america in space the first decade. putting satellites to work

8/8/2019 America in Space the First Decade. Putting Satellites to Work

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TOWORKby Will iam R. Corliss

Nationa l Aeronautics and Space Administration, Washington, D.C., 20546


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Introduction

Ten years ago NASA was brought into being to explorespace. As stated in the National Aeronautics and SpaceAct of 1958: ". . . it is the policy of the United Statesthat activities in space should be devoted to peacefulpurposes for the benefit of all mankind." One of themajor avenues of intellectual and program effort thathas guided us at NASA has been the concept, at firstunproved but now clearly valid, that space systems canprovide unique, direct benefits to man, benefits notbefore possible or economically feasible.We do not yet know the full range and scope of thepossibilities that spacecraft open for the service of man.Those few particular applications upon which we haveconcentrated in the past and which are described in"Putting Satellites to Work" have borne out thatpromise. Communications, navigation, geodetic, andn~eteorological pace systems are operational today,

and their existence, once the subject of science fiction,is now a practical fact. It is clear that many potentialapplications exist: the one most clearly on the horizonis the possibility of surveying the Earth's resources fromspace. We are really just beginning to develop thepossibilities in this area of research, but we can clearlyforesee that during the next decade NASA can, inbuilding on its past accomplishments, provide toolswhich may significantly affect the efficiency and thusthe quality of our life here on Earth.

Leonard JaffeDirector, Space Applications Program


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TableOfContents

The Weather Watchers 1A Better View of the Weather 1Pictures by the Thousand 3Hot Spots Below 7Pictures on Request 10Extraterrestrial Relays 10The Advantages of Height 10Balloons in Orbit 12Active Cdmmunication Satellites 13

The Figure of the Earth 17Better ~ r i c d oons 20Testing ~a b b r at o r ~n Space 22The Eqrth: The Big Picture 24


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PuttingSatellitesToWorkTheWeatherWatchersA Better View of the WeatherGood weather forecasting is worthmoney. An accurate five-day forecastwould probably save between $2.5 and $5.5 billionyearly in the U.S. alone; perhaps $15 billion for theentire world. Most of the savings would be in agricul-ture, the construction industry, and governmentoperations, such as flood control. The capital invest-ment for a good global weather forecasting systemshould cost less than a half billion dollars; the potentialpayoff is impressive.Besides the time-honored approach of watching theskies for weather signs, what can we do to improveweather forecasting? Four possibilities come to mind,and we have been trying all four for over a century:1. Rather than looking for red sunsets or mackerel

skies, we can record more objective data.Examples are: temperature, wind direction andvelocity, humidity, and so on. These are allclassical measurements that have been recordedregularly at weather stations since the early1800s.

2. A weather station operator can expand his fundof basic information further by lofting instru-ments on balloons, or, as was popular in the lastcentury, by flying them on large kites. Betterknowledge of the air aloft leads to a betterunderstanding of what is happening to air closeto the ground.

3 . As meteorologists realized that weather patternswere actually continent-sized rather than localin nature, they began to link individual weatherstations into networks. Just before the CivilWar, Joseph Henry, of the Smithsonian Institution,began to collect weather observations takenconcurrently over a broad area. This synopticinformation was relayed to Henry by telegraphfrom stations all over the eastern U.S. Now,many countries have well-integrated networksof stations and there is considerable internationalexchange of weather data.

4. Another improvement in weather forecasting hascome as meteorologists have learned more aboutthe natural forces that actually make weather.Representing these forces mathematically, theyhave constructed equations that describethe movements of weather fronts and themechanism of the hurricane. By feeding theseequations into a high speed computer and addingthe data taken by weather stations and weathersatellites, meteorologists believe they caneventually generate accurate two-week forecasts.

Long before the National Aeronautics and SpaceAdministration was conceived, sounding rockets werecontributing to our knowledge of the upper atmosphere.NASA, of course, has continued this line of attack,but the Earth satellite is the most valuable meteorologi-cal tool contributed by the space program. Like


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1 A surface weather station usually records only highlylocalized info rmatio n. Station networks extend coveragegeographically. Satellites produce c loud cover and infrar edinformation over the entire globe.2 TlROS 111 before launch. Cameras are centered in thebottom. Solar cells cover the top and all sides.We ight of TlROS 111 was about 280 pounds.

rockets and balloons, satellites carry instruments farabove the layer of air hugging the ground; like bignetworks of stations, they afford a wide geographicalperspective of world weather. In fact, no combinationof surface station networks can match the panoramaof world weather radioed back from satellite cameras.The meteorologird satellites presently operating dohave limitations. They orbit so high up that theycannot make direct measurements of temperature andpressure with ordinary thermometers and barometers.They can only "see" what is going on far below.Seeing alone, however, is of great value to meteorolo-

gists. Cloud pictures-the stock-in-trade'of the weathersatellite-show the great weather systems forming,swirling, and dissolving against the backdrop of theoceans and continents. By taking pictures of theEarth in the infrared portion of the electromagneticspectrum, weather satellites give the meteorologistinformation about the heat added to and lost from theEarth and its atmosphere. Since the vast cyclones andanticyclones that roll across the globe are reallymonstrous heat engines, this heat budget informationhelps forecast weather. Weather satellites by themselvescannot provide all the information needed for goodforecasts, but they can help significantly.


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Pictures by the ThousandAnyone who has flown in an airplaneand looked down on the great white cloudbanks marching across the landscape has seen only asmall fraction of what a satellite sees. An airplanedoes not fly high enough to see the big picture.Meteorologists did not see truly large-scale picturesof cloud patterns until cameras were flown on highaltitude rockets in the late 1940s and early 1950s.What they saw whetted their appetites for more.The Army, Navy, and industry began studying weathersatellites in the early 1950s. In particular, the Radio

Corporation of America (RCA) applied to meteorologythe experience it had gained studying television-equipped satellites for the Air Force. After itwas created in 1958, NASA supportedthe RCA work. The RCA weather satellite concepteventually became known as TlROS (TelevisionInfrared Observation Satellites).The first TIROS satellites had the proportions of ahatbox, aithough they were considerably larger: 42inches in diameter and 22.5 inches high. Theyweighed about 270 pounds. The top of the hatbox andeach of the 18 facets around the circumference were


