ,r1 we know1 from the law of reflection1 that the angle of incidence1 i1 made by the sunlight to this line will be equal to the angle of reflection1 i1 between the reflected light and the same line4 ,the law of reflection also tells us that the sun1 the line ,,orb1 and the observing station all must now lie in the same plane4 ,and1 since we can calculate where the satellite is in its orbit at this exact moment1 we can locate line ,,orb4 ,but what about the spin axis8 ,we know where on the satellite our reflector ,r is located1 so we know ahead of time what the angle between ,,orb and the spin axis1 ,,op1 will be4 ,we call it the flash angle4 ,thus we can tell that the spin axis will be somewhere on the surface of an imaginary cone ,,opp that has ,,orb as its axis and #b as its vertex angle#c4 ,illustration3 #a4 ,solar aspect cells on the satellite report via telemetry the amount of sunlight they receive2 from these data we can calculate the angle between the satellite's spin axis1 ,,op1 and the satellite-sun line1 ,,os4 ,this means that ,,op can be anywhere on the surface of cone ,,opp4 ,illustration3 #b4 ,when sunlight is reflected to observing station ,t on the earth1 we know that the angle of incidence i must be equal to the angle of reflection i1 and1 if ,,orb is a line perpendicular to the reflector ,r1 we know that the sun1 the observer1 and line ,,orb must all lie in one plane4 ,since we also know the position of the satellite in its orbit and the distance from it to the earth1 we can locate line ,,orb precisely4 ,the reflector ,r is set at an angle of #fh from the spin axis ,,op4 ,this tells us that the spin axis must lie on the cone ,,opp1 which is now precisely determined by its axis ,,orb and its vertex angle #b1 equal to #acf4 ,illustration3 #c4 ,cones ,,opp and ,,opp intersect along the two lines ,,op and ,,oq1 so these are the only possible spin axis locations4 ,from our general knowledge of the situation 7or from any third measurement of glint time71 ,,oq can be ruled out1 and we conclude that only ,,op can be the true spin axis4 ,in ,diagram #c we have combined our two measurements of the satellite's spin axis4 ,you can see that the two cones will intersect along two straight lines1 ,,op and ,,oq2 these are thus the only possible positions that will satisfy both our measurements4 ,actually1 of course1 only one of these lines is the true location of the spin axis4 ,and it is usually obvious which one it is1 when we consider all our other data about the satellite's position4 ,using this technique1 if we measure the exact times when we see flashes of reflected sunlight from ,telstar1 we can combine that information with data from our six solar aspect cells and get a good plot of the position of the satellite's spin axis4 ,in theory1 this looked like a very promising idea4 ,but finding a satisfactory way to put it into practice was something else again4 ,our first thought was simply to make use of the light reflected from the sapphire covers on the satellite's solar cells4 ,however1 these covers have a low coefficient of reflection and do not form a completely flat surface4 ,this means that the light reflected from them is very much reduced in intensity and spreads out too much to give us the precise readings we want4 ,on the other hand1 if we attached a plane mirror with a high reflection coefficient to the satellite1 we thought we could pick up the minute flashes of reflected light from a distance of as much as a few thousand miles4 ,so we decided to press ahead with this scheme and install one or more reflectors on the satellite4 ,by the time we started work on the mirrors1 the final design of ,telstar ,i was almost complete2 this meant that we had to squeeze our mirrors aboard it as best we could4 ,the most stringent physical requirement in designing them was weight2 they could not add more than half a pound to ,telstar's total load4 ,nor could they project more than one-eighth inch from the satellite's surface1 or they might interfere with the radiation pattern for the main antenna4 ,we also decided to make the mirrors out of highly polished metal1 since any other possible material might break too easily4 ,and the mirrors had to be as flat as possible1 so the beam of reflected sunlight would not diverge by more than one degree4 ,thus we had to design mirrors that would be very thin1 very shiny1 very flat1 very light1 and almost unbreakable4 ,after much experimenting1 we solved this rather tricky problem4 ,the mirrors we added onto ,telstar ,i1 as you can see in the illustration below1 were machined from aluminum alloy sheet1 carefully polished by hand with abrasive papers1 and buffed on a cloth wheel4 ,finally1 we evaporated a thin layer of pure aluminum onto their surfaces to improve their reflection coefficients and make them resistant to corrosion4 ,the three mirrors were fastened to the surface of the satellite with small screws1 which had to be tightened and shimmed very carefully so that the mirrors stayed as flat as possible4 ,locating the ,mirrors on the ,satellite ,as we mentioned above1 the flash angle between the satellite's spin axis and a line perpendicular to the mirror is very important in our calculations4 ,we made detailed studies of the various flash angles that would be possible during the first #fj days after launch4 ,we plotted the times when the satellite would be above the horizon while our ,crawford's ,hill1 ,new ,jersey1 observing station was in darkness1 and we made allowance for satellite orbits that might deviate slightly from the planned one4 ,these calculations told us that the best flash angle for the mirror would be #fh degrees--which is the angle made by the first facets above ,telstar's equatorial antenna band4 ,so we located a flat mirror on one of these facets4 ,because one of the solar aspect cells was already installed in the center of this facet1 we were forced to cut a circular hole out of the center of the mirror4 ,but we knew that one mirror could not do the whole job4 ,after ,telstar ,i had been in orbit more than #cj days1 the #fh-;degree mirror would only be in position to give infrequent flashes1 and one at about #ie degrees would be more useful4 ,this presented two problems4 ,first1 no facet on the satellite makes a #ie-;degree angle with the spin axis4 ,however1 we could use one of the facets just below the equatorial antenna1 which makes a #aab-;degree angle1 and groove or facet the mirror so that its reflecting faces became narrow strips slanted #ag degrees away from the base at the angle of #ie degrees 7#aab - #ag #ie74 ,our second problem was space--since there was not enough room left on any of the #aab-;degree facets to mount a second large mirror1 we substituted two smaller mirrors and mounted them #abj degrees apart4 ,this arrangement lets us know from which mirror we see flashes--the plane mirror gives one flash for each revolution of the satellite2 the faceted mirrors give two flashes for each revolution of the satellite4 ,illustration3 ,sketches of three reflecting mirrors and their locations on the ,telstar satellite4 ,the upper plane mirror is set at #fh to the spin axis2 the lower ones are faceted to give reflecting surfaces at #ie4 ,two of the satellite's six solar aspect cells can be seen within the circular cut-outs in the mirrors4 ,how ,we ,record ,flashes from the ,mirrors ,now we had finally found a satisfactory way to reflect a train of tiny flashes--much too faint to be seen by the naked eye--from ,telstar as it passed across the sky during the night4 ,but our main aim was to record the exact times when these flash bursts occurred4 ,with this information1 we could1 using the method we described above1 tell very accurately both the satellite's spin axis and its rate of spin4 ,we do not have space to describe the many problems that had to be solved in setting up the equipment to record the flashes4 ,let us merely outline the procedure that we finally devised3 #a4 ,to pick up the satellite's flashes we use a #ab-;inch-aperture photoelectric telescope mounted on a radar trailer 7shown in illustration below74 ,it is pointed by means of prediction drive tapes produced by an electronic computer2 these are based on data from previous passes4 #b4 ,on clear1 dark nights when the satellite is at relatively short range1 we can see it with an auxiliary finder telescope1 and then adjust the large telescope precisely4 ,or1 if the satellite's high-frequency beacon has been turned on1 the ,holmdel microwave antenna can automatically point our large telescope4 #c4 ,when flashes of light are picked up by the telescope1 they fall directly onto the cathode of a photomultiplier tube4 ,they are then filtered out from the random light in the night sky and amplified4 ,illustration3 ,twelve-inch telescope and electronics box mounted on a radar antenna pedestal at ,crawford's ,hill4 ,three-inch sighting telescope mounted on top has since been replaced by six-inch telescope4 #d4 ,rather than make a continuous recording of the output--one night this would have produced a record twelve miles long for us to pore over--we use an electronic trigger4 ,this is the time base of an oscilloscope1 whose sawtooth output is set in operation only if a signal of four volts or more is received 7photo below74 #e4 ,a pen recorder makes a continuous line on a revolving drum1 with a heated stylus connected to a galvanometer marking a permanent record on heat-sensitive paper4 ,any signal output from the oscilloscope is picked up by the galvanometer and causes the pen to make a sawtoothed mark2 when the paper is unrolled from the drum1 these marks are clearly visible as notches in a series of otherwise straight lines4 #f4 ,a synchronous timer marks the chart every ten seconds1 and we are able to time individual pulses with a precision of one tenth of a second4 ,because the beginning and end of a train of pulses are not always distinct1 we can only determine the center of a