Spacechronicle: JBIS. Vol.59. Suppl.1. pp.56-62, 2006 RELAY IN THE SKY: THE SATELLITE DATA SYSTEM DWAYNEA. DAY SpaceSlLidies Boatd,500 Fifth Street, NW, Washington,DC 20001,USA. Era : [email protected] One of the key developments in real-time satellite reconnaissance was the Satellite Data System, or SDS.The SDSis a constellationof communicationssatellites placed in highly-elliptical,highly-inclined orbits that relay imagery from low-altitude reconnaissance satellites back to the United States. The original concept for satellitedata relay datesto the late 1950s,but the modernconcept of the systemwas conceivedin the mid- '1960sand the satellitesentered development in the early 1970s.The satelliteswere built by Hughesand at leasta dozenof them were builtand launchedthrough the mid-1980s.They have apparently been replaced by newerand largersatellites starting in the early 1990s. Ket/words! Data-relay,reconnaissance, National Reconnaissance Office, communications 1. INTRODUCTION In fall '1963Albert "Bud" Wheelon was sitting in his overcome: getting the data back to a ground station. living room in a suburb of Washington, DC,watching "This was a major issue because the opticaldata were a football game being played in San Francisco.At the so vast in each frame thatwe could not afford to store it time Wheelon was the Deputy Director for Science on board the satellite", Wheelon explained. "The data and Technology at the Central Intelligence Agency. simplywould pile up and overflowthe limitedavailable He was building his directorate into a powerful re- storage devices". In fact, an early attemptatdoing this search and development organization that initiated with the Samos E-1 and E-2 satellites was limited to the and funded aerial and space reconnaissance sys- transmission of only a few photographs during each tems for the United States [1]. pass over a ground station, "We could notdownlink the data as it was being collected, because it would have While watching the football game Wheelon had an to be received in denied territory". But Wheelon had a epiphany. "lt suddenly struck me that if I could do that, solution for that as well. "The rightway to do this was to the technology was available to view the Earth's sur- uplink the data to a relay satellite in much higher orbit, face from orbit and to observe that scene as it was which could then pass it on to the ground station" [2]. being received in the spacecraff', Wheelon said. "ln other words, to develop a near-real-time imagery sys- 2. THE BEGINNINGSOF tem". The nextday Wheeloncalled one of his deputies DATA RELAY and assignedhim to work fulltime on the development of such a system "This turned outto be one of the best Wheelonwas not the first person to develop the idea decisionsof my career", Wheelonsaid. "We ended up of a data relay satellite. In August 1958 Lockheed creating a new class of satellite imaging systems that Missile Systems Division prepared a slide depicting revolutjonized intelligence collection". a "Sentry Data Relay Concept". The slide depicted data and commands being relayed between three It took a decade to develop the technology for satellites in low Earth orbit and a ground station. such a near-real-timesystem, known as the KH-11 This was a means of connecting a satellite with a KENNAN and first launched in late 1976. The clA ground station that was far below the horizon. But helped fund work at Bell Telephone Laboratories on this method of relaying data between satellites in what eventually became charge-couple devices, or low Earth orbit was not practical in 1958 with satel- CCDS,which today are readily available in commer- lite communications and electronics in their infancy. '1990s cial digital cameras. In fact, it was not until the late that the tech- nology was perfected and utilizedfor the lridium low But there was anolher hurdle that thev needed to Earth orbit communicationssystem [3]. 56 Belay in the Sky: The Sate ite Data System The biggest problem was that of locating another in a 12-hour Molniya orbit would spend eight hours satellite to relay data through. Because all of the satel- flying above the northern hemisphere, but only four lites were in low orbitand moving relativelyfastto each hours flying over the southern hemisphere.From the other, no satellite would be visible to another for very satellite's perch approaching apogee, it could see long before moving below the horizon. Before the days much of the northern hemisphere.A satellite in such of sophisticated computers it was impossible to com- an orbit could maintain a line of sight with both a pute what satellites would be in range, where they reconnaissance satellite low over the Soviet Union would be located, and then point an antenna at them, and a ground station in the United States. The Soviet switching