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TRANSPARENT FACEPLATECONDUCTIVE, TRANSPAREfiT COATING

t f f ' ELECTRON BEAM

ELECTRON GUN

covered with solar cells, which generated electricalpower for the TV camera and the radio transmitterthat relayed pictures to antennas waiting on Earthbelow. The TIROS satellites were spin-stabilized; theyspun at about ten revolutions per minute. Thesatellite's angular momentum kept it from tumbling inspace, just as the spin of a rifle bullet stabilizes itsflight. On TIROS I, the axis of the hatbox remainedapproximately fixed in space. As the satellite circledthe Earth, its cameras, which were mounted on thebottom of the spacecraft, pointed wherever thesatellite axis pointed, which more often than not wastoward empty space. Later TIROS satellites carrieda coil of wire to improve the cametas' view of theEarth. When electric current was sent through the coil,the satellite became a weak electromagnet and turnedin the Earth's magnetic field like the armature of anelectric motor. Nevertheless, the early TIROS satellitessaw and photographed the Earth only part of the time.A rather frustrating situation, but a complete solutionto the problem of camera pointing had to be deferredto the second-generation Nimbus weather satellites.

The heart of a TIROS satellite is its complement oftelevision cameras. Ordinary television cameras, whichtake many frames a second to give the illusion ofsmooth motion, cannot be used because TIROS couldnot possibly transmit that much information to theground.* A special television tube called a vidiconis used on most U.S. picture-taking spacecraft whetherthey photograph the Earth's clouds, the lunar surface,or Mars. Replacing conventional film, the vidicon hasa sheet of photo-conductive material, which becomesa good conductor of electricity wherever light hits it.Before the shutter of the vidicon is opened to take apicture, the photoconductive sheet is sprayed uniformlywith electrons. Then, the shutter is opened and thescene is focused on the sheet. Bright areas in thepicture activate the photoconductive material, causingthe deposited electrons to be drained away. Electronsremain in the dark areas of the picture. Now anelectron gun sweeps across the photoconductive sheetin a pattern or raster consisting of several hundredlines. The electrons from the electron gun will berepelled from dark areas but not from bright areas.The current of electrons flowing out of the electrongun onto the photoconductive surface is thus a

* The transmission of information requires power; doublethe number of television frames per minute and the powermust be doubled.


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3 Schematic of a vidicon camera.4 Nimbus I1 APT photo of two 1967 hurricanes, takenSeptember 1 3. Hurricane Doria is about 300 milessoutheast of Cape Cod and Chloe is roughly 1000 milessoutheast of New England. The storms are about 700miles apart.

measure of the brightness of the image being scanned.This fluctuating current is turned into a signal that canbe converted into a television picture back on theEarth.Between 1960 and 1965, ten TIROS satellites werelaunched without a failure. All ten radioed backpictures of the Earth's cloud cover-hundreds ofthousands in all. The TIROS pictures presented agrand global panorama of the Earth, grander by farthan the sights seen by the first balloonists and aeronauts.The most striking and exciting features of the TIROSpictures have been the large-scale cloud patterns,which show a degree of organization never realizedfrom terrestrial and aircraft observations. In particular,huge vortices-some 1000 miles across-wheelacross the oceans and the continents, makingweather as they go.TIROS photos clearly show weather fronts and otherpatterns that coincide closely to the maps drawn forthe newspapers from accumulated surface stationreadings. This proven correspondence gives meteorolo-gists confidence that they can employ satellite cloud-cover pictures to draw weather maps in portions ofthe world where ground weather stations are sparseor nonexistent. The correspondence between the

satellite panorama and ground-level direct measure-ments is not perfect. These discrepancies, though small,have led to modification of cyclone theory.In the pre-TIROS days, hurricanes used to sweep infrom the unpatrolled oceans and slam into land areaswith little warning. Destruction and loss of life havefrequently been high; much higher than they would havebeen with ample warning time. TIROS has changedall that by constantly monitoring cloud cover overthe desolate reaches of the oceans. Anyone whowatches TV news programs during the hurricaneseason has seen TIROS pictures of these intense stormsand followed their progress along the U.S. Atlanticcoast. Satellite pictures often catch these storms intheir formative stages, showing the prehurricanesquall lines that ring the growing nucleus. Some-times, a hurricane interacts with a jet stream, givingmeteorologists a ringside seat for the battle betweenthese two powerful weathermakers. Without the highvantage point of the weather satellite this drama wouldgo unseen.As the atmosphere swirls across the surface of theEarth, it encounters land formations that deflect the


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air currents and cause turbulence. The patternscreated are intriguing as well as instructive. TheSierra wave, for example, manifests itself as a longlinear cloud created as air is pushed up as it tries toslide over the Sierra range. The Andes form similarcloud patterns. In a similar vein, ocean islandswith high mountains create strange eddies of cloudsthat reveal large-scale turbulence, which except for sizeresembles the turbulence formed behind rocks in abrook.The first eight TIROS satellites were very much alike.Although they were very successful, NASA and RCAengineers wanted to try some new ideas. TIROS I Xintroduced the so-called "wheel" configuration in 1965.Instead of mounting the cameras so that they pointeddown from the bottom of the hatbox, two were placedon the satellite rim facing outward, 180" apart. AfterTIROS IX was launched, its axis was twisted so thatas it spun it essentially rolled around the Earth,pointing one camera and then the other at the Earth.In the wheel configuration, the TIROS camerascan take more pictures of the Earth.

5 In a Sun~ synchronou s rbit, the orbital planerotates (precesses) about a degree per day to keepthe plane of the orbit (shown edgewise) pointed at theSun. With no perturbing forces, the plane of the orbitwould remain fixed in inertial space. Irregularitiesof the Earth cause the precession.

6 TIROS IX, Showing the wheel configuration. Sate!litespins like a wheel around the Earth, pointing its twocameras (180; apart) at the Earth one after the other.

