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SIGINT Overview

Signals intelligence satellites are designed to detect transmissions from broadcast communications systems such as radios, as well as radars and other electronic systems. The interception of such transmissions can provide information on the type and location of even low power transmitters, such as hand-held radios. However, these satellites are not capable of intercepting communications carried over land lines, such as under-sea fiber optic cables (nor can they detect non-electronic communications, such as the spoken word).

Signals intelligence (SIGINT) consists of several categories. Communications intelligence (COMINT) is directed at the analysis of the source and content of message traffic. While most military communications are protected by encryption techniques, computer processing can be used to decrypt some traffic, and additional intelligence can be derived from analysis of patterns of transmissions over time. Electronic intelligence (ELINT) is devoted analysis of non-communications electronic transmissions. This would include telemetry from missile tests (TELINT), or radar transmitters (RADINT).

The United States operates four constellations of signals intelligence satellites in geostationary, elliptical and low Earth orbits.

The geostationary SIGINT constellation consists of three or four satellites. The first generation of these satellites, known as Rhyolite, were launched in the early 1970s, and had a receiving antenna with a diameter of over ten meters. The next generation of these satellites, known as Chalet or Vortex, were first orbited in the late 1970's, and had an antenna with a diameter of several tens of meters. The most recent models, known as Magnum, were first launched in the mid-1980s, and have a very large deployable antennae with a diameter of approximately 100 meters. Satellites with an even larger antenna are currently under development. Increasing the diameter of the antennae of these satellites makes it possible to detect lower power transmissions, as well as to determine the position of a transmitter with increased precision.

The notional geostationary SIGINT constellation consists of three or four satellites. The National Reconnaissance Office and National Security Agency launched a third Magnum signals intelligence satellite on the Space Shuttle on 14 November 1990. This spacecraft joined the Chalet (also known as Vortex) launched on 10 May 1989, and the second Magnum launched on 23 November 1989. In addition to these 1989 launches the constellation also includes the first Magnum, launched on the Shuttle in 1985. The older Chalet launched in 1981 has probably left service, having long surpassed its five year design life.

In addition to these geostationary SIGINT satellites, two Jumpseat SIGINT satellites, launched in 1985 and 1987, remained in service throughout the end of the 1980s.(1) These satellites, in highly elliptical Molniya-type orbits, provide specialized coverage of the far Northern regions of the Soviet Union.

Signals intelligence provided one of the first warnings that the Iraqi invasion of Kuwait was likely, when a Soviet-built TALL KING radar resumed operation 29 July 1990. The 350 mile range radar had been out of service for a number of months prior to the invasion.(2) By early October US electronic intelligence had some success in monitoring Iraqi military communications, but the Iraqi army was also using underground cables to communicate, making it difficult to determine Iraqi military intentions.(3)


1. Richelson, J., The U.S. Intelligence Community, (Ballinger, Cambridge, MA, 1985), p. 122.

2. "Invasion Tip," Aviation Week & Space Technology, 6 August 1990, page 15.

3. Gertz, Bill, "US Breathes Easier as it Spots Iraq's Jamming Gear," The Washington Times, 9 October 1990, page A8.

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Implemented by Christina Lindborg, 1997 Scoville Fellow
Maintained by Robert Sherman
Originally created by John Pike
Updated Sunday, March 09, 1997