burst of flashes--which we use as our most important time indication--to within two seconds4 ,however1 this is accurate enough1 for a change of only one degree in the orientation of the satellite's spin axis would change the time of the flash burst center by about half a minute 7see below74 #g4 ,we use a second oscilloscope to check on whether the signals we receive are genuine flashes or just accidental stray light4 ,this oscilloscope has a long-persistence screen1 which we use as a temporary memory4 ,the pulses traced on its cathode ray tube are automatically photographed by a #ce-mm camera while they persist on the screen4 ,we can then examine the photograph to see if the pulses are genuine1 which we ascertain from 7a7 their shape and size and 7b7 the intervals between successive pulses4 ,looking at the photographic record also confirms whether we are observing flashes from the #fh plane mirror or the #ie faceted mirrors4 ,we can calculate the satellite's spin rate by measuring the intervals between individual flashes4 ,illustration3 ,general view of amplifying1 monitoring1 and recording gear that picks up glints of sunlight at the ,crawford's ,hill observation station4 ,illustration3 ,enlarged portion of a typical pen record of flashes of sunlight from ,telstar mirrors1 showing a burst of #ba glints from the #fh mirror recorded at #jc3dj3eh ,greenwich ,mean ,time on ,august #i1 #aifb4 ,synchronizing vibration mark seven lines above the recorded burst indicates the time #jb3ei3jj4 ,measuring the horizontal distance between consecutive sawtooth marks tells us that the spin rate is between #afc and #afd revolutions per minute4 7,precise measurements of the oscilloscope traces fixed the exact spin rate at the time of this burst at #afc4fd revolutions per minute47 ,results ,telstar ,i was launched on ,july #aj1 #aifb4 ,that evening1 beginning on the satellite's seventh pass1 we were able to detect trains of flashes from the mirrors4 ,we assumed that1 since ,telstar had been launched almost exactly according to plan1 its spin axis would be perpendicular to the plane of the earth's orbit1 and we calculated when we should see the flashes4 ,and1 each time1 we actually saw them within two minutes of the times we had predicted--so we knew that the spin axis was almost exactly where it should be4 ,our measurements have continued whenever the weather and other conditions permitted4 ,combining readings from the bursts of flashes and telemetry data from the solar aspect cells1 we have accurately plotted ,telstar's spin axis2 it has continued to precess very much as we predicted it would4 ,we have also seen what happens to the spin axis when the satellite's torque coil is turned on4 ,and1 by measuring the intervals between flashes1 we have made very precise measurements of the spin rate1 which is gradually decreasing mostly according to schedule4 ,however1 the plot is showing some small unexplained variations of spin decay rate1 and a study of them will1 we hope1 throw light on some of the variations of the earth's magnetic field4 ,for future communications work1 particularly with satellites at longer ranges1 it would seem to be preferable to use stiffer1 flatter mirrors and to make them from beryllium rather than aluminum alloy4 ,more accurate tracking means1 more observatory sites1 and more powerful telescopes will also be needed4 ,but for this first experimental use our little mirrors have worked very well4 ,jeofry ,s4 ,courtney-,pratt was born in ,hobart1 ,tasmania1 ,australia1 and received a ,bachelor of ,engineering degree from the ,university of ,tasmania in #aidb and a ,ph4,d4 from ,cambridge ,university in #aidi4 ,he was also awarded an ,sc4,d4 by ,cambridge in #aieh4 ,he joined ,bell ,telephone ,laboratories in #aieh1 and has done research in high-speed photography1 optics1 optical masers1 the properties of materials1 and the physics of the contact of solids4 ,,case ,,history ,,no4 #d ,how ,do ,we ,keep ,solar ,cell ,power ,plants ,working in ,space8 ,kenneth ,d4 ,smith ,electronics ,engineer--,member of ,staff1 ,semiconductor ,device ,laboratory ,,the ,,problem ,before we learned about the ,van ,allen belts1 we expected that the solar cells used to power satellites would last for many years in space4 ,we thought they would be damaged only by cosmic rays1 micrometeorites1 and occasional bursts of particles from the sun4 ,but when the solar plants of some ,american satellites went out of action after only a few weeks in orbit1 we realized that in the future solar cell power units would need better protection from radiation damage4 ,we had learned that satellites--and particularly medium altitude communications satellites--must spend a lot of time in regions where they will be struck by thousands or even millions of high-speed radiation particles each second4 ,this fact forced us to change almost all our thinking about solar power plants for satellites4 ,to make sure they would last for several years1 we had to design new types of solar cells and devise new ways of mounting them4 ,we also had to revise our estimates of how much power we could expect to get from our cells4 ,if a communications satellite is to go into regular commercial service1 it must continue working for several years in space4 ,the ,telstar satellite1 however1 was designed as an experimental project1 and we decided that two years would be a reasonable lifetime to plan for4 ,when ,project ,telstar began1 our problem was to develop solar cells that would operate in an environment subject to strong radiation effects--and keep on operating there for two years4 ,organizing the ,work ,our work on the solar cells for ,telstar ,i began in ,october1 #aifj4 ,with just a little more than a year to go before the satellite had to be ready1 there was no time to lose4 ,so we decided to break down the over-all problem into three parts3 --,finding out how radiation would affect various kinds of solar cells2 --,making experimental cells and1 when the best had been picked1 determining the best ways to make them in the large quantities we would need2 and --,developing ways to mount the cells on the ,telstar satellite so that they would withstand the stresses of being launched1 the effects of radiation particles1 and extreme changes in temperature4 ,a different group of people began work simultaneously on each of these three parts of the problem1 with each of them going ahead under the assumption that the others would be successful4 ,each group had to find the answers to many very interesting questions1 but since our space is limited we can only discuss some of them here4 ,before doing so1 however1 we must say something about what a solar cell is and how it works4 ,technical ,background on ,solar ,cells ,there are two ways of making a silicon solar cell4 ,in one1 the body of the cell is what we call n-type silicon--that is1 pure silicon that has been doped with a small number of impurity atoms of an element such as phosphorus or arsenic 7from group ,v of the periodic table74 ,this kind of semiconductor#d conducts electricity by means of a supply of free-to-move electrons 7negative charges7 caused by the presence of these impurity atoms4 ,to make a workable solar cell from n-type silicon1 a thin surface layer of p-type silicon is formed by diffusing atoms of a material from group ,,iii of the periodic table--usually boron--into the silicon4 ,metallic contacts then are made to these two regions4 ,this kind of cell is known as a p-on-n cell4 ,the second type of solar cell is just the reverse4 ,it begins with a body of p-type silicon 7with impurity atoms from a group ,,iii element7 and conducts electricity by means of 8holes0--vacant sites where electrons might be but are not4 ,these holes act as free-to-move positive charges4 ,we can make a solar cell from this material by diffusing a layer of n-type impurity1 such as phosphorus1 into it4 ,we call this an n-on-p cell 7see the figure below74 ,illustration3 ,construction of a silicon solar cell of the n-on-p type 7thickness of n-layer greatly exaggerated74 titanium-silver evaporated contact with solder dip finish antireflection coating contact gridding for lower series resistance #ae mil wafer1 p-type #j4d micron front layer1 n-type ,the key to the operation of either type of solar cell is the junction between the regions of n-type and p-type material--what we call the p-n junction4 ,in an actual n-on-p cell this junction is only about twenty millionths of an inch below the surface1 since that is the thickness of the n-layer4 ,at this point1 where the hole-rich p-region meets the electron-rich n-region1 there is a permanent1 built-in electric field4 ,as shown in the figure below1 the n-layer has many free electrons 7indicated by minus signs7 and a few holes 7circled pluses71 while the p-region has many holes and a few electrons4 ,when the cell is in equilibrium1 thermal agitation causes some holes to diffuse into the p-region4 ,we call these stray holes and electrons minority carriers 7the circled pluses and minuses in the figure74 ,thus1 the n-layer has a slight positive charge and the p-body has a slight negative charge2 this results in a difference in potential across the junction1 which in silicon amounts to about seven-tenths of a volt4 ,illustration3 ,schematic diagram of an n-on-p solar cell4 ,in the n-layer1 minuses represent free electrons1 circled pluses are minority-carrier holes2 in the p-type body1 pluses represent holes1 circled minuses are minority-carrier electrons4 ,sunlight is made up of individual corpuscles of energy called photons4 ,when these photons are absorbed in or near a cell's p-n junction1 they liberate both a free-to-move negative charge and a free-to-move positive charge--this is called generating a hole-electron pair4 ,the electric field across the p-n junction causes the holes to flow to the p-side and the electrons to the n-side of the barrier4 ,this flow tends to make the p-side positive and