TIROS X sdlved another problem: the fact that theangle with which dunlight hits the Earth below thesatellite is often poor for picture taking. We cannotcontrol the S U ~ r the orbit of the Earth, but we canplace the satellite in a Sun-synchronous orbit. Inthis kind of orbit, the satellite is injected into a near-polar orbit. The plane of the satellite orbitcontains both the karth and the Sun. If this configura-tion could be maihtained, the satellite would cross theequator at just about local noon on the sunlit sideof the Earth and local midnight on the dark side. TheEarth would rotati: under the satellite orbit, whichis fixed in space, zit the rate of 15" per hour. In thisway, the equatorial and temperate zones of the Earthcould be photographed with the Sun highin the sky all of the time. However, as theEarth rotates around the Sun, it disturbs thisideal situation. The satellite orbital plane remainsfixed in space solthat a quarter of a year later, it willbe perpendicular to the plane containing the Earth andSun. To qaintain the Sun-synchronous condition,the plane of the satellite orbit has to be rotated360/365 degrees per day. If the satellite is in just theright orbit, the karth's equatorial bulge will deflect thesatellite orbit just this amount. TIROS X demonstratedthe practicality of this type of orbit.While the TIROS program was proving the valueof the weather satellite, NASA also worked on theNimbus weather satellite program. Basically, theNimbus program is aimed at improving the instrumentsand spacecraft components used on operationalweather satellites.The Nimbus satellites are large automated spacecraft.For example, Nimbus I weighed 912 pounds, overthree times the weight of the early TIROS satellites.Nimbus is fully stabilized; that is, the satellite isoriented so that its instruments always point toward


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the Earth. The Nimbus attitude control schemeemploys three flywheels plus nozzles that squirtFreon gas into space to obtain thrust. Between theflywheels and the jets, the satellite can be kept pointedto within l o of the center of the Earth's disk.Nimbus I, launched August 28, 1964, proved thebasic spacecraft design, especially the oriented solarpanels and the attitude control system. Nimbus Icarried three important experiments as well : (1) A newhigh resolution TV cloud mapping system (theAdvanced Vidicon Camera System, or AVCS);(2) An Automatic Picture Transmission (APT)system that allowed local stations to receive weatherpictures directly; and ( 3 ) A high resolution infraredinstrument that allowed nighttime cloud mapping on aglobal scale. Nimbus 11, launched May 15, 1966,carried the same instruments as Nimbus I plus amedium resolution infrared instrument. FutureNimbus satellites will prove out a great variety ofoptical equipment as well as a nuclear power supplyfor augmenting the solar panels.The wheel configuration of TIROS IX, the Sun-synchronized orbit of TIROS X, and the Nimbuscamera technology were adopted by the U.S. WeatherBureau for its TIROS Operational Satellites (TOS) .

The Weather Bureau is now part of the EnvironmentalScience Services Administration. Its satellites arecalled ESSAs, for Environmental Survey Satellites.Three ESSAs were launched by NASA for the WeatherBureau in 1966 and two more in 1967. The successesof the TIROS and Nimbus programs can be gagedby the adoption of their technologies for operational,routine weather satellites.

Hot Spots BelowEach hurricane is created and sustainedby a colossal heat engine that we are justbeginning to understand. Somehow, energy from theSun starts these atmospheric machines turning over.The same is true for the much bigger, but less intensecyclones and anticyclones that make most of ourweather. Since weather is really atmosphericturbulence created by too much solar heat at theequator and too little at the poles, measurements ofthe Earth's heat inflow and outflow should be usefulto meteorologists. For this reason, most NASA weathersatellites have carried infrared radiometers to record thethermal radiation emitted from the cloud tops and thevisible land surface below the satellite. The thermalradiation emitted by the Earth falls mainly in the


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7 Wave clouds over the Appalachian Mountains,photographed by TIROS VII.8 The Nimbus weather satellite carries attitude co ntroldevices (gyros and gas nozzles) that keep its camerasin the base pointed at the Earth. The solar cell paddlesare driven by a motor that keeps them perp~ndicularto the Sun's rays.

range from 5 to 50 microns." These wavelengths arefar longer than the long wavelength limit of the eyeat about 0.7 micron. Unlike the vidicons which operatewith visible light, the infrared radiometers are usefulboth day and night in determining cloud cover and thespeed with which the Earth cools once the Sun has set.In infrared radiometers, lenses focus the infraredradiation on a detector made from a photoconductingmaterial, such as lead selenide, which becomes a goodelectrical conductor when illuminated by infrared light.The amount of current passed by the detector isproportional to the intensity of the infrared lightand therefore the amount of heat being radiated fromEarth to outer space.Infrared radiometers can be made sensitive to variouswavelength ranges or channels through the use offilters. For example, the high resolution infraredradiometer on Nimbus I was sensitive to only thatradiation between 3.4 and 4.2 microns. Radiation ofthis wavelength is emitted from cloud tops and gaveNimbus I a way of mapping cloud cover at night. Aninfrared channel between 6 and 7 microns helpsdetermine the amount of absorption caused by watervapor in the air. Data from such a channel aidinconstructing worldwide. humidity charts of the upperatmosphere. Analysis of the radiation emitted by the* A micron is one-millionth of a meter.


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warm Earth by radiometers, spectrometers, and otheroptical equipment can provide the following kinds ofmeteorologically useful data:Atmospheric temperature and humidity profilesVertical water vapor distributionVertical ozone distributionSurface temperatureWhen added t o photos taken in visible light byweather s atellite cameras, meteorologists see the world'sweather from a superb vantage point at wavelengthsthey have never been able to use before.

Pictures on RequestWill the Boy Scout hike be rained ou ttomorrow? D o the smudge pots have tobe lit in the oran ge groves tonight? This is the kindof weather information that most people want to know;tha t is the local forecast, the local situation. Th elocal weather forecaster would like very much to seewhat is going on in his area as he prepares hispredictions. Th e AP T (A utoma tic Picture Trans-mission) system gives him local cloud pictures witha minimum investment in equipment.

9 ESSA I took this picture of North America onFebruary 5, 1966. It shows a heavy rain producing stormalong the west coast, ice in Lake Erie, snow cover overNortheastern United States, clouds in cold air flow offthe east coast.10 The High Resolution Infrared Radiometer on NimbusI1 was able to distinguish the Gulf Stream along theU.S. east coast by virtue of its warmer temperature.

