Cobra Mist 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 . Cobra Mist was part of a small number of " Cobra " long - range surveillance radars operated by the U . S . Originally intended to be mounted in Turkey and offer coverage of most of European Soviet airspace , when Turkey objected to the site it was moved to the UK and sited to offer a view of most of western Europe . Built through the late 1960s and into 1970 , when the system was first turned on it proved to be 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 radio transmitting station used mainly for the BBC World Service until 2011 . History . Background . Cobra Mist was based on the Naval Research Laboratory ' s experimental MADRE radar , which was able to reliably detect aircraft at ranges up to 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 bids followed the next year for the actual system itself . However , Turkey refused to provide a base 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 . In particular , 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 acceptable and the USAF accepted the new location . Construction . 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 with the buildings and support systems which had to be carefully shielded to avoid contamination from signals being reflected locally . Many of the buildings had to be built on short stilts because the site was below the maximum water height recorded some years earlier . 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 as well as tests from distant aircraft . These were completed by September and attention moved 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 . The original plan to go operational in July was delayed until January 1973 , in spite of a truncated testing period that combined the " Design Verification System Test " and " Initial Operational Test and Evaluation " periods . Problems and closure . 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 appear 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 . It is estimated to have cost between $ 100 and $ 150 million . As part of the ABM Treaty , large long - range radars were limited to operation in the home countries of the operators . Whether this had an effect on Cobra Mist is unknown . Description . 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 long , supported on masts from to 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 . The minimum range was about due to the maximum elevation of the beams , while the maximum range was about 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 . At high frequencies the active portion would be close to the antenna hub and it would move out toward the larger dipole elements as the frequency was lowered . 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 around 500 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 extract a signal out of the background clutter . The system relied on ultra - linear 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 and capture only the objects of interest . This is accomplished 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 by eliminating any signals lying outside a particular time window ; thereby selecting targets within a particular area . Then it was 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 .