SDS-1 Satellite Communications System

The Satellite Data System (SDS-1) was composed of two operational satellites that had supported near-real time communications between low altitude photographic intelligence satellites and ground control stations, making it a critical part of the US Air Force Satellite Communications (AFSATCOM) System.

In particular, SDS-1 served as a communications link between the Air Force Satellite Control Facility at Sunnyvale, California, and seven remote tracking stations located at Vandenberg AFB, California; Hawaii; Guam; Nahe Island; Thule, Greenland; Oakhanger, England; and Boston, Massachusetts. (1)

Furthermore, the SDS-1 enabled almost two-way real-time command, control, and Ultra-High Frequency (UHF) communications (C³) in the polar region for Strategic Air Command (SAC) forces. The North polar region was not covered by communications satellites in geostationary orbit, and therefore the SDS-1 filled this gap.

A third and unacknowledged function of the SDS-1 was to provide a relay for information from the Keyhole-11 photoreconnaisance satellite to a ground station, which was probably the Defense Special Missile and Astronautics Center at Fort Meade in Maryland. (2)

The SDS-1 had two additional possible functions. Because the SDS-1 orbit, which was highly elliptical and semi-synchronous, was similar to that of the Soviet Molniya communications satellite, the SDS-1 might have been used to intercept signals from the Soviet satellite. (3) Another possibility was the use of the SDS-1 for early warning of ballistic missile launches from the Northern Soviet Union. (4) The SDS-1, however, did not have a signals intelligence (SIGINT) class receiving dish, nor did it have have an infrared package (deduced from the fact that it was too small). Therefore, these two speculative ideas are still fairly weak.

The SDS-1 spacecrafts flew in 63 degree orbits, having high apogees over the North Pole and low perigees. This gave them a long stay time over the polar region, enabling them to relay communications to aircraft in those areas. (5) Two of the SDS-1 satellites had to be in orbit for the communications to be constant. Each SDS-1 spacecraft had 12 communications channels in the ultra-high frequency bands.

The main transmitting antenna of the SDS-1 was over 10 feet in diameter. The body itself, based on the design of the US military's TACSAT, was cylindrical and about 13 feet long and 10 feet in diameter. The outside cylinder, called the solar array, was made of a silicon skin and spun for thermal distribution of the heat load. The SDS-1 had a mass of 1,385 pounds and was a total of 25 feet long. The primary electrical power for the SDS-1 spacecraft was provided by solar charged batteries with a total of 980 watts of power.

The development of SDS-1 began in 1973 by the Hughes Aircraft Corporation, with the first launch occuring on the 2nd of June 1976. Each satellite was worth about $100,000,000. Seven satellites were launched on Martin Marietta/Lockheed Titan 3B Ascent Aegena-D vehicles from the Western Test Range at Vandenberg Air Force Base in California. A Titan 3B launch is shown here on the left, with a payload that was most likely the SDS-1. SDS F-5 and F-5A, launched in 1983 and 1984 respectively, probably remained in service through the end of the decade. The last of the series was launched in February 1987 and survived until November 1995.