Th e basic idea is to have special cameras on th eweather satellites (TIROS,Nimbus, ESSA) thatcontinually transm it cloud cover pictures as they aretaken. Anyone on the Earth below within 15 00 milesof an AP T-equip ped satellite can receive these pictureswith modest equipment that he ca n purchase o r buildhimself. Every time an AP T satellite passes overhead,the owner of an A PT gro und station can collect up tothree o verlapping pictures of w eather systems withinabo ut 10 00 miles of his station.The APT concept is particularly helpful to foreignweather forecasters who cannot get the maps andcloud cover photos that the U.S. Weather Bureautransmits to many U.S. locations. Hundreds of APTground stations have been set up all over the world, no tonly by professional weathermen but also by radioamateurs and high school science classes.ExtraterrestrialRelaysThe A dvantages of HeightMore and more TV programs cometo us from the far corners of the worldvia satellite, as the subtitles sometimes say. W hat is no t


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so obvious is the immense commercial and militarycommunication traffic carried between continentsby satel'lite. Not only do people talk to people, butcomputers and data handling equipment talk amongthemselves in their own languages.The possibility for this global conviviality was firstdescribed back in 1945, when the Britishwriter Arthur C. Clarke, better known for hisscience fiction, published an article entitled"Extraterrestrial Relays" in the magazine WirelessWorld. Clarke pointed out that small artificial satellitesin orbit high above the Earth could relay messagesbetween continents and greatly improve long distancecommunication. As befits a writer of science fiction,Clarke was ahead of his time, but only by aboutfifteen years.Radio waves travel away from their transmittingantennas in straight lines at the speed of light. Unlesssomething changes their direction of travel, radiocommunication beyond line of sight is impossible.The Earth's ionosphere some fifty miles above the

surface reflects some radio waves back to Earth,making long distance communication possible. Butthe ionosphere is fickle, moving up and down anddisappearing when we don't want it to. Further, itreflects only those wavelengths longer than roughly100 feet.* Dependable, long range radio communica-tion requires an artificial radio wave reflector high inthe sky.Covering the whole sky with radio wave reflectors isout of the question; but, as Clarke suggested,satellites can do the job. The most primitive kind ofcommunication satellite is passive in character; thatis, it only reflects the signals hitting it, like a mirror.In contrast, active communication satellites rebroad-cast signals with greater strength. Signal ampliticationtakes electrical power, of course, but the relayedsignals are easier to detect.* Equivalently, frequencies below 10 MHz (10,000,000 Cyclesper second) are reflected.


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11 Echo I, a 100-ft diameter balloon satellite. Echo Iwas a passive communication satellite.12 NASA has experimented with passive and activecommunication satellites at various altitudes. The activesynchronous satellite was ultimately selected forcommerical use.

Height above the Earth's surface makes communicationsatellites useful. Antennas well over the horizon atground level can see a satellite in a 100-mile orbit. Acommunication satellite in a synchronous orbit 22,300miles high can be seen by nearly half the radio antennason Earth. A system of three or more evenly spacedsynchronous communication satellites can relaymessages between any two points on the inhabitedparts of the globe.Balloons in OrbitOne prominent communication satelliteis rarely mentioned: our natural satellite, theMoon. During he 1950s, the U.S. Army bouncedradio signals off the Moon in long range communicationexperiments. And while artificial satellites were gettingall the glory, the Mpon gave the Army an operatingcommunication link between Washington and Hawaii.This was the world's first operational space communi-cation system; it was called CMR, for Communicationby Moon Relay.

would be negligible compared to the 2Y2 secondsfor Moon bounces. To test the artificial moon idea,NASA launched two metalized plastic balloons thatinflated once in orbit. Echo I (100 feet in diameter)was launched in 1960; Echo I1 (135 feet in diameter)in 1964.By aiming transmitting and receiving antennas at theballoons, two-way conversations between the U.S. andEurope proved feasible. There was no limit to thenumber of users. This multiple access feature is oneof the big advantages of the passive communicationsatellite. Other pluses are their long life and highreliability. With no parts to fail or wear out, balloonsatellites should last forever. They don't; they getpunctured by meteoroids; they get wrinkled andeventually the tiny bit of atmosphere remaining willslow them down and bring them back to Earth.This is just what happened to Echo I on May 23,1968 when it reentered the atmosphere and burnedup over South America.

Would an artificial moon be any better than the natural Despite all their virtues, passive communicationone? It would certainly be much closer, reflected satellites were bypassed in favor of active repeaters.signals would be much stronger, and signal delay times One reason is that a great many balloons-perhaps

5 0 w o u l d have to be launched into low orbits to makeworldwide communication possible. Orbits had to below because signals reflected from high altitude


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ECHO II

PASSIVE ACTIVE (LOW & MED. ALTITUDES) ACTIVE (SYNCHRONOUS)Long lifeMult iple AccessLarge AntennasLarge TransmittersLow AltitudesLarge NumbersCapacity Low

Active (Low & Med. Al t i tudes)Simplest ActiveCapaci ty HighGround Station ComplexityLarge Numbers(Mult iple Launch)Traf f~cManagement Complex

Ac t i v e ( ~ ~ nc h r o r i ous )Smal l NumbersSimple Ground StationsCapacity HighTraffic Management SimplerComplex SpacecraftStation KeepingComplex Launch

satellites would be too weak. Because the balloonsatellites do not amplify the signals, ground stationshave to have big antennas and high power transmitters.Active Communication SatellitesThe U.S. Army in its perpetual searchfor more reliable and more secure long-distance communications built the first activecommunication satellite. SCORE (an acronym forSignal Communication by Orbiting Relay Equipment)was launched on December 18, 1958. SCORE relayedvoice conversations, code, and teletype directly. Thesatellite also carried a tape recorder that storedmessages and repeated them when triggered by asignal from the ground. President Eisenhower's 1958Christmas message was carried around the world bySCORE in one of its more dramatic performances.Score ceased operation after twelve days.The Army followed up its success with SCORE byorbiting Courier in 1960. Courier was a largesatellite by U.S. standards; it weighed 500 pounds.It was covered with 20,000 solar cells, and carried fourreceivers, four transmitters, and five tape recorders.During its 18 days of active life, Courier received andretransmitted 1 18 million words.Despite these successful demonstrations, the commercial

feasibility of the communication satellite wasunproven. How long could an active communicationsatellite operate? Should it be at high, low, orintermediate altitude? How many should there be?Three programs were designed to answer questions likethese: the NASA Relay and Syncom programsand the joint NASA-AT&T Telstar program.The Relays were medium altitude (about 4600 miles)active repeaters; while the Syncoms were injected intosynchronous orbits at about 22,300 miles. The twoTelstars were designed by AT&T, with NASAproviding the launching rocket and the ground trackingfacilities on a reimbursable basis. They were placedin orbits similar to those of the Relays. The Telstarsforeshadowed the fact that the success ofcommunication satellites would inevitably bring privateindustry into the picture.When an engineer tries to answer a complex questionlike: What kind of communication satellite is best?he thinks in terms of tradeoffs. He can, for example,put his satellite in a higher orbit to gain a better viewof the Earth's surface in trade for a loss in signalstrength from the more distant satellite. Or, he canadd more solar cells to the satellite to increasetransmitter power at the expense of discarding one ofthe extra transmitters he wanted to make the


