Imaging and Signals Intelligence Space Systems

The major effort within the US satellite reconnaissance program in the 1960s and 1970s featured overhead visual imaging systems, which produced information not obtainable any other way. (CORONA, GAMBIT, and HEXAGON, the early filmbased satellite systems, have already been well documented.) But there were important intelligence questions that could not be answered with pictures alone. The first question involved determining the location and characteristics of Soviet radars that could detect American strategic bombers. The second involved the performance capabilities of Soviet missiles—ICBMs and ABM systems. These two problems led the list of reasons favoring SIGINT satellites that could listen to and record the signals of Soviet radars, radio communications, and telemetry systems.

A SIGINT satellite system had many of the same elements as an imaging satellite system, but with important differences. Instead of a camera and film, a SIGINT satellite mounted antennas, receivers, and, sometimes, tape recorders. Instead of sending its information down on film in a reentry vehicle, a SIGINT satellite transmitted its findings by radio link in real-time or shortly after passing over the target area. On the ground, instead of a photo-processing laboratory, technicians used a SIGINT processing system, usually computerized and immensely complicated, to translate the raw electronic signals into intelligence listings and reports for release to analysts. The targets of the SIGINT systems were the actual radio signals radiated by Soviet transmitter equipment, which meant that the satellites had to be in the right place, looking in the right direction, tuned to the right frequency, at the very time the Soviet transmitters were on the air. This was an entirely different game from the photo-collection business, but one with the potential to get different and extremely important information. A number of different types of SIGINT satellites were employed to gather this vital information.

First launched on 22 June 1960 in a 70-degree-inclined, circular orbit about 500 miles above the Earth, the Navy’s POPPY satellites searched for the main beams of Soviet scanning radars and provided wide-area coverage of and locations for radars on the surface of the Earth. POPPY satellites acted as “repeaters,” encoding each radar pulse as it was received and then retransmitting the pulse stream in realtime to US-manned ground STATION LOCATED AROUND THE PERIPHERY OF THE Soviet Union [REDACTED EO 13525]

The SIGINT satellites most nearly like the photo satellites in their appearance and orbits were the WS-117L family, the Lockheed Agena-based low-orbiters called SAMOS F-1, 698BK, MULTIGROUP, and STRAWMAN. Starting with SAMOS F-1 on 31 January 1961, these satellites orbited at about 275 miles in 67-degree inclined, circular orbits and searched for Soviet radars of all types, attempting to intercept the Soviet radars from high overhead and from a direction the Soviet radars were not “looking” (i.e., the “sidelobes” of the enemy radar antenna patterns). They operated by reading in and recording the radar information while over the Soviet Union and reading out that data, by playback of onboard tape recorders, when they passed over the ground tracking stations of the US Air Force Satellite Control Facility (AFSCF) stations in California, New Hampshire, and Hawaii. These satellites, developed by the Air Force, were the first successful orbital collectors of the EOB for SAC. They provided ELINT technical performance details and locations of radars that could threaten our strategic bomber forces. Phased out in 1972, these low-altitude satellites were conceptual pioneers succeeded by more powerful vehicles in different Earth orbits.

A variety of small electronic boxes were attached to the Agena SIGINT and Photographic satellites. These boxes, sometimes with antennas of various sizes—special kinds of SIGINT collectors and experiments—were called AFTRACK payloads, due to their positioning on the aft rack of the Agena launch vehicle. The first, named SOCTOP, designed to detect tracking of the host vehicle by Soviet radars, was launched on 10 August 1960 on DISCOVERER 13, a CORONA photo mission that had a one-day mission life and was the first to achieve successful reentry of a photo payload from orbit. From this beginning, a single day of operation, came a succession of these small SIGINT payloads, for many different purposes, usually designed and flown on short notice for little cost each remaining attached to the host satellite and usually operating for the life of the host satellite. [REDACTED EO 13525]

The launch rate of AFTRACK payloads peaked in the 1960s. By the 1970s. all the AFTRACK payloads were “vulnerability”-type payloads, used for detection of hostile radar activity [REDACTED EO 13525]

In 1945 Arthur C. Clarke described two key types of Earth satellite orbits: the high, geosynchronous orbits suitable for communications, where the satellite’s orbital motion coincided with the Earth’s rotation and would enable the satellite to remain motionless over one point above the Earth’s equator; and near-polar orbits, which would allow reconnaissance satellites to cover the whole Earth in successive passes as the Earth rotated beneath it each pass occurring at the same local time of day on the ground. 11 These near-polar, sun-synchronous orbits were chosen for the photo satellites so that the target areas could be viewed in sunlight. Low orbiting SIGINT satellites, which did not need to have their targets in sunlight used lower inclination (about 67 degrees}, non-repeating Earth orbits to get the best coverage of the target areas over a period of days or weeks. At the geosynchronous equatorial orbit (22,000 miles high), perceived by Clarke as the orbit most suitable for relaying of communications from one point on the Earth to another, SIGINT satellites became signal interceptors. [REDACTED EO 13525]

By 1975, the US employed [REDACTED EO 13525] and [REDACTED EO 13525] high orbiters [REDACTED EO 13525]. Collectively, they represented an extraordinary, complementary set of reconnaissance satellites.

11. Davies and Harris, pp. 15-17. See also Arthur C. Clarke, “A Short Pre-History of Comsats, Or How I Lost a Billion Dollars in My Spare Time,” in Voices from the Sky: Previews of the Coming Space Age, New York: Harper & Row, 1965, pp. 119-128.

NRO APPROVED FOR RELEASE 10 FEBRUARY 2016

Source: National Reconnaissance Office, The SIGINT Satellite Story