to the next available satellite when needed. communacations system required three satellites for a full day of coverage, but because the American Another problem at the time was bandwidth, or reconnaissance system only operated during the day, the amount of information that could be sent over a only two SDS satellites would be necessary to sup- data link, somethingthat every user of the Internet is port a reconnaissancesatellite in low Earth orbit. familiarwith. The communicationssystems used with the early Samos satellites were severely limited in 3. THE KH-II REGONNAISSANGE how much information they could transmit. lt took SATELLITE many minutes to transmit a single photograph. For the remainder of the '1960sthe CIA sponsored Intelligence officials desperately wanted a,,near the developmentof new technologies to make "near real-time" reconnaissance satellite that could take real-time" reconnaissance possible. Until the basic pictures and relay them to the ground within a few image creation and processing technologies were minutes because itcould be used to warn of immedi- developed,there was not really a need for communi- ate events, such as Soviet tanks about to roll into cations systems to relay the imagery to a distant Czechoslovakia.But without a relay system, a satel- ground station. lite over the Soviet Union would have to travel a quarter of the way around the world or more before ln June 1971 President Richard Nixon approved the coming within range of an American ground station. development of what was soon designated the KH-11 KENNAN reconnaissance satellite. The KH-11 was a Wheelon's original idea was to place the relay big telescope with an image-forming electronic device satellite in geosynchronous orbit above the equator. at its focal point. The tirst KH- l1s launched used light But the problem with this solution was that a single sensitive diodes, but the later ones used a linear CCD relay satellite would be insufficient to beam the data array [6]. Once the KH-11 was appmved, a data relay back to the United States and it would have to trans- system was also necessary mit it through another satellite or a ground station. The key was finding a way of sending the information The exact origins of the relay satellite are not through only a single relay satellite before sending it currently known due to continued classification.Ap- back to the ground. parently in the early 1970s the U.S.Air Force devel- oped a requirementfor providing command and con- According to Wheelon,Alexander Flax,the Direc- trol informationto nuclearforces in the Arctic where tor of the NationalReconnaissance Office from 1965 the propagation of radio waves is poor. The Air Force until 1969- after Wheelon had left the government - proposed a Data Relay Satellite System, or DRSS, came up with a superior solution. Although it is not which would transmit data to nuclear forces ooerat- clear how Flax got the idea, it is possible that he had ing in the Arctic, such as Strategic Air Command B- help from the Soviet Union [4]. 52s attacking the Soviet Union during a nuclear war, and also relay data from satellites over the horizon In April 1965 the Soviet Union taunched its first to a ground station. The early Defense Support Pro- successful Molniyacommunications satellite [5]. The gram missile warning satellites were proposed as satellite was placed in a new orbit, which was soon candidates for this data relay mission [7]. named the Molniyaorbit. lt was highly inclined to the equator, but also highly eccentric, meaning that it At the same time, the CIA component of the Na- swung low over the Earth in the southern hemisphere tional ReconnaissanceOffice required a relay sateF before heading toward a distant apogee high over lite for its KH-11 data. This satellite, known as the the northern hemisphere - like tossing a tennis ball Satellite Data System, or SDS,was undoubtedly ap- high into the air, it would slow down on its way up and proved sometime in late 1971 and it is possible that speed up on its way down, but appear to spend most for a short period of time the American military was of its time in a small area above one sDot.A satellite studying both DRSSand SDS simultaneously[8]. Dwayne D. DaY satellitesand was DRSSwas formally cancelled in May 1973, but it sible for developmentof Air Force from Program A. lf the seems likely that its missions were transferred over located across the street SAMso, it would provide to the SDS by early 1972. DRSS had formally been satellite was developed by mission. An early plan designated "Program 313" and this designation was useful cover for the classified was that it could be apparently then given to the SDS [9]. One mission for the near-real-time system previous satellites, that was not given to SDS was that of relaying DSP covert.