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13 A commu nication satell i te ground terminal atAndover, Maine. The antenna consists of a large hornthat collects the incoming radio waves. This facilitybelongs to the Bell Laboratories.14 Artist's concept of the Syncom satellite.

satellite more reliable. What the engineer wants to dois optimize the entire communication system fromground station to satellite to ground station. Becausecommunication is a salable commodity, optimizationusually means transmitting the most informationfor the least cost in terms of satellites, ground facilities,and operations.The question of communication satellite altitudeprovoked the biggest battles among the engineers.The altitude options were three: low (100 to 500 ~miles) ; medium (2000 to 12,000 miles) ; andsynchronous (22,300 miles) . Altitude buys visibility.Also, the higher the satellite the more slowly it movesfrom horizon to horizon and the easier it is to followwith antennas. The visibility of low orbit satelliteswas so poor that fifty to one hundred would be requiredfor good worldwide coverage. Further, they wouldfly over so quickly that ground station operatorswould have to pick up a new satellite every fewminutes. For these reasons, low altitude communicationsatellites were eliminated from the competition early.

By this reasoning, the synchronous satellites shouldhave been adopted forthwith. With three synchronoussatellites spaced equally around the equator, almostevery inhabited spot on Earth would be covered.Ground station antennas could be aimed at the satelliteand locked into position because synchronous satellitesover the equator rotate at the same speed as the Earthand would appear to be fixed in space. Actually, thesynchronous solution was not obvious because highaltitude is good for some things but bad for others.To illustrate:1. The launching and positioning of a synchronoussatellite is difficult. The altitude has to be justright. To launch an equatorial satellite fromCape Kennedy, the launch rocket trajectory hasto take the satellite south and then turn("dogleg") when over the equator and injectthe satellite into orbit.2. Furthermore, once the tough synchronous

equatorial orbit has been achieved, the satellitehas to run continually just to stay in one place-


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U. S. COMMUNICATION SATELLITESLaunch Injected Orbit (miles)Sate llite Date Weight (Ibs) Apogee'IPerigee Remarks

SCORE 12-18-58 8750 914 115 First active comsat.Transmitted for 13 days.Echo I 8-12-60 166 1052 941 First passive comsat.Relayed voice and TV.Courier 1B 10- 4-60 500 767 586 Functioned 17 days. Active.Telstar 1 7-10-62 170 3053 593 Medium altitude active comsat.Relay I 12-13-62 172 4612 819 Medium altitude active comsat.Telstar 2 5- 7-63 175 6713 604 Medium altitude active comsat.Syncom I 2-14-63 86 22953 21195 In near-synchronous orbit.Communications lost at injection.Syncom I1 7-26-63 86 22750 22062 First successful synchronous comsat.Relay II 1-21-64 172 4606 1298 Medium altitude active comsat.Echo II 1-25-64 547 816 642 Passive comsat.Syncom Ill 8-19-64 86 22312 22164 First Geo-stationary cornsat.LES 1 * 2-11-65 545 393 343 Air Force all-solid-state comsat.INTELSAT I ' 4- 6-65 85 22733 21740 Owned by INTELSAT '(Early Bird ) Based on Syncom technology.LES 2 5- 6-65 82 9384 1757 Air Force comsat.LES 3 12-21-65 35 . 18000 100 Air Force comsat.LES 4 12-21-65 115 20890 124 Air Force comsat.IDCSP 1-7 6-16-66 100 al l near- Seven Air Force comsats launchedsynchronous together.INTELSAT ll F- 1 10-26-66 190 23300 2020 Not in planned synchronous orbit.INTELSAT I F-2 1-1 -67 192 22257 22254 For transpacific commercial service.(Pacific 1)IDCSP 8-15 1-18-67 100 al l near- Eight Air Force comsats launched

synchronous together.INTELSAT I F-3 3-22-67 192 22254 22246(Atlantic 2)IDCSP 16-18 7- 1-67 100 al l near- Eight Air Force comsats launchedsynchronous together.INTELSAT I F-4 9-27-67 192 22245 22220(Pacific 2)IDCSP 19-24 6-13-68 100 all near- Eight Air Force cornsats launchedsynchronous together.

1 Includes last stage of launch vehicle.LES = Lincoln Experimental Satellite, built by Lincoln Laboratory.INTELSAT = nternational Telecommunications Satellite Consortium, an organization of more than60 countries; also INTELSAT spa cecraft.4 IDCSP = nitial Defense Communication Satellite Program.

just like Alice in Wonderland. Natural forces, station keeping, and it is accomplished by smallsuch as the pressure of sunlight and the nozzles that squirt charges of cold gas whenevergravitational attraction of the Sun and the Moon, the orbit needs correcting.keep pushing the satellite from its assigned 3. Synchronous comm unication satellites are smallerposition . Keeping it in the same spot is called than medium orbit communication satellites


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because rocket payload is sacdced to reachthe higher altitude and perform the doglegmaneuver.

4. Signals relayed from 22,300 miles are sometwenty times weaker than those from 4,000miles, assuming the same transmitter powerlevels. In addition, there are significantpropagation delays resulting from the fact thatradio waves do not travel iniinitely fast.