the n-side negative1 so that1 when a load is connected between them1 a useful external voltage 7amounting to about six-tenths of a volt7 is produced1 and electric current will flow4 ,thus1 we have converted light energy into electrical energy4 ,only part of the energy in light can be used to generate an electrical output1 since a good deal of the light striking a cell is absorbed as heat or is reflected from its surface4 ,the percentage of solar energy that can be converted into usable electric power is called the cell's conversion factor or efficiency4 ,although this can theoretically be as high as #bb1 the best cells we have made in the laboratory have conversion factors of only about #ae1 and the better commercial cells have efficiencies of #ab or more4 ,although both p-on-n and n-on-p cells were made in early laboratory studies1 the p-on-n cells gave a somewhat higher output4 ,as a result1 all the ,american commercial solar cells up to #aifj were of this type1 and they were used on all satellites before ,telstar ,i4 7,russian satellites1 we believe1 have used n-on-p cells from the beginning47 ,the ,u4,s4 ,army ,signal ,corps ,research and ,development ,laboratory1 however1 decided to make both p-on-n and n-on-p cells and compare their performance4 ,this laboratory work led to a surprising discovery3 ,the n-on-p cells were several times as resistant to energetic particle radiation as were comparable p-on-n cells4 ,these results were announced in #aifj1 and confirmed by our measurements and those of other laboratories4 ,the timing was very fortunate1 since we had just learned of the greatly increased radiation hazards presented by the ,van ,allen belts4 ,finding ,out ,about ,radiation ,damage ,now1 having given you a very brief account of how a solar cell works1 let us return to our three-part problem4 ,the first objective was to study all the aspects of radiation damage4 ,to do this1 we had to find out how much radiation the ,telstar satellite would encounter2 we needed to estimate the concentration of high-energy particles--both electrons and protons--at various altitudes and locations4 ,several government agencies are now carrying on research in this important area1 but at the time of the ,telstar ,i launch we did not know exactly how much radiation the satellite would run into4 ,and the high-altitude nuclear explosion of ,july #i1 #aifb 7the day before ,telstar ,i went into orbit7 may have increased the quantity of high-energy electrons injected into its path4 ,we also wanted to find out whether electrons and protons would do the same damage to solar cells4 ,several kinds of cells were exposed at ,bell ,laboratories and at various university research laboratories to a wide range of radiation dosages4 ,the experiments showed1 generally1 that the damage effects of electrons and protons should be about the same4 ,although protons are #ahdj times as massive as electrons1 there are a great many more electrons in the ,van ,allen belts1 so that an unprotected solar cell would be much more likely to be injured by electrons than by protons4 ,in fact1 we found that the ,van ,allen belt protons have so much energy that they can go through transparent shielding material as much as several centimeters thick and still damage a solar cell4 ,thus1 to screen our cells from protons we would need very thick transparent cover plates1 and this added weight would be intolerable4 ,so we decided to use no proton shielding at all4 ,with electrons1 the situation is different2 they are much lighter and have much less energy4 ,also1 if their energy is reduced below a certain level 7about #ahj thousand electron volts7 electrons will not be able to knock silicon atoms out of position1 and thus cannot harm a solar cell4 ,we experimented with a number of different kinds and thicknesses of cover plates1 and found that transparent material with a mass of #j4c gram per square centimeter would slow down electrons enough to make them no problem4 ,another radiation study helped us take advantage of the fact that solar cells respond differently to light of different wave lengths4 ,if the surface layer of a cell is extremely thin1 it will absorb blue1 green1 and yellow light well1 but may be much less sensitive to the deeply penetrating red and infrared waves4 ,we experimented with n-on-p cells having very shallow p-n junctions1 exposing them to an extremely strong radiation dosage4 ,the cells still responded very well to blue and green light1 even though most of their response to infrared and red light was lost4 ,these findings convinced us that we should work to make our new cells as blue-green sensitive as possible1 since they were going to be exposed to heavy radiation4 ,designing and ,making the ,best ,solar ,cells ,after it was discovered that the n-on-p cell was more resistant to radiation1 we decided to make an all-out effort to develop an n-on-p cell that could be manufactured in quantity for our new satellite4 ,since we didn't know whether we could solve this problem in time to meet the ,telstar ,i launch date1 we 8hedged0 by designing the new n-on-p cells to be the same physical size 7one by two centimeters7 as conventional p-on-n cells4 ,thus1 if the n-on-p project hit a snag1 we probably could use regular p-on-n cells4 ,as you can imagine1 making a solar cell to fit the very high requirements we had set for the ,telstar satellite is not an easy job--and making these cells by the thousands is even more of a task4 ,during ,october1 ,november1 and ,december of #aifj we carried on a crash program in which we made hundreds of experimental cells in our laboratories1 using a variety of materials and many different manufacturing techniques4 ,we perfected a phosphorus diffusion process to develop the very thin n-layer 7about one forty-thousandth of an inch thick7 that we needed for our special blue-sensitive n-on-p cells4 ,we also had to devise an entirely new way to attach the metallic contacts to the highly polished surfaces of our cells1 using a combination of titanium and silver4 ,some tricky manufacturing problems also had to be solved once the ,western ,electric ,company began to make the large quantity of cells needed for the ,telstar program4 ,for example1 during the diffusion of the n-layer of the cell1 the silicon slice is surrounded by phosphorus pentoxide vapor1 which covers the entire slice with an 8n-skin40 ,this skin must be removed from the bottom of the cell by etching or grit blasting before the p-contact is applied4 ,another difficult problem occurred when we decided to give our cells an anti-reflection coating4 ,because polished silicon has a refractive index near #d and space has an index of #a1 silicon will reflect about #cd of visible light from the sun4 ,however1 if we apply an anti-reflection layer onto the silicon this percentage of reflection can be considerably decreased4 ,we found that the best substance for this purpose was a layer of silicon monoxide only three-millionths of an inch thick4 ,but it was only after quite a bit of trouble--and scrapping several thousand cells--that we were able to get this coating to adhere properly in the right thickness4 ,mounting the ,cells on the ,satellite ,the third part of our problem had to do with finding the best ways to mount and protect the cells on the ,telstar satellite itself4 ,since a satellite's solar power plant usually has several thousand cells1 we find it best to mount the cells in groups1 or modules4 ,these can be pretested as a unit after individual interconnections have been made4 ,for ,telstar ,i1 we decided to mount the #cfjj solar cells in #ab-;cell modules like those shown in the figure below4 ,illustration3 ,the satellite uses #cjj modules of twelve solar cells1 in groups of six or three modules per facet4 ,illustration3 ,lengthwise diagram of a solar cell module1 showing how individual cells were fixed in place4 ,each of the cells has a top contact along one edge and a bottom contact all over its base1 so we were able to assemble the #ab-;cell groups like shingles1 with the bottom edge of one cell covering the top edge of the next1 leaving only the active area of each cell exposed4 ,but this meant that each module would be over four inches long and only #ad thousandths of an inch thick--far too weak to withstand stress and vibration4 ,to support the cells1 we decided to mount them on a metallized ceramic base4 ,but this presented a problem3 ,if the cells were soldered directly to the base1 the different thermal expansion rates of the silicon and the ceramic would cause the structure to break during the cycles of extreme changes in temperature that ,telstar would pass through4 ,we remedied this by connecting each cell to the ceramic support by a thin ,u-shaped strip of silver 7see above74 ,since silver has a much higher thermal expansion coefficient than silicon1 we added tiny sandwiches of ,nilvar or ,invar 7#cf nickel1 #fd iron7 where the cells were attached4 ,with this mounting method1 the cell modules withstood thermal and mechanical shocks much more severe than those they would undergo in actual use4 ,in one test1 for instance1 an entire cell module with its cover plates was first dipped in hot water1 then plunged into liquid nitrogen at a temperature of -#aie ,centigrade4 ,in orbit1 the temperature range for the satellite was not expected to be more than from #hj to -#ajj;,c1 with a rate of change of no more than three degrees a minute4 ,finally1 we needed to find the right kind of transparent protective cover for the ,telstar solar cells1 both to keep micrometeorites from damaging the sensitive and very thin diffused layer and to slow down the incoming electrons to nondestructive energy levels4 ,for micrometeorite protection1 only a thin layer of hard transparent substance was needed2 for electron protection1 the cover plates should have a mass of no less than #j4c gram per square centimeter 7as we explained above74 ,and there were two other