AN/FPS-95 (Cobra Mist)

Monday, December 27, 2010

Also known under the code name of Cobra Mist, the AN/FPS-95 was an-over- the-horizon tracking radar built on the English North Sea coast in the late 1960s to overlook air and missile activity in Eastern Europe and the Western areas of the Soviet Union during the Cold War. Up to that time, the AN/FPS-95 was the most powerful and sophisticated radar of its kind. The design incorporated rather coarse spatial resolution and relied upon ultralinear wide dynamic range components and complex sygnal processing in attempting to achieve extreme subclutter visibility of 80 to 90 dB needed to separate target returns from the strong ground clutter.

Nevertheless, the detection performance of the FPS-95 was spoiled, since the actual subclutter visibility achieved was only 60 to 70 dB, the limitation being due to a noise with approximately flat amplitude-versus-Doppler frequency which appeared in all range bins containing ground clutter and aircraft returns. The experiments carried out at the site failed to determine the source of the noise, either in the equipment or in the propagation medium. The project was shut down in 1973. The site and buildings are now occupied by a broadcast transmitter for the BBC World Service.

The AN/FPS-95 antenna consisted of 18 individual strings radiating outward from a single point near the eastern shore of Orford Ness. Each string was 2,040 feet (620 m) long, supported on masts from 42 feet (13 m) to 195 feet (59 m) high, with multiple active elements hung from the strings. The strings were arranged 8 degrees 40 minutes apart, covering an arc from 19.5 to 110.5 degrees clockwise from true north. Beneath the antenna was a large wire mesh screen acting as a reflector. The mesh extended past the hub to the east. Operating the AN/FPS-95 radar took considerable pre-observation setup. To select a particular region of the sky, six adjacent antennas strings were connected to the electronics using a switch matrix hidden underground at the antenna hub. Using beam steering, the operators would select a 90 degree wide fan-shaped area to investigate.