Despite these objections, synchronous communicationsatellites finally emerged victorious. The last halfof the table on page 16 attests to the completeness ofthis victory. The NASA Syncoms paved the wayby proving that synchronous orbits could be attainedand maintained on an operational basis. Withoutthis practical proof of feasibility, the debate mightstill be going on.Once NASA proved the feasibility of the synchronouscommunication satellite, the technology was usedby a commercial enterprise called the CommunicationsSatellite Corporation-Comsat for short. Comsatwas created by an act of Congress on August 31, 1962.It was formally incorporated on February 1, 1963.The large communication companies and the publichold stock in the Corporation, which is closelyregulated by the Government.Comsat's two major tasks were to establish anoperational system of communication satellites andenlist the support of foreign governments in setting upinternational service. The latter was accomplishedthrough organization of the International Tele-communications Consortium (INTELSAT) which nowincludes more than 60 countries. The Consortiumowns the communication satellites, called INTELSATs.(See table, page 16.) Regular transoceanic servicecommenced in 1967, with synchronous communicationsatellites stationed over the Atlantic and Pacific oceans.The FigureOf theEarthSatellites have once more proved that the Earth isround, but how round is it? Answering this questionis the first task of geodesy, the science that dealswith the shape or "figure" of the Earth and thenuances of its gravitational field. The Earth's field isfar from uniform. Large sections of the Earth's crust

15 The gravitationa l pull of t he Earth's bulge deflectsa satellite slightly westward each pass.

have different densities; heavier sections locallystrengthen the Earth's field, lighter sections weaken it.Such differences in the gravitational field are termed"anomalies," and many are detectable by satellites.For decades, small-scale anomalies have guidedgeologists to local mineral deposits. Eventuallysatellites may be able to do the same. In additionto describing the shape of the Earth and the structureof its field, geodesy is concerned with accuratelylocating points on the Earth's surface with respect toone another. The precise whereabouts of pointson the Earth is basic to map makers, navigators onships and planes, and NASA itself, which must knowexactly where its spacecraft tracking stations arelocated.The orbital path of a satellite is not a perfect ellipse;The satellite weaves sideways and up and down as itplies its course around the Earth. These small orbital


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perturbations are measured only in feet, but they canbe detected by Earth-based tracking stations and madeto reveal new facts about the Earth's structure.The larger the distortion of the globe, the bigger theeffect on satellite orbits; for example, the equatorialbulge of the Earth causes the entire plane of thesatellite orbit to rotate in space.The bulge perturbation can be understood byconsidering a satellite approaching the equator fromthe northwest. The extra mass in the bulge givesit an extra southward pull. This deflects the satelliteorbit into a more southerly path. After it passes overthe equator, the bulge pulls the satellite north andstraightens the srbit out. But it is too late, the planeof the orbit has already been shifted westward. To aground observer the plane of a satellite's orbitseems to shift westward 15" each hour, as theEarth rotates under the orbit. The effect of the Earth's

bulge is added to the 15" per hour. Otherimperfections of the Earth, such as the inexplicableconcentration of continental land mass in the northernhemisphere, cause other orbital perturbations thatare superimposed upon the "normal" 15" per hourwestward drift.A geodesist needs extremely accurate ground-basedtracking equipment that tells him where a satellite isat every moment. Tracking data will showperturbations from the ideal mathematical ellipse.From the total perturbation, he must subtract thosedeviations due to the pressure of sunlight on thesatellite, the drag of the tiny amount of air remainingat satellite altitudes, and the gravitational attractionsof the Sun and Moon. The remaining perturbationsshould be due to the Earth itself.For high precision satellite tracking, NASA takes


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photographs of the satellite against the accuratelyknown background of fixed stars with large Baker-Nunn cameras, operated by the SmithsonianAstrophysical Observatory. Localized perturbations toa satellite's orbit can often be measured better withradar or other radio equipment. For example, NASAhas experimented with tracking satellites by laser.Explorers XXII, XXVII, and XXIX carried specialreflectors that mirrored laser light flashes, permittingaccurate range determinations. Eventually, lasers andhigh precision microwave equipment may be able to fixa satellite's position to within a foot or so.Precision in satellite tracking leads to precision interrestrial map making. On the North Americancontinent, surveyors have laid out a grid enabling themto locate any point with respect to another to withinabout 30 feet. There are similar grids in other well-developed countries, but they are not tied together. A

16 Map of the Earth's major gravitational anomaliesexpressed in terms of height above (+ I or depressionbelow (-1 an ideal Earfh shaped like an oblatespheroid. Contours are in meters.17 In the Secor approach to tying geodetic gridstogether, known and unknow n stations observe rangeof the same satellite simultaneously. The satellite is,in effect, a known landmark.

surveyor cannot see over the ocean with his transitto make the connections. The locations of manyislands in the Pacific were not known to better than afew miles before satellites were developed. Again,it is satellite height that makes it a valuable tool.Observers several thousands miles apart can see ahigh satellite simultaneously. By making simultaneousmeasurements with optical and radio trackinginstruments, they can determine just how far apart theyreally are. The current goal of satellite geodesy is to tieall geodetic grids together with an accuracy of 30 feet.The U.S. Army is expected to make important progressalong this line with its Secor (Sequential Collation ofRange) satellites. By island hopping across the oceans,using high satellites as geodetic markers, the world'scontinents will eventually be tied together to onecommon reference system.Almost all satellites are valuable to geodesy. The


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BetterBrickMoons

most useful ones are those that are easy to track.The Echo communication satellites, for example,were very useful because they were so easy to seewith optical instruments. Pageos is another balloonsatellite orbited by NASA in 1966 specifically to helpthe geodesists. To help make truly simultaneousobservations, flashing lights were installed on several"active" geodetic satellites. The lights flash in codedsequences so .that widely separated ground stations cancompare time exposure photographs taken againstthe background of the fixed stars. The Department ofDefense satellite Anna lB* carried the first opticalbeacon into orbit in 1962. Geos I (Explorer XXIX),launched November 6, 1965 by NASA also included aflashing light in its payload. Radio beacons of varioustypes are placed on many satellites to aid tracking.NASA's two Beacon Explorers (Explorers XXIIand XXVII) carried both radio beacons and laserreflectors. GEOS I1 (Explorer XXXVI), launched onJanuary 11, 1968, is now fully operational. Inaddition to a full complement of geodeticinstrumentation, it has C-band radar transpondersto determine if the approximately 65 C-band radartracking stations can track with geodetic accuracy.The preliminary results are far better than wereexpected.The United States has centralized its satellitegeodetic activities in the National Geodetic SatelliteProgram. NASA has overall responsibility, and theDepartments of Defense and Commerce participate.