important considerations3 ,the material we used should not be darkened or discolored by prolonged exposure to ultraviolet radiation1 and it should have good thermal conductance1 so that some of the heat absorbed by the solar cells could be conducted out to the cover plates and re-radiated4 ,all these requirements led us to the choice of clear1 man-made sapphire4 ,although sapphire is more expensive and difficult to make than the equivalent quartz or glass1 it only has to be #cj mils 7three hundreds of an inch7 thick4 ,twice this thickness would be required if quartz or glass were used4 ,we have had space to describe only a few of the things involved in designing a solar cell power plant that would work unattended out in space4 ,we have not mentioned a good many of the tough problems that had to be worked on4 ,but we are glad to report that we could find answers to almost all our questions4 ,and the most significant answer is shown in the figure below1 where you can see how the ,telstar ,i solar power plant slowly diminished in power almost exactly as we predicted it would4 ,illustration3 ,very gradual decay due to radiation effects of the ,telstar ,i solar cell plant in the first months after the satellite went into orbit2 it was extremely close to the predicted rate 7solid line74 ,kenneth ,d4 ,smith was born in ,galesburg1 ,illinois1 and received a ,b4,a4 from ,pomona ,college in #aibh and an ,m4,a4 from ,dartmouth ,college in #aicj4 ,he joined ,bell ,telephone ,laboratories in #aicj1 and has worked on the development of proximity fuzes1 radar bombing systems1 broadband microwave radio systems1 and various semiconductor devices1 including radiation-resistant solar cells for the ,telstar satellite4 ,,case ,,history ,,no4 #e ,would ,time ,delay ,be a ,problem in ,using a ,synchronous ,satellite8 ,peter ,d4 ,bricker ,psychologist--,member of ,staff1 ,behavioral ,research ,laboratory ,,the ,,problem ,one of the satellite communications systems that has been proposed would make use of stationary synchronous satellites4 ,these would be precisely located above the earth's equator in orbits #bb1cjj miles high1 where they would circle the earth once every #bd hours1 and thus appear to remain stationary over a point on the earth4 ,there are several advantages to this type of system--the most important being that we would need only three satellites for communications between almost all the inhabited regions of the earth4 ,on the other hand1 there are several problems in establishing a synchronous system4 ,just getting the satellites into exactly the right places and keeping them in position is a formidable one4 ,we also have something of a mystery to contend with1 because of the tremendous distances that would be involved4 ,although we can communicate at speeds close to that of light--#ahf1jjj miles per second--we cannot go any faster than that4 ,you might think that #ahf1jjj miles a second was fast enough for us1 and most of the time it is4 ,however1 if you send signals #bb1cjj miles up into the sky1 transmit them back to earth1 perhaps send them up again to a second satellite1 and finally bring them #bb1cjj miles back down to earth1 even the speed of light may not be fast enough4 ,the delay will be only about a second or so1 but it may--for some kinds of communications--be long enough to cause trouble4 ,how much trouble1 we don't yet know4 ,for one-way signals such as television1 a transmission delay of about one second obviously makes little or no difference4 ,but for two-way conversations on the telephone1 where there is rapid back-and-forth talking1 even this tiny amount of time delay may be a problem4 ,and then again1 it may not be4 ,there have been a lot of experiments to find out something about this delay problem1 and these have given us a lot of different answers4 ,work is still going on1 and there is still much to find out4 ,in this chapter1 we tell you about one small1 early experiment4 ,its results were not conclusive1 but they should give you one example of how to set up and carry out a typical experimental study on human behavior4 ,illustration3 ,typical circular satellite orbit3 r is distance from center of earth to satellite2 ,r is radius of earth4 ,how a ,synchronous ,satellite ,would ,work ,for our purposes1 we will not be concerned with all the problems of launching a synchronous satellite into its proper orbit4 ,but you may be curious why we know that this orbit must be #bb1cjj miles high4 ,it can be calculated by using two basic formulas from elementary physics4 ,from ,newton's ,law of ,gravitation we know that the velocity1 v1 of a satellite moving in a circular orbit#e will be v 77g,r7_/7r771 where ,r is the radius of the earth1 r is the distance from the center of the earth to the satellite1 and g is the acceleration due to gravity 7see diagram above74 ,we also know that this velocity must be v 7#br7_/7,t71 since the distance the satellite travels to complete an orbit is #br1 and ,t is the time of one complete revolution4 ,thus we have the equality 77g,r7_/7r77 7#br7_/7,t71 and1 solving for r1 we get r 7g,r,t7_/7#d777 ,since we are interested in a synchronous satellite1 ,t in this case will be #bd hours4 ,we can now find r 7using g #cb feet per second per second and ,r #cifj miles71 and then obtain the distance r - ,r1 which will be #bb1cjj miles4 ,by using our previous formulas1 we also can find the velocity of a satellite moving in this orbit1 which will turn out to be v #fhgj miles per hour4 ,illustration3 ,one possible method of using synchronous satellites4 ,signals from ,new ,york 7,n7 to ,paris 7,p7 would go via satellite ,s2 signals from ,new ,york to ,calcutta 7,c7 would go via satellites ,s and ,s4 ,the illustration above gives a rough idea of how a synchronous satellite system might be set up4 ,three communications satellites1 ,s1 ,s1 and ,s1 are above the equator in fixed positions equal distances apart and #bb1cjj miles up4 ,located in this manner1 they would cover the major part of the earth's surface4 ,from a point directly beneath it1 the distance would be #bb1cjj miles to a satellite2 from other points the slant range would be greater4 ,signals sent from1 say1 ,new ,york 7point ,n7 to ,paris 7point ,p7 would be reflected via satellite ,s4 ,in doing this1 they would travel a total distance of about #df1jjj miles4 ,because we can't send signals any faster than the speed of light 7#ahf1jjj miles per second71 it would take at least a quarter of a second for a signal to go this far4 ,for communicating a much greater distance1 say from ,new ,york to ,calcutta 7point ,c71 the signal path would use two satellites1 ,s and ,s4 ,in this case1 the total distance traveled by a signal would be more than #ij1jjj miles1 and the one-way time delay would be about half a second4 ,the ,effects of ,time ,delay ,delays of a quarter- or half-second have different effects on various kinds of communications4 ,however1 we are concerned here only with what they might do to telephone conversations4 ,time delay will affect conversations in two ways4 ,one of these--pure delay--depends on the nature of speech and the way people use it to converse2 the other--echo--has to do with the nature of the world's telephone systems4 ,the first effect can be illustrated by an example4 ,suppose that ,george in ,paris is talking to me in ,new ,york4 ,he says1 8,do you want to go80 and ,i answer 8,yes0 immediately upon hearing the word 8go40 ,but that word didn't arrive in ,new ,york until a quarter of a second after ,george said it1 and my reply was delayed another quarter-second1 so ,george hears my instantaneous reply a half-second late4 ,under some circumstances1 he might interpret this delay to mean that ,i was less than enthusiastic about going4 ,we don't know exactly what response times people expect in conversation1 or how much variation in such intervals they can tolerate4 ,but it has been assumed that delays of a half-second or more would make a noticeable and perhaps disturbing difference4 ,a little later on1 ,i will describe an experiment dealing with this first effect4 ,but first we must briefly discuss the second effect of delay on telephone conversation1 to show why we decided to try to isolate the first effect and study it separately4 ,the ,echo ,problem ,all the world's telephones are individually connected to the rest of the system by what we call two-wire local loops4 ,speech travels in both directions on the same wires over these local parts of the circuit4 ,in other parts of the system1 where speech travels farther and must be amplified1 it is carried over four-wire circuits4 ,these consist of two pairs of wires1 one for transmission in each direction4 ,at the junctions where the two-wire and four-wire parts of the telephone system meet1 specially designed transformers1 called hybrid coils1 are used4 ,it is impossible to have these junctions between two-wire and four-wire circuits always in perfect balance1 so part of the speech that reaches a local loop will be reflected back along the path on which it arrived4 ,unless a circuit has been specially treated1 this reflected speech will get all the way back to where it started1 and the talker will hear an echo of his own voice4 ,when the circuit is short enough1 the echo is heard almost instantaneously1 and is not bothersome4 ,but when the echo is delayed by a twentieth of a second or more1 it can become extremely annoying1 and even temporarily destroy one's ability to speak coherently4 ,telephone engineers have long been aware that this echo effect was present on their long-distance circuits1 and they have not let it go unchecked4 ,devices known as echo suppressors are installed on circuits that have more than a critical amount of delay4 ,they are placed in a four-wire part of the circuit1 where there is one-way transmission over each pair4 ,since incoming and outgoing sounds are using separate paths1 an echo suppressor can attenuate or shut off the return