SDS

In the late 1960s, the CIA office within the NRO was running a program named ZAMAN, which studied the options to develop a real-time photo-reconnaissance satellite system. ZAMAN would eventually lead directly to KENNEN, the first electro-optical imaging satellite. However, there was the unsolved problem of how to get the image data back to earth. Most of the time, the low-flying reconnaissance satellite would not be in line of sight to a ground station on friendly territory. And storing the data on board was not a good option either, because the amount of data was beyond the capacity of then state-of-the-art digital storage devices. The envisioned solution was to develop a specialized communincation satellite, which would serve as a relay between the imaging satellite and the ground station(s).

In 1969, the NRO decided to give the development of the communications relay satellite system, openly named SDS (Satellite Data System), to the USAF's Space and Missile Systems Office (SAMSO). SAMSO was not part of the NRO, and until that time not involved with secret reconnaissance satellites. The move had several reasons, the most important one being fiscal-political: The NRO wanted to keep a relatively low profile budget-wise, and therefore it was desirable to shift funding of the expensive SDS to an open Air Force program. Also, as a USAF program, SDS was less likely to be connected by the Soviets to covert satellite reconnaissance. However, the fact that a non-covert Air Force office was now managing the program led to some CIA/USAF inter-service rivalries. There were people in the USAF, who wanted to fly at least one, and possibly more, secondary payloads on SDS, in addition to the primary mission of reconnaissance satellite relay. The CIA, on the other hand, feared that the mass and power requirements of any secondary payload could impede the overall performance of the primary mission. In the end, a compromise was agreed upon: SDS would also incorporate secondary missions, but the USAF guaranteed that the relay mission would always have top priority.

Even though SDS was in principle a "white" program (i.e., its mere existence was not a secret), the security and classification procedures were effectively defined by the NRO. Information about the relay mission were compartmentalized by NRO's BYEMAN framework, and all standard SECRET classified documents would only mention the secondary USAF missions.

In 1972, the prime contract to build the SDS satellites was awarded to Hughes. The design was reportedly based on Hughes' successful Intelsat IV commercial communications satellite. By that time, the KENNEN program had been started, and was expected to launch its first satellite in 1976. This put a firm deadline on SDS, because without the relay, KENNEN wouldn't be able to transmit its data to the receiver stations. In the end, the Air Force managed to launch the first two SDS satellites in June and August 1976, just in time for KENNEN's first launch in December that year.

Until 1987, a total of seven SDS satellites of the first generation were launched on Titan 3(34)B boosters. All were placed into highly elliptical Molniya orbits with an inclination of 57°. This meant that for most of the time, the satellites were high above the northern hemisphere, in line of sight to the KENNEN satellites as well as a US-based ground station. Geosynchronous orbits had been preferred during early stages of the program, but had been ruled out at that time as too expensive and risky. The secondary payloads were an S-band datalink for communication with a remote tracking station in Greenland, SIOP (Single Integrated Operational Plan) communications with strategic bomber (B-52) and tanker (KC-135) aircraft, and a NUDET (Nuclear Detection) payload to register above-ground nuclear test detonations.

Between 1989 and 1996, four second-generation SDS satellites were launched. They are referred to as SDS Block 2, SDS-2 or SDS-B. Also built by Hughes, they were derived from the Leasat (a.k.a. Syncom 4) communication satellite. The first three SDS-2 satellites were launched with the Space Shuttle, and the last one on a Titan 405A. The NRO used the name QUASAR for SDS-2 (and possibly also for the earlier Block 1 and later Block 3 satellites). The second Shuttle-launched SDS-2, USA 67, was the first one to be put into a geostationary orbit. In public announcements of the launch, it was designated as Air Force Program (AFP-) 658. It is unclear, if this numeric designator applied to all SDS-2 satellites, or only to USA 67. Beginning in the late 1980s, SDS also served as a relay for the ONYX and TOPAZ radar imaging satellites.

Image: NRO

SDS-3 (probably)

In 1998, the NRO published an image of a satellite, which was apparently a communications satellite based on Hughes' Intelsat VI. It is generally assumed to show a 3rd-generation SDS satellite, the first of which was most likely launched as USA 137 in January 1998. Most SDS-3 missions were launched by Atlas boosters, initially Atlas IIA/Atlas IIAS and later Atlas V. Only one mission used a Delta 4 booster. USA 137, USA 179 and USA 198 were placed in Molniya orbits, while all the other SDS-3 satellites are geostationary.

SDS Launch List

• No.: Sequential flight number for the SDS program
• Name: Unclassified name of the satellite. The OPS number ("OPS" standing for "Operations") is a random number given to all military-related satellite launches between 1963 and May 1984. Since June 1984, U.S. military satellites get a sequential "USA" number. "NROL" stands for National Reconnaissance Office Launch, a number assigned by the NRO to its satellite launches since 1996.
• Block: The SDS "Block" or "Generation" of the satellite
• COSPAR ID: International designation of the satellite

SDS launches

^* Part of the first block of SDS was contemporary with the JUMPSEAT series of SIGINT satellites. JUMPSEAT satellites used Titan 34B launch vehicles and were placed in Molniya orbits, just like SDS-1. Therefore it was hard for observers to tell the two types apart. The first two SDS-1 launches are effectively confirmed, but some of the later ones could have been confused with JUMPSEAT launches in the same general timeframe. The list of seven SDS-1 missions presented here is the current consensus among reputed sources.
^† Some sources attribute USA 137 to the SDS-2 series, which can not be completely ruled out.
^‡ The final two launches, USA 269 and 279, might belong to a 4th generation of SDS satellites, but this is unconfirmed.

Main Sources

[1] Dwayne A. Day: Relay in the sky: The Satellite Data System, JBIS, 2006
[2] Dwayne A. Day: Spinning out of the shadows, The Space Review, March 2017
[3] Dwayne A. Day: Spinning in the black: The Satellite Data System and real-time reconnaissance, The Space Review, May 2025
[4] Gunter Krebs: Gunter's Space Page (for launch lists)

Back to Directory of U.S. Military Rockets and Missiles, Appendix 3

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Last Updated: 21 July 2025