About a century ago, Edward Everett Hale wrotea short story entitled "The Brick Moon." In it, thehero proposed launching four satellites, 200 feet indiameter, into polar orbits passing over Greenwich,England, and New Orleans. The rockets of the 1870scould scarcely lift Hale's brick moons, so he hit uponthe idea of flinging them into orbit with huge flywheels.Once the artificial moons were in orbit, navigators at seacould determine their longitudes by measuring theelevation of the moons above the horizon. Itwas a precocious plan, although Hale overlooked thefact that satellites cannot remain in orbit over agiven meridian because of the Earth's rotation.Today we need better brick moons. to help aircraftand ships fix their positions to within a mile or two.Aircraft traffic is congested and safe control of this

18 Navigators on the Earth's surface can locatethemselves by measuring the ranges of two satellites insynchronous equatorial orbits. The navigator must beat one of the two intersections of the circles.

* Anna = Army, Navy, NASA, Air Force; th e cooperating Agencies.


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traffic depends upon pilots and traffic controllersknowing precisely where aircraft are located.The U.S. Navy has already launched its system ofnavigation satellites: the Transit satellites. TheTransit system is complex and expensive; too muchso for commercial aircraft and ships. Nevertheless,its features are interesting because they offer ideas foreconomically practical systems.A ground rule that makes the Transit systein expensiveis that a ship trying to fix its position shouldnot transmit signals that would reveal its location toothers. With this in mind, the satellite does two thingsfor the ship navigator below:( 1) It transmits a radio signal at constant frequencyso that the navigator can obtain a Doppler curve* and(2 ) It transmits in code the latest informationabout its own orbit. This information was insertedin the satellite's memory when it last passed overa Transit ground station. From ( 1 , a shipboardcomputer calculates the satellite's distance of closestapproach. Knowing this distance, the orbital datarelayed by the satellite, the time, and the ship'sspeed, the computer gives the navigator his positionto within one mile.A less expensive plan of possible commercial interestwould place satellites in stationary orbits above the

* A Doppler curve is a graph of the apparent frequency of thesatellite transmitter. The frequency is higher when thesatellite is approaching, lower when receding (like a trainwhistle).

equator where their positions would be relatively fixedand where they could be seen by most potentialusers. A navigator wishing to fix his position addressessatellite #1 by radio in a known code. Satellite#1 responds. By timing the transit of signals, therange of satellite #1 can be determined. Next,satellite #2 is addressed, and its range is found in thesame way. If the navigator is on a surface vessel, hecan now draw two circles on his map. Each circleis the locus of points at the just-measured rangefrom the satellite. The ship is at one or the other ofthe two points where the circles intersect. Normally,the navigator can resolve this ambiguity from theknowledge that he is at sea rather than in the corn-fields of Iowa. An airplane navigator, of course, needshis altimeter reading before he can draw his circles.Another navigation scheme under study by NASAinvolves more difficult geometry. Imagine threesatellites in stationary orbits over the equator.Satellite #1 (the master satellite) emits a radio signal.At two different, precisely known intervals afterwards,satellites #2 and #3 (the slave satellites) emit theirsignals. The signal cycle repeats, and the navigatorreceives sequences of three signals. Each signal containsinformation identifying the satellite that originatedit. The true time intervals between signals fromsatellites #1 and #2, # 1 and #3 , and #2 and #3 areknown, but the navigator will receive them at slightly


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19 ATS I carried a wide variety of weather, communication,and attitude control experiments.20 THE ATS-I view of the Pacific basin fromsynchronous orbit (about 22,000 miles altitude.)

different intervals because of the different timesrequired for the radio waves to flash from thesatellites to the ship. Knowing these time discrepancies,he can calculate the difterences between the distancesof the three possible pairs of satellites from his ship.The locus of points which have a constant differencein distance from a pair of satellites is a hyperboloid.There is a hyperboloid for each of the three satellite-pair combinations. The navigator finds hisposition from the common intersection of all threehyperboloids. Though this approach sounds complex,it is really only an extension of a two-dimensionalsystem used for ship navigation since World War 11.This system is caiied loran (Long Range Aid toNavigation). There are many land-based loranmaster-slave transmitters in the better developed

parts of the world. The proposed satellite navigationsystem extends the loran concept to three dimensionsand, in the process, the rest of the world.TestingLaboratoryIn SpaceOnce NASA has developed weather and communicationsatellites, the responsibility for operating them on aregular basis falls to other agencies of the Governmentor to private industry. However, NASA retains thejob of finding better ways to do these tasks. Bettercameras, better means for maintaining stationaryorbits, better navigation transponders; all are typicalof the new technology NASA is developing.---It often happens that the best way to test a new


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camera or any other piece of space hardware is to putit on a satellite and try it out. NASA has built aseries of Applications Technology Satellites (ATS) forthis purpose. The ATS satellites are multipurposetestbeds.ATS I, the first in the series, looks like a largeversion of the Syncom communication satellite. Itwas launched on December 6, 1966 and successfullymaneuvered into a synchronous equatorial (stationary)orbit over Christmas Island in the Pacific. The mostdramatic results have been the remarkably sharppictures of the entire Earth . From 22,300 miles up,the ATS special ~$11-scan camera has been able totake thousands of pictures of planet-wide circulationpatterns in the atmosphere. Because the ATS I is fixedin one spot over the Earth, it has been able toprovide long sequences of snapshots of weather showingthe time development of convection cells, typhoons,

and jet streams. In effect, ATS takes a movie of theweather over nearly 40% of the Earth, a perspectivethe lower, fast-moving TIROS satellites cannot see.ATS I is also involved with the Weather DataRelay Experiment, more commonly known asWEFAX. The WEFAX experiment has demonstratedthe feasibility of disseminating weather data from acentral facility to widely scattered weather stationsvia a synchronous satellite relay. In other words, ATSI acts like a Syncom to relay weather satellite data.Many communication experiments have also beenconducted using the special radio equipmentinstalled on ATS I. ATS I was the first satellitepermitting two-way very high frequency (VHF)communication between aircraft and the ground-animportant accomplishment for traffic control over theoceans and sparsely populated areas.