path when speech is coming in on the other path4 ,unfortunately1 echo suppressors have effects of their own on transmission4 ,they may1 for example1 cut off some speech that should be getting through1 because they can't distinguish it from echo4 ,echo suppressors can be made more sophisticated1 but whether they can be made to operate more successfully than present ones is not clear4 ,and the problem of adapting them to the long delays of synchronous satellite circuits will require a great deal of research and development effort4 ,experimenting ,with ,pure ,delay ,although we don't know how good echo suppressors can get1 we do know that a long circuit with the best possible suppressors could never be better than a circuit of the same length that had no echos4 ,this brings us back to the problem of how serious the effect of delay alone is on conversations4 ,if the delay in a synchronous satellite system1 even without any echo1 made conversation all but impossible1 there would be little point in developing echo suppressors for such satellites4 ,this question looked like one that we could answer1 at least in part1 by experimenting with special four-wire circuits that had delay but no echo4 ,the strategy we adopted1 then1 was to do some experiments on pure delay while other people at ,bell ,telephone ,laboratories began to attack the problem of testing and improving echo suppressors4 ,in the pages that follow1 ,i will describe one of our experiments on the pure delay problem4 ,more elaborate ones have been performed since1 and there will be more to come4 ,it should now be clear how this sort of experiment might be helpful to the development of a synchronous satellite communications system4 ,if it showed pretty convincingly that conversation was extremely difficult with a pure echo-free delay of about a second1 synchronous satellites for two-way conversations would be less practical4 ,on the other hand1 if the experiment were to show that some conversation1 at least1 could be carried on without too much difficulty1 our results would be less decisive4 ,we would know only that echo-free delayed circuits might sometimes be all right4 ,but we would not know how bad they were under a variety of conditions or how closely they resembled a real circuit with echoes and echo suppressors4 ,in either case1 the experiment would have no bearing on the use of synchronous satellites for one-way purposes1 such as television4 ,designing the ,experiment ,illustration3 ,recording equipment used in the experiment4 delay 77_/7#cfj7 1 where is the period of rotation in seconds 8,can people converse over an echo-free1 four-wire circuit that has delay like that of a synchronous satellite80--that is one way of putting the question we isolated to study4 ,the next problem was to find a way of setting up this question in the form of an experiment whose results might be interpreted as a meaningful answer4 ,the problem of apparatus1 fortunately1 was fairly simple4 ,two telephone sets in separate rooms were connected by four wires1 with one pair going directly from each transmitter to the other receiver1 so that no echo would go back the way it came4 ,this gave us the 8echo-free1 four-wire circuit0 we wanted4 ,to simulate the satellite1 ,i inserted a magnetic delay device 7see sketch7 between one of the transmitters and the other receiver4 ,this had a revolving drum on which speech could be recorded and then played back a short interval later4 ,by moving the playback head1 ,i could produce any amount of delay up to two satellite bounces' worth4 ,at this point1 the equipment was ready1 but there were still two major problems3 7#a7 how to get people to converse over the circuit in a natural way1 and 7#b7 what to observe and measure that would give us an answer to our question4 ,when you think about the first problem1 you soon realize that it is hard to say exactly what a 8natural0 conversation is4 ,but even if we can't describe it1 we can try to find examples of it4 ,i experimented with several ways of making people talk that could be recognized as unnatural3 word games1 list-checking1 shape description and recognition1 and a system of rewards for spurts of talk4 ,none of these schemes seemed to generate the real conversational interplay we wanted4 ,finally1 ,i noticed that my coworkers often got involved in vigorous conversations on political and social issues at the lunch table4 ,so ,i circulated a questionnaire to help me pick pairs of people who might enter into lively discussions on one or more topics4 ,i arranged seven conversations of this sort1 and they became the basic material of my study4 ,only one of these lacked sufficient spirit to yield good data1 and six out of seven is a pretty good percentage when you try to study human behavior in such a free situation4 ,note that the conversers expressed ideas that came from within them at the time--not from any external materials--and that they usually felt rather strongly about what they were saying to the other fellow1 who disagreed and therefore needed some convincing4 ,of course1 these conversations do not represent the whole range of possible conversations2 they are only a small sample of one type4 ,this doesn't limit the truth of the particular result we got1 but it does limit how far we may generalize from these results4 ,what ,should ,we ,observe8 ,you might think we could solve this problem simply by asking the conversers their opinions4 ,we found out long ago1 however1 that the opinions you get are affected by a lot of things3 how you ask the question1 the attitude of the respondent1 and his unrelated experiences outside the experiment4 ,so we usually try a more subtle approach4 ,in this case1 my basic observation was of what psychologists call escape behavior4 ,the conversers were told that they would start talking over a normal circuit1 and that delay would be introduced at some point4 7,the delay was inserted in such a way that an abrupt change could not be noticed47 ,all the conversers had pushbuttons for signaling the experimenter4 ,if they thought they noticed a delay1 they were told that it would be removed if they pushed the button4 ,thus they could always escape from this possibly unpleasant condition4 ,my reasoning was this3 ,if the conversers found it very difficult to talk with delay in the circuit1 they would surely push the button soon after the delay was introduced4 ,on the other hand1 any time when they continued to converse without pushing the button--while delay was in the circuit--was obviously also a time when the delay did not make conversation impossible4 ,so we had at least one measurable quantity--the time taken to detect delay--which we could interpret as an answer to our question4 ,note that we could tell if people pushed the button 8just to be on the safe side0 by seeing how often they did this when there actually was no delay in the circuit4 ,there are just a few more necessary details before we discuss the results3 --,i recorded the conversations for later analysis1 but the conversers knew that the recordings would be held in confidence4 --,the amount of delay used was #a4b seconds1 which represents the total round-trip delay for a circuit using two satellites1 including the delay in typical end connections on the ground4 ,we used this much delay because our preliminary tests indicated that it would be more likely to produce an effect than would the #j4f second delay in a one-satellite link4 --,the entire #a4b seconds of delay was put into one of the lines1 since we had discovered that1 where there is no echo1 conversers cannot tell the difference between a delay of #bt seconds in one line and a delay of t seconds in each of two lines4 ,i did this for the sake of convenience1 so that ,i could introduce delay in the quiet line while the other one was active4 --,after someone detected delay1 ,i removed it immediately and then waited at least a minute before putting it back in4 --,altogether1 ,i collected about two hours of conversation and introduced delay #bb times4 ,the ,results and ,what ,they ,mean ,now we could answer the question1 8,how long does it take people to detect #a4b seconds of delay80 ,as you can see from the table opposite1 the times ranged all the way from #bj seconds to over #aj minutes1 and1 in two cases delay was not detected at all4 ,the results in the table are also shown in the histogram on the next page1 which depicts how broadly the detection times were distributed4 ,to me1 one of the most interesting things is that even people who were able to detect delay quickly sometimes did not detect it for a couple of minutes4 ,for example1 the pair ,k_/,g had two times under a minute1 one of #adc seconds1 and one of #dba seconds4 ,i interpret their two short times to mean that they knew what to look for1 since they made no incorrect responses while delay was not present4 ,however1 their long times seem to mean that they sometimes didn't notice delay for quite a while4 ,incidentally1 only two responses were made during the total of about #dj minutes when ,i did not introduce delay1 and these 8false alarms0 were by two of the fastest pairs at true detection--,f_/,k and ,s_/,h4 ,length of ,time ,before ,seven ,pairs of ,talkers ,could ,detect #a4b ,seconds of ,delay ,,pairs ,,of ,,talkers ,,number ,,of ,,seconds ,,before ,,talkers ,,detected ,,delay ,g_/,h #afa #bbd #ajg ,f_/,k #hg #fe #dc #bbj false alarm ,a_/,l #fah #ie #cfg ,f_/,t no detection after #ied seconds ,s_/,h #bbg false alarm #ij #ge #hc ,k_/,g #ch #dba #bj #adc ,s_/,w #beg #bbi no detection after #bfj seconds ,illustration3 ,data from table have been arranged in this histogram to show wide range of delay detection times that were recorded in the experiment4 ,each block represents a single delay detection1 identified by initials of conversers4 number of cases #f ,s_/,h #e ,s_/,h #d ,s_/,h ,s_/,w #c ,k_/,g ,a_/,l ,s_/,h #b ,k_/,g ,f_/,k ,k_/,g ,f_/,k #a ,f_/,k ,f_/,k ,g_/,h ,g_/,h ,s_/,w ,a_/,l ,k_/,g ,a_/,l #j #j #ej #ajj #aej #bjj #bej #cjj #cej #djj #dej #ejj #eej #fjj #fej detection time 7seconds7 ,now1 to answer our question about whether people can converse over our circuit1 we can say something like this3 ,we have found some cases 7of a certain type of conversation7 where people can use a circuit with #a4b seconds of echo-free delay for the amounts of time listed in our table4 ,but there are two important things to remember about our results3 --,the findings are a non-negative answer to the original question1 8,can people converse over a four-wire1 echo-free circuit that has delay like that of a synchronous satellite80 --,the experiment applies only to four-wire1 echo-free circuits1 and does not help with the problem of improving echo suppressors or with that of finding out how good synchronous satellite circuits with the best possible echo suppressors would be if they were used to interconnect the world's telephones4 ,since we didn't get a 8no0 answer to our question1 we have been encouraged to do more experiments with other subjects and other types of conversations4 ,we also have begun to look for more than a yes-or-no answer2 we now want to find out how serious various amounts of pure delay would be4 ,some of my colleagues have been working on this problem by furnishing special four-wire telephones to a group of people1 so that echo-free delay can be inserted in the line when one member of this test group calls another member4 ,their experiment has confirmed our finding that conversation with a round-trip pure delay of #a4b seconds is not impossible1 but it has also shown that the degradation of conversation that results is not trivial4 ,recently an international committee on commercial telephone standards set the maximum permissible echo-free delay 7round-trip7 at #j4g seconds4 ,however1 the search for a more precise evaluation is still going on4 ,peter ,d4 ,bricker was born in ,scranton1 ,pennsylvania1 and received an ,a4,b4 from ,bucknell ,university in #aiej and an ,m4,a4 in #aieb and a ,ph4,d4 in #aied from the ,johns ,hopkins ,university4 ,he joined ,bell ,telephone ,laboratories in #aied1 and has been engaged in psychological studies of telephone color preferences1 pushbutton set designs1 voice identification1 and transmission quality evaluation4 ,,case ,,history ,,no4 #f ,how ,can ,we ,repair an ,orbiting ,satellite8 ,e4 ,jared ,reid ,electrical ,engineer--,member of ,staff1 ,satellite ,design ,department ,,the ,,problem ,it is hard enough to fix a new piece of scientific equipment when it goes out of order in the laboratory4 ,and when the equipment is sailing around the earth a couple of thousand miles up in the sky a repair job ought to be impossible4 ,but during the last two months of #aifb we found this not to be true at all4 ,that was when the ,telstar ,i satellite began to misbehave and eventually would not obey the commands we sent it from the ground4 ,this presented us with a nice little problem3 ,we had to find out exactly what was wrong with the satellite and then--the really tough job--devise a way to cure the trouble4 ,we were able1 finally1 to do both these things1 after a combination of logical deduction1 trial-and-error experimentation1 and laboratory testing--plus a certain amount of plain good luck4 ,our 8cure10 unfortunately1 turned out to be only a temporary one1 for our patient had a relapse some weeks later4 ,however1 the story of how we went about doing our never-before-attempted task should give you an idea of the things you have to improvise in the laboratory when an experiment doesn't work out exactly as you had planned4 ,the ,telstar ,command ,circuit ,as shown on pages #cb and #cc1 the operation of the ,telstar ,i satellite was controlled by orders sent from the ground on a frequency of #abc megacycles4 ,fifteen different commands could be given to the satellite1 each a coded signal made up of a series of ones and zeros4 ,the signals1 as you can see in the table below1 turn on or off the radiation experiments1 the telemetry1 the communications equipment1 and the orientation coil4 ,these are important functions1 and we wanted them to be going on when they were needed4 ,but we did not want them to be operating continuously4 ,command was the only ,telstar ,i function that we felt had to be 8redundant10 so two duplicate chains of components were provided4 ,as you can see in the block diagram on the next page1 the satellite has two radio receivers in parallel1 so that one can operate if the other fails4 ,there are also two command decoders1 which take the pulse-coded signals from the receivers and translate their zeros and ones into usable instructions4 ,in the command switching control1 these instructions operate nine relays that turn on or off the power to all the electronic circuits except the command receiving chain1 which operates continuously4 ,telstar ,i's telemetry unit reported back #aab measurements every minute over the #acf-megacycle frequency4 ,these told both what the satellite encountered in space and the condition of the satellite's own components--as indicated by a variety of different sensors4 ,the telemetry also gave a check on whether the commands sent to the satellite were actually obeyed4 ,the ,fifteen ,telstar ,commands ,,command ,,function ,a ,turns on traveling-wave tube filament voltage ,b ,turns on traveling-wave tube helix and collector voltages2 energizes all transistor circuits associated with communications experiments ,,aa ,turns off traveling-wave tube helix1 collector1 and filament voltages2 de-energizes transistor circuits ,c ,turns on traveling-wave tube by applying anode voltage ,,cc ,turns off traveling-wave tube anode voltage ,d ,turns on telemetry and energizes radiation experiment circuits ,,dd ,turns off telemetry and de-energizes radiation experiment circuits ,e ,turns on current in orientation torque coil ,,ee ,turns off current in orientation torque coil ,f ,connects telemetry encoder ,no4 #a to circuit ,,ff ,connects telemetry encoder ,no4 #b to circuit ,,ss ,performs duties of ,,aa1 ,,cc1 ,,dd1 ,,ee1 and ,,ff2 de-energizes #acf-mc beacon transmitter and removes all load from storage battery ,s ,connects storage battery back into the circuit and energizes the #acf-mc beacon transmitter ,t#a ,turns off command receiver and decoder ,no4 #b for #ae seconds1 so that command receiver and decoder ,no4 #a can be tested ,t#b ,turns off command receiver and decoder ,no4 #a so that ,no4 #b can be tested ,illustration3 ,this block diagram of ,telstar ,i's command circuit shows the redundant receiver and decoder chains1 ,no4 #a and ,no4 #b1 which are fed the command signals from the ground that are picked up by the ,,vhf antenna4 ,decoded instructions go to the command switching control1 which operates relays to turn equipment on or off4 ,reports on the operation of this control are sent back by telemetry to the ground station along with information from the satellite's sensors4 ,,vhf antenna diplexer command receiver ,no4 #a command decoder ,no4 #a ,t#a command command receiver ,no4 #b command decoder ,no4 #b ,t#b command command switching control 7operates relays7 ,s-,,ss relay ,d-,,dd relay other relays #acf-mc beacon transmitter ,,ss open ,s close telemetry ,,dd open ,,ss open ,d close sensors 7radiation detectors1 particle counters1 solar aspect cells1 thermistors1 etc47 power supply solar cell power plant storage battery power plant ,,ss open ,s close through other relays to communications circuits and orientation torque coil ,what ,went ,wrong ,with ,telstar ,i ,during ,telstar ,i's first two months in orbit1 the only indication of trouble cropped up in one of the command operations4 ,telemetry told us that the satellite was no longer executing the ,t#b command4 ,this meant that we could not temporarily disconnect command chain ,no4 #a to check the performance of chain ,no4 #b4 ,then1 a short while later1 ,no4 #b began to give intermittent operation4 ,finally it failed completely4 ,at the time1 we didn't know why this had happened1 but1 since the satellite's other command chain still seemed to be operating normally1 we were not very worried4 ,however1 in the middle of ,november #aifb command chain ,no4 #a also began to be intermittent4 ,we would send a command but get no response from the satellite2 only after we repeated it a few times would the satellite finally do what it had been told to do4 ,now there was something to be concerned about4 ,and1 if chain ,no4 #a should fail1 we had to make sure that ,telstar would be left in a favorable operating condition4 ,we didn't want the satellite's communications equipment to be left on without our being able to turn it off--this would keep a continuous drain on the power supply4 ,as we feared it would1 the other command circuit went out of commission on ,november #bcrd4 ,however1 when this happened1 the communications circuits had been turned off1 although the command chains themselves and the telemetry remained on4 ,this meant that we could still try to send commands1 the condition of the satellite could still be monitored by telemetry1 and the solar cells could still supply useful power4 ,but1 since we could not turn the communications equipment on1 ,telstar ,i could no longer be used for transatlantic television or any of the experiments we had been carrying on successfully since ,july4 ,looking for the ,trouble ,spot ,at this point a number of ,bell ,telephone ,laboratories engineers began to analyze ,telstar's troubles4 ,as you can imagine1 we had a rather difficult problem4 ,we obviously could neither go up and look at ,telstar nor bring it down for an overhaul on the ground4 ,we could only send different commands to the satellite and watch the telemetry data to see what1 if anything1 happened4 ,after checking the satellite's other equipment1 we were happy to find that everything except the command chains was in good condition4 ,so we decided the trouble had to be