Cobra Mist

Under the Joint Electronics Type Designation System (JETDS), all U.S. military radar and tracking systems are assigned a unique identifying alphanumeric designation. AN/FPS-95 represents the 95th design of a "Fixed, Radar, Search”. ==History==

Background Cobra Mist was based on the Naval Research Laboratory's experimental Magnetic-Drum Radar Equipment ('MADRE'), which was able to reliably detect aircraft at ranges up to from its base on the 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. 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. The site and buildings were then occupied by a radio transmitting station used mainly for the UK Foreign Office and the BBC World Service from September 1982. When the BBC stopped using the transmitters in 2011, they were brought back into service and briefly used by the Dutch Government to broadcast to Holland during problems with their national transmitters. In August 2015, the site and all the facilities previously held by the UK Foreign Office and the BBC (and prior to them the Ministry of Defence) were acquired by Cobra Mist Limited, a privately owned company. Subsequently in May 2017 Radio Caroline was given a licence to use the 648 kHz frequency previously used by the BBC and now broadcasts from the site. ==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. 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. It was 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. ==See also==