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The Earth:The BigPicture

21 The U.S. Geological Survey has used Nimbuspictures to update and correct its maps ofAntarctica. More advanced survey satellites will becapable of muc h greater resolution.22 Schematic showing how satellites can be used torelay data taken by unmanned instrument platforms tocentralized data collection points.

The payload of ATS 11, launched April 5, 1967, alsocontained a wide variety of experiments and equipment.The primary objective was to test a gravity-gradientattitude control device. Also included were twoweather cameras,, a microwave communicationexperiment, and eight scientific experiments.Unfortunately, ATS I1 did not attain the desiredcircular orbit because the second stage enginefailed to restart. Nevertheless, data were obtained frommany of the experiments.ATS I11 was launched November 5, 1967. Anotherspin-scan camera, able to take full color pictures,was carried along this time, plus an ImageDissector Camera System. The ATS role is to testsuch equipment for possible use on operational weathersatellites. A third experiment concerns satellitenavigation systems-more specifically the OmegaPosition Location Experiment (OPLE) . The OPLEnavigation concept is not directed toward aidingnavigators aboard aircraft and ships but rather in thedirection of locating unmanned instrument platformssent out for the purpose of gathering meteorologicaland oceanographic data. OPLE can interrogate suchinstrument platforms for data and relay the data to acentral facility. Eventually, however, OPLE could alsobe applied to air traffic control, since an airplanecan also be thought of as a moving instrumentplatform.

From their vantage points 100 miles and more up,satellites can see large-scale patterns and phenomenathat Earthbound observers cannot-either becausethey are too close to them or are actually immersedin the phenomena. Cloud patterns, already mentioned,are obvious examples, but we can add agriculturalpatterns, ocean currents, geological formulations,buried archeological sites, and many more.One of our problems in studying the Earth is thatwe are too close to it; this is certainly the case inmapping large weather systems where high altitudemeteorological satellites are unbeatable. Aircrafthelp some in seeing the Earth in the large, butsatellites fly considerably higher and see the big picturemuch better. The questions are: What cansatellites see of importance from so high up, and howcan this information be put to practical use?Besides scrutinizing the Earth below in visible light,satellite infrared and microwave sensors can scaniheoceans and continents at longer wavelengths. Theinfrared and microwave radiation emitted by the Earthdepends upon surface temperatures and the composi-tion of surface materials. The use of infrared sensorson weather satellites to monitor atmospheric processes,such as the Earth's heat budget, has already beendescribed, but the same sensors can also be used tostudy the Gulf Stream, detect forest fires, and locatesubterranean heat sources.


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The key elements in seeing more than weather fromsatellites are: (1) Look harder; that is, use moremagnification; ( 2 ) Study the infrared and microwaveemissions from the Earth; and ( 3 ) Subtract out theeffects of the Earth's atmosphere. Once these things aredone, some surprising possibilities emerge, although itshould be stressed that considerable research anddevelopment work lies ahead before these applicationscan be realized.Ma p preparation. Transportation networks and urban/rural settlement patterns, at some future time, may bequickly and economically mapped to facilitate highwayand pipeline routing.Agricultural census and crop prediction. Differentcrops may be identified by satellite sensors, leading tofrequent and economical forecasts of the world's foodsupply. Furthermore, invasions of insects and diseasemay be spotted early and countermeasures organizedon the basis of satellite data.Detection o f forest fires. Satellite infrared sensors mayscan the forests in the U.S.Wate r resource surveys. Satellites may help keepaccurate inventories of fresh water supplies for the entirecountry. Droughts may be predicted and, eventually,water flow may be controlled to offset droughts.

Satellite surveys may also help plan new dams andartificial watercourses.Mineral resource surveys. Satellites may survey quicklyand cheaply the huge areas of the world that areinadequately explored geologically. Satellites mayidentify particularly promising areas that it might takeground survey crews many years to find.

Disco very o f archeological sites. For the same reason,buried cities, ancient roads, mound sites, and otherpartially obscured relics of past civilizations may bediscovered.Air pollution surveys. Like forest fires, sources of airpollution may be spotted from a satellite, mainly throughthe way they absorb light. The spread of pollutantsunder various wind and terrain conditions could also bestudied with an eye to better control.Location o f hydrothermal energy sources. Infraredsensors may spot geothermal anomalies on the Earth,such as those at Yellowstone, where natural heat may beconverted into electrical power.Oceanography, Despite the mare than 100 milesbetween an earth satellite and the sea, the satellite mayturn out to be one of the most powerful oceanographictools devised, Satellite cameras can monitor such

factors as pollution patterns, beach erosion, river run off,


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and sedimentation patterns. Temperature patterns inthe sea have already been distinguished by weathersatellites, and knowledge of temperatures can improvethe fish harvest. Wave heights can be mapped overlarge areas of the ocean by measuring the "Sun glint" offthe sea surface. In a different vein, satellites can receiveradio transmissions from large numbers of unmannedoceanographic buoys and relay their data to centralland facilities.The practical value of the satellite as a monitor andexplorer of the Earth's surface is just beginning to beappreciated.On the basis of preliminary conclusions drawn fromstudies and the results obtained from remote sensorexperiments on aircraft, an Earth Resources TechnologySatellite (ERTS) Program plan has been drafted byNASA in cooperation with several other interesteddepartments and agencies of the Government. Testingof an early version of such an experimental satelliteshould be possible in the 1970s.

AdditionalReadingFor titles of books and teaching aids related to thesubjects discussed in this booklet, see NASA'seducational publication EP-48, Aerospace Bibliography,Fourth Edition.Information concerning other educational publicationsof the National Aeronautics and Space Administrationmay be obtained from the Educational ProgramsDivision, Code FE, Office of Public Affairs, NASA,Washington, D. C. ,20546.


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T L 7 9 6 . C6 1 9 6 9Corliss, William R .

Putting satellites to work

For sale by the Superintendent of Documents, U.S. GovernmentPrinting Office, Washington, D.C. 20402 - Price 5 0 cents*US. OOYERNMENTPRINTING OFFICE: leMtoszk429


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