one of five possibilities3 --excessive electronic 8noise0 in the satellite1 which had blocked the command receivers2 --extreme temperature variations1 which had caused a joint to expand1 contract1 and finally break2 --a loose connection2 --slow aging of an electronic component2 --deterioration of a component from excessive radiation bombardment4 ,we could quickly narrow this list down4 ,telemetry indicated that the receiver was not being blocked by noise4 ,reports from the temperature-measuring thermistors told us that inside the satellite the temperature was #ge degrees ,fahrenheit1 just as it should be4 ,a loose connection was very unlikely1 because every one had been made by expert wiremen1 examined by trained inspectors1 and then completely encapsulated in polyurethane foam4 ,aging also seemed very improbable1 since all the components had been individually tested and selected for the highest reliability and longest life4 ,the ,villain3 ,radiation ,this left only radiation damage4 ,we had other good reasons to suspect this1 too4 ,as far back as ,october #aifa1 scientists at ,bell ,labs and ,brookhaven ,national ,laboratories had made an important discovery about the effect of radiation on a transistor4 ,they found that1 when radiation penetrates the outer shell of a transistor and ionizes the gases inside1 electrically charged particles 7ions7 tend to collect on the surface and change the transistor's electric properties4 ,this effect is particularly noticeable when a transistor is operating under reverse bias voltage4 ,we knew that some of the #cg transistors used in each ,telstar ,i decoder circuit were operating under continuous reverse bias and that they also had less metal shielding than did those semiconductors in the telemetry and receiver circuits4 ,telstar's radiation detectors had been telling us that the concentration of high-energy electrons near the inner edge of the ,van ,allen belt was greater than we had expected4 ,we now know this may have come as a result of man-made high-altitude nuclear explosions1 one of which took place the day before ,telstar ,i was launched4 ,but we had had no reason to anticipate this extra radiation--it was more than one hundred times the predicted level--when we tested the transistors to be used in ,telstar ,i4 ,so we were not too surprised that they were more susceptible to radiation damage than we had thought they would be4 ,all this seemed to give us a theoretical explanation for the trouble4 ,so1 after ,november #bcrd1 we began looking for laboratory evidence to confirm our radiation theory4 ,first1 two engineers traveled down to ,johannesburg1 ,south ,africa4 ,at this time the highest point of ,telstar's orbit--when it passes through the least ,van ,allen belt radiation--was over the southern hemisphere1 and it seemed a good idea to command the satellite when it was under the condition of lowest radiation and see if anything would happen4 ,however1 everything the engineers tried proved fruitless4 ,illustration3 ,a duplicate of one of ,telstar ,i's command decoders1 containing #cg transistors and #aia diodes1 being placed in a small elevator that will lower it into a gamma radiation chamber4 ,at our ,murray ,hill1 ,new ,jersey1 laboratories we worked on a different approach4 ,we exposed transistors like those used in the decoders to large doses of radiation4 ,we also exposed entire spare decoder units to accelerated radiation to find out where their weakest points were 7see illustration74 ,and then we built and tested decoders using radiation-resistant transistors to see if they worked better4 ,after a week of intensive laboratory work1 we had some pretty good evidence4 ,the tests of individual transistors definitely showed that heavy radiation would cause them to deteriorate4 ,testing of the complete decoders also led to some failures1 and1 when we analyzed them1 they turned out to be the kind that would be caused by faulty transistors4 ,we also discovered that the most sensitive part of a decoder circuit was the zero gate1 which recognizes the zeros in the one-and-zero code that commands the satellite4 ,illustration3 ,a typical command signal sent to ,telstar ,i4 ,it consists of seven pulses3 a three-unit 8start0 pulse and a binary code made up of three two-unit 8one0 pulses and three one-unit 8zero0 pulses4 ,fooling the ,decoder ,now we thought we knew the guilty component1 but the hardest job still lay ahead of us4 ,we had to do something to the commands so that they would bypass this troublesome zero gate4 ,each of the fifteen satellite commands is a binary code of seven pulses1 as illustrated in the diagram above4 ,the first--the start pulse--is three units wide4 ,then follow six more pulses of which three are two units wide 7one pulses7 and three are one unit wide 7zero pulses74 ,the arrangement of this group of six ones and zeros determines the particular command4 ,each time a one pulse arrives at the decoder1 a one gate counts the pulse and stores a one in its memory4 ,a zero gate counts the zero pulses1 but does not store anything4 ,so1 if the zero gate is blocked1 the decoder will not count the zeros in any of the coded commands and thus cannot decode them properly4 ,illustration3 ,the special 8notched one0 pulse that was invented to fool ,telstar ,i command decoder4 ,what could be done about this8 ,the answer seemed to be to devise a new type of pulse--a pulse that would be enough like a one so that it would pass through the one gate and advance the counter1 but1 at the same time1 be enough unlike a one so that the one gate would not store it in its memory4 ,this led to the invention of the special long pulse with a dip or notch in it that is shown in the diagram above4 ,when we tested it in the laboratory on one of the duplicate decoders we had exposed to radiation1 this new notched pulse worked as we hoped it would4 ,it passed through the one gate and advanced the counter1 but was not stored as a one in the one gate's memory4 ,thus it fooled the decoder by doing just what a zero is supposed to do1 even though it had gone through the one gate rather than the zero gate4 ,illustration3 ,magnetic tapes of special codes using notched ones in place of zeros being prepared for use4 ,but the real test was yet to come4 ,special modified signals for two of the fifteen ,telstar commands1 using our new notched ones in place of the usual zeros1 were put on magnetic tape 7see photograph74 ,then1 on ,december #bjth1 when ,telstar made its #adibnd pass over ,andover1 ,maine1 a group of tired engineers huddled about the mass of equipment they had assembled4 ,finally1 on the third try1 the notched pulses were successful2 ,telstar's telemetry flashed back the word that the proper relay had operated upon command4 ,removing the ,ionization ,we now wanted to get ,telstar to do something that had seemed to work in the laboratory4 ,the transistors most affected by radiation were those operating under continuous reverse bias1 to whose surfaces unwanted ions were attracted4 ,if we removed the voltage from these transistors1 we felt that the ionization layer would be dissipated1 and they would act normally again4 ,our plan was to prepare a complete taped program of all fifteen commands1 and carefully disconnect ,telstar's storage battery 7using command ,,ss74 ,then1 when the satellite went into eclipse1 there would be no power available from the solar cells either1 and--if our calculation was right--the complete lack of voltage ought to restore the transistors to working order4 ,this was a hazardous procedure1 for if something went wrong we might have a completely silent satellite on our hands4 ,as it turned out1 an accident did happen--but one of a different and much more fortunate kind4 ,on ,december #bgth ,telstar misinterpreted our 8trick0 commands and disconnected its own battery before we asked it to4 ,then1 as the satellite went into the earth's shadow1 we held our breath while all its power stopped and the telemetry went silent4 ,but1 as we had hoped1 a rest period with all power removed from the deteriorated transistors apparently made them work almost normally once again4 ,on ,january #a1 #aifc1 we were able to disconnect the battery in regular fashion--that is1 using the one-and-zero code4 ,after this was done1 and all power had been removed1 both decoders again would operate when given normal commands 7actually1 the first one restored to duty was decoder ,no4 #b1 which had gone out of order first1 back in ,august74 ,back to ,normal--,for a ,time ,for more than a month ,telstar ,i behaved as it should1 and our communications experiments1 including television broadcasts1 were resumed on ,january #crd4 ,during this time we used both normal commands and our special notched-pulse modified commands4 ,whenever normal commands became intermittent we used the modified commands to disconnect the battery for several eclipses4 ,our good fortune1 however1 did not last4 ,continued exposure to radiation apparently led to further damage to ,telstar ,i's transistors4 ,by ,february #adth1 disconnecting the storage battery no longer returned the decoder to normal1 and we could operate only with our modified commands4 ,and1 on the #bast1 the satellite apparently misinterpreted a command1 disconnected its storage battery1 and went silent4 ,since then1 none of our modified commands has been able to bring back its voice4 ,there is still a possibility that ,telstar ,i may recover if it remains out of the high-radiation part of space for a long enough period--but as time goes by this appears less likely4 ,however1 our work was not in vain4 ,because we pinpointed the effects of radiation on the transistors in ,telstar ,i1 this problem was counteracted on the ,telstar ,,ii satellite launched on ,may #g1 #aifc 7see page #ca74 ,to avoid the worst of the radiation effects1 the second ,telstar is in a considerably larger orbit1 which causes it to spend less time in the heaviest high-energy ,van ,allen belt regions4 ,it carries new radiation detectors with much greater measuring capacity4 ,and in one of ,telstar ,,ii's command decoders we are using a new type of transistor1 which we hope will not be affected nearly as much by radiation as were the ones in ,telstar ,i's ill-fated decoders4 ,e4 ,jared ,reid was born in ,hartford1 ,connecticut1 and received a ,b4,s4 from ,trinity ,college in #aief1 a ,b4,e4,e4 from ,rensselaer ,polytechnic ,institute in #aieg1 and an ,m4,e4,e4 from ,new ,york ,university in #aiei4 ,he joined ,bell ,telephone ,laboratories in #aieg1 and has worked on the design and testing of the ,time ,assignment ,speech ,interpolation 7,,tasi7 system for the transatlantic cable1 as well as on transistor circuits for the ,telstar satellite4 ,a ,final ,note to the ,reader ,now1 having read ,part ,,ii of ,satellite ,communications ,physics1 you should have an idea how we predict the orbit of an artificial satellite and how we find out where it points while passing a thousand miles above our heads4 ,you can see how we pick the best material to cover its surface with and how we protect its solar cells from the hazards of space4 ,and you have watched the steps we would take when our satellite stops working properly4 ,it would1 we admit1 take a little more experience to solve problems like these on your own--and to deal with all the other complications of satellite communications4 ,but we hope our brief glimpses into the laboratory have shown what this experience might be like4 ,our six case histories have only scratched the surface1 but they should give you a good idea of the fascinating work that goes into practical science and engineering4 ,they should show that something like ,project ,telstar doesn't succeed only because of far-sighted1 imaginative thinking--nor only because of ingenious engineering4 ,it draws upon the best of both of these4 ,along the way1 we hope you have noticed some important guideposts--things like ,newton's law of gravitation1 the law of reflection of light1 the ,stefan-,boltzmann law4 ,they typify the basic principles of physics that engineers and scientists1 whatever they do1 must always keep in mind4 ,no matter how exotic or up-to-the-minute the application1 the ground rules of physics must be followed4 ,if we have convinced you of this1 we have done what we set out to do6 ,suggested ,reading ,if you would like to read further about satellite communications in general or get some information about the case histories in ,part ,,ii1 you may be interested in using the following reading list4 ,the references under each of the subheadings are listed chronologically2 they include books1 reports1 technical papers1 and magazine articles4 ,as you can see1 some of these ought to be understandable by almost anyone1 but others are quite technical in nature4 ,for further background in the basic physical principles that are discussed in ,part ,,ii1 you may refer to many good high school and college physics texts4 ,an increasing number of useful physics books--both originals and reprints--are now being published in paperback form4 ,satellite ,communications ,arthur ,c4 ,clarke1 8,extra-,terrestrial ,relays--,can ,rocket ,stations ,give ,world-,wide ,radio ,coverage810 ,wireless ,world1 ,october #aide1 page #cje4 ,john ,r4 ,pierce1 8,orbital ,radio ,relays10 ,jet ,propulsion1 ,april #aiee1 page #aec4 ,john ,r4 ,pierce and ,rudolf ,kompfner1 8,transoceanic ,communication by ,means of ,satellites10 ,proceedings of the ,i4,r4,e41 ,march #aiei1 page #cgb4 ,john ,r4 ,pierce1 8,exotic ,radio ,communications10 ,bell ,laboratories ,record1 ,september #aiei1 page #cbc4 ,steven ,m4 ,spencer1 8,dial #8,s' for ,satellite10 ,the ,saturday ,evening ,post1 ,january #ad1 #aifj1 page #ac4 ,space ,electronics ,issue1 ,proceedings of the ,i4,r4,e41 ,april #aifj4 ,william ,meckling1 8,economic ,potential of ,communication ,satellites10 ,science1 ,june #af1 #aifa1 page #ahhe4 ,special ,issue on ,project ,echo1 ,bell ,system ,technical ,journal1 ,july #aifa4 ,c4 ,c4 ,cutler1 8,radio ,communication by ,means of ,satellites10 ,planetary and ,space ,science ,journal1 ,july #aifa1 page #bed4 ,w4 ,c4 ,jakes1 ,jr41 8,project ,echo10 ,bell ,laboratories ,record1 ,september #aifa1 page #cjf4 ,john ,r4 ,pierce1 8,communication ,satellites10 ,scientific ,american1 ,october #aifa1 page #ij4 ,united ,states ,senate1 ,committee on ,aeronautical and ,space ,sciences1 ,communication ,satellites3 ,technical1 ,economic1 and ,international ,developments 7staff report71 ,u4 ,s4 ,government ,printing ,office1 ,washington1 #aifb4 ,l4 ,j4 ,carter1 editor1 ,communications ,satellites1 ,academic ,press1 ,new ,york and ,london1 #aifb4 8,situation ,report on ,communications ,satellites10 ,interavia1 ,june #aifb1 page #gdi4 ,leonard ,jaffe1 8,communications by ,satellite10 ,international ,science and ,technology1 ,august #aifb1 page #dd4 8,communicating by ,satellite10 ,business ,week1 ,october #bg1 #aifb1 page #hf4 ,project ,telstar ,rowe ,findley1 8,telephone a ,star10 ,national ,geographic1 ,may #aifb1 page #fch4 ,louis ,solomon1 ,telstar1 ,mc,graw-,hill ,book ,company1 ,new ,york1 #aifb4 ,special ,telstar ,issue1 ,bell ,laboratories ,record1 ,april #aifc4 ,special ,telstar ,issue1 ,bell ,system ,technical ,journal1 ,july #aifc4 ,satellite ,communications ,case ,histories #a4 ,how ,do ,we ,calculate a ,satellite's ,orbit8 ,mario ,iona1 8,satellite ,orbits10 ,the ,physics ,teacher1 ,may #aifc1 page #ee4 ,a4 ,j4 ,claus et al41 8,orbit ,determination and ,prediction and ,computer ,programs10 ,bell ,system ,technical ,journal1 ,july #aifc1 page #aceg4 #b4 ,what ,color ,should a ,satellite ,be8 ,p4 ,t4 ,haury1 8,thermal ,design of the ,electronics ,canister10 ,bell ,laboratories ,record1 ,april #aifc1 page #afa4 ,j4 ,w4 ,west1 8,space ,hardware ,aspects of the ,satellite10 ,bell ,laboratories ,record1 ,april #aifc1 page #afg4 ,peter ,hrycak1 et al41 8,the ,spacecraft ,structure and ,thermal ,design ,considerations10 ,bell ,system ,technical ,journal1 ,july #aifc1 page #igc4 #c4 ,how ,do ,we ,make ,optical ,measurements on a ,satellite8 ,w4 ,c4 ,jakes1 ,jr41 8,participation of the ,holmdel ,station in ,project ,telstar10 ,bell ,system ,technical ,journal1 ,july #aifc1 page #adba4 #d4 ,how ,do ,we ,keep ,solar ,cell ,power ,plants ,working in ,space8 ,d4 ,m4 ,chapin et al41 8,the ,bell ,solar ,battery10 ,bell ,laboratories ,record1 ,july #aiee1 page #bda4 ,g4 ,r4 ,frost1 ,from ,sun to ,sound1 ,bell ,telephone ,laboratories1 ,new ,york1 #aifa4f ,f4 ,m4 ,smits1 ,k4 ,d4 ,smith1 and ,w4 ,l4 ,brown1 8,solar ,cells for ,communications ,satellites in the ,van ,allen ,belt10 ,journal of the ,british ,i4,r4,e41 ,august #aifa1 page #afa4 ,d4 ,m4 ,chapin1 ,energy from the ,sun1 ,bell ,telephone ,laboratories1 ,new ,york1 #aifb4f ,r4 ,e4 ,d4 ,anderson et al41 8,the ,satellite ,power ,system10 ,bell ,laboratories ,record1 ,april #aifc1 page #adb4 ,k4 ,d4 ,smith et al41 8,the ,solar ,cells and ,their ,mounting10 ,bell ,system ,technical ,journal1 ,july #aifc1 page #agfe4 #e4 ,would ,time ,delay ,be a ,problem in ,using a ,synchronous ,satellite8 ,g4 ,m4 ,phillips1 8,echo and ,its ,effect on the ,telephone ,user10 ,bell ,laboratories ,record1 ,august #aied1 page #bha4 ,w4 ,a4 van ,bergeijk1 ,j4 ,r4 ,pierce1 and ,e4 ,e4 ,david1 ,jr41 ,waves and the ,ear1 ,anchor ,books 7,science ,study ,series paperback71 ,doubleday ,company1 ,new ,york1 #aifj4 ,r4 ,p4 ,haviland1 8,the ,synchronous ,satellite10 in ,communications ,satellites1 ,l4 ,j4 ,carter1 editor1 ,academic ,press1 ,new ,york and ,london1 #aifb1 page #aac4 #f4 ,how ,do ,we ,repair an ,orbiting ,satellite8 ,d4 ,s4 ,peck et al41 8,surface ,effects of ,radiation on ,transistors10 ,bell ,system ,technical ,journal1 ,january #aifc1 page #ie4 8,fixing ,up ,telstar10 ,time1 ,january #ah1 #aifc1 page #dh4 ,e4 ,p4 ,moore and ,w4 ,j4 ,maybach1 8,satellite ,command and ,telemetry ,systems10 ,bell ,laboratories ,record1 ,april #aifc1 page #aef4 ,j4 ,s4 ,mayo et al41 8,the ,command ,system ,malfunction of the ,telstar ,satellite10 ,bell ,system ,technical ,journal1 ,july #aifc1 page #afca4 ,note3 ,the ,bell ,laboratories ,record is published by ,bell ,telephone ,laboratories1 ,incorporated1 #dfc ,west ,street1 ,new ,york #ad1 ,new ,york4 ,the ,bell ,system ,technical ,journal is published by the ,american ,telephone and ,telegraph ,company1 #aie ,broadway1 ,new ,york #g1 ,new ,york4 ,the ,editor ,ronald ,m4 ,foster1 ,jr41 was born in ,plainfield1 ,new ,jersey1 and received an ,a4,b4 degree from ,harvard ,college in #aidh4 ,he joined ,bell ,telephone ,laboratories in #aief1 and is a member of the ,educational ,aids ,department of the ,public ,relations and ,publication ,division4 ,he is engaged in development of material for the ,bell ,system ,aid to ,high ,school ,science ,program4 ,footnotes #a,this is obtained from k g,r1 where g is the acceleration due to gravity and ,r is the radius of the earth4 7,here1 we can use k #if1ejj miles per second47 #b;,donald ,r4 ,herriott of ,bell ,labs had suggested using plane reflectors on satellites as long ago as #aieg--;although his idea was that this would increase their visibility1 rather than aid in determining their attitude4 #c,this method was developed by ,d4 ,w4 ,hill of ,bell ,telephone ,laboratories4 #d,we will not attempt to go into all the details of semiconductor physics here4 ,if you would like to know more about how solar cells work1 refer to the ,suggested ,reading on page #hh4 #e,see pages #db and #dc4 #f,published as part of the ,bell ,system ,aid to ,high ,school ,science ,program4 ,illustration3 ,wraparound cover image ,transcriber's ,notes --,silently corrected a few typos4 --,modified some image references to reflect the pageless flowable e,book format4 --,retained publication information from the printed edition3 this e,book is public-domain in the country of publication4 --,in the text versions only1 text in italics is delimited by underscores4