AN/FPS-95 COBRA MIST System 441a

System 441a, known as COBRA MIST, was initiated to aquire, install, and test the AN/FPS-95 OTH radar set in an operational overseas environment at Orford Ness, England. The 441A contractor was RCA. The missions of the 441A AN/FPS-95 over-the-horizon (OTH) backscatter radar were to detect and track aircraft; detect missile and earth satellite vehicle launchings; fulfill current and critical intelligence requirements; and to provide a research and development (R&D) testbed for determining optimum backscatter techniques for other operational missions.

COBRA MIST was built on the English North Sea Coast in the late 1960's to overlook air and missile activity in Eastern Europe and the western areas of the USSR. The AN/FPS-95 was expected to detect and track (a) aircraft in flight over the westerly part of the Soviet Union and the Warsaw Pact countries and (b) missile launches from the Northern Fleet Missile Test Center at Plesetsk. Aircraft detection and tracking at ranges of 500 to 2,000 nmi, corresponding to one-hop ionospheric propagation, were considered feasible. Missile launches from Plesetsk were also within one-hop range from the radar. A searchlight mode was provided for high-priority targets whose approximate locations were known a priori. These targets could be single aircraft, compact formations of aircraft, or missile launches. In this mode, the radar continuously illuminated a small geographical area to obtain the maximum data rate on the selected targets. As an alternative, a scanning mode was provided, which allowed the radar to search in azimuth and range over any chosen sector of the radar coverage.

COBRA MIST was the most powerful and sophisticated radar of its kind up to that time. The design, which emulated Naval Research Laboratory's Madre over-the-horizon radar, incorporated rather coarse spatial resolution and relied upon ultralinear, wide dynamic range components and complex signal processing in attempting to achieve the extreme subclutter visibility (scv) of 80 to 90 dB needed to separate target returns from the strong ground clutter - a goal well beyond the 60-odd decibel subclutter visibility previously achieved. The detection performance of the radar was spoiled, however, because the actual subclutter visibility achieved was only 60 to 70 dB.

To achieve sufficient signal-to-noise ratios against the predicted noise background, the radar was capable of very high transmitted power output. A peak power of 10 MW and an average power of 600 kW were originally specified, although these figures were not achieved in practice. Such high powers were incorporated in the design to compensate for the relatively low antenna gain of approximately 25 dB.

System 441A comprised a huge fan shaped array of aerials supported on masts from 42 feet to 195 feet high. The antenna consisted of 18 log-periodic antenna strings, which radiated like spokes in a wheel from a central "hub." Each string was 2,200 ft in length and carried both horizontal and vertical radiating dipoles. The strings were separated by 7 deg in angle, and they thus occupied a 119-deg sector of a circle. The complete antenna was located over a wire-mesh ground screen, which extended beyond the strings in the propagation direction.

By 10 July 1971 the 18th (and last) antenna string was installed. Phasing at high power began on 17 July. A large steel blockhouse on short legs was behind this array. It was connected to the aerials by cables which ran to an underground chamber, lined with copper, in front of the array.

The Rome Air Development Center (RADC) aircraft conducted antenna pattern measurement tests 5-17 August, with the radar operating 10 decibels down. All 18 strings were tested at all ranges, including the side lobes and back lobes. The aircraft long range tests were completed early in September 1971.

By 3 September it appeared that problems in integrating all end items into a workable system would delay system test another two or three weeks. The problems did not appear major when considered individually, but the system did not perform in the sense that meaningful data could be retrieved at the displays. Most of the problems appeared to be in the receiver and signal processing areas, since it was known that the transmitters were working and the antenna was radiating at expected power levels. RCA provided four modification kits for the screen regulators, and system functional tests began on 29 September, except for Reliability and Maintainability (R&M) test. R&M, which requires operation 24 hours a day, was delayed until 15 October because repair work on the damaged masts had to be done during the daylight hours. On 9 February 1972 the radar was accepted from the contractor.

System turnover was scheduled for 1 July 1972. Based on guidance by Hq USAF, the one year Design Verification System Tests planned for this system were terminated after six months on 10 August 1972 and Initial Operational Test and Evaluation (IOT&E) of the system was begun with the Using Command, USAFE in an effort to accelerate the system operational date to January 1973.

In the interim it was decided to combine the Design Verification System Test (DSVT) and Initial Operational Test and Evaluation (IOTandE) in order to expedite the test program. In the combined DVST and IOTand E during the summer and fall of 1972, a severe noise problem was encountered which resulted in a reduction in detection capability. The range related noise had a median value of -65 decibels; in addition, there were noise modulation sidebands 10 to 30 hertz from the carrier with a median value of -60 decibels.

The testing revealed that the system was plagued by excessive noise of then undetermined origin which prevented the system from meeting its operational performance requirements. Various on-site and prime contractor efforts to alleviate the problem were unsuccessful and resulted on 29 December 1972 in direction by Hq USAF to terminate the IOT&E and to establish a joint US/UK blue-ribbon scientific team of OTH experts to determine the cause of the noise problem and recommend appropriate fixes.

Much work was done to establish the source of this noise and to try and eliminate it, but the source of the difficulty that caused Cobra Mist's demise was never found. The noise problem could be over come by certain system modifications and that the system could be brought into an operational state with certain limitations on its expected capabilities. In the end it was decided that the economics of the situation were unjustifiable and the project was terminated. On 19 June 1973, the UK was advised that the US has decided to close down the site and deactivate the system on 30 June 1973.

The imposing building is now used as a BBC World Service transmitting station. 'Cobra Mist' is well known for its alleged associations with UFOs.

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