Cobra Mist
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Cobra Mist was the codename for an Anglo-American experimental over-the-horizon radar station at Orford Ness, Suffolk, England. It was known technically as AN/FPS-95, and sometimes referred to as System 441a, a reference to the project as a whole. The project was plagued by noise problems that could not be identified, and the project was shut down in 1973. The site and buildings are now occupied by a broadcast transmitter.
Cobra Mist was based on the Naval Research Laboratory's experimental MADRE radar, which was able to reliably detect aircraft at ranges up to 2,000 nautical miles (3,700 km) from its base in Chesapeake Bay. With prior setup, MADRE was even able to detect rocket launches at Cape Canaveral, and atomic tests in Nevada.
With this successful demonstration, the US Air Force started plans to deploy a similar system in Turkey, providing coverage of much of the western part of the Soviet Union. Tenders for the system outline were placed in 1964, and followed the next year for bids for the actual system itself. However, Turkey refused to provide basing for the system, and a search started for a new location.
After some time, the British offered a site in Suffolk. From this location the radar would be able to see almost all of Eastern Europe, as well as the western parts of the Soviet Union, and of particular interest it would be able to track missile launches from the Northern Fleet Missile Test Center at Plesetsk. Although not as useful as the original site in Turkey, the UK site was nevertheless quite useful, and the USAF accepted the new location.
In 1966 a new series of bids were sent out for a system to be installed at this site, which was won late in the year by RCA. Construction of the site started in mid-1967, starting with the buildings and support systems, which had to be carefully shielded to avoid "contamination" from signals being reflected around the site. Oddly, the buildings were located in "front" of the antenna.
The main antenna consisted of a series of eighteen 2,200 ft (670 m) long cables strung out in a pattern of "rays" from a central pole, 7 degrees apart and thus covering 119 degrees in total. Each of the cables contained a number of smaller elements "hung" from it, tuned to radiate in particular orientations or at different frequencies. Under the array was a large wire-mesh reflector on the ground.
Using beam steering, the operators could select a particular angle across a fan-shaped area about 90 degrees across. The minimum range was about 500 nm due to the maximum elevation of the beams, while the maximum range was about 2000 nm using one-hop off the ionosphere's F-layer. Ranges between this could be selected by changing the broadcast frequency from 6 to 40 MHz, and "gated" by varying the pulse repetition frequency. Longer ranges were possible under certain conditions by allowing for multi-hop propagation.
The antenna was not particularly effective, with a gain of about 25 db, so to receive a useful signal from such ranges an enormous broadcast signal was needed. The system could develop a peak power of up to 10 MW, which can be compared to the most powerful commercial radio stations at 50 kW. Reception of the signal was a complex affair, as very little signal would be returned after a round-trip of several thousand kilometers, requiring a receiver sensitivity of 80 to 90 dB to pull a signal out of the background clutter. The system relied on "ultralinear" amplifiers that could amplify the signal across the entire frequency range without introducing distortion.
Key to the operation of any backscatter radar is the ability to filter out the huge return from the ground and sea, which is handled by using the Doppler effect and "gating" out the vast majority of the signal. In the case of Cobra Mist, the signal was first gated for range (to select targets within a particular area), and then fed into a series of frequency filters tuned to the expected frequency shifts from various sorts of common targets: lower speed for ships, high-speed for aircraft, and constant-acceleration for missiles.
The site was completed on 10 July 1971, and testing began a week later. Tests of the broadcast system were extensive, including both local measurements made on-site, as well as tests from distant aircraft. These were completed by September, and attention moved on to the reception systems. Minor storm damage in October slowed this phase somewhat, but RCA turned the system over to the USAF in February 1972, with the original plan being to go operational in July, but now delayed until January 1973 even with a truncated testing period.
Through the early part of 1972 testing found a considerable amount of unexpected noise, which appeared as frequency shifting of the signal. This made "targets" appears in all of the filters (high-speed, low-speed, etc.) even when it seemed there was nothing of the sort in that area. For instance, the system would often report missiles being launched no matter where the radar was looking.
A lengthy series of investigations into the source of the noise followed, and in desperation the USAF eventually turned over the testing to a panel headed by SRI. The new team continued testing from January to May 1973, but no convincing explanation was ever found. Internal problems with the equipment were eliminated as a potential source. One particularly interesting finding was that the distortion only occurred over land. Deliberate jamming was not ruled out.
Although the noise was never identified, the panel concluded that the system could still be made operational by further improvements in the receivers, although the resulting system would only be marginally useful. Instead the USAF simply gave up, and on 30 June 1973 the system was shut down, never having been used operationally. The system is estimated to have cost between $100 and $150 million.
