page_title: Cobra Mist
section_title: Description
hierarchical_section_title: Cobra Mist / Description
caption_attribution_description: English: Orfordness transmitting station
context_page_description: 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. Cobra Mist was 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 Eastern 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 were then occupied by a radio transmitting station used mainly for the UK Foreign Office and the BBC World Service until 2011. In August 2015 the site and all the facilities previously held by the UK Foreign Office and the BBC were acquired by Cobra Mist Limited, a privately owned company. The main building and 12 towers remain. Five are 340 feet high.
context_section_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 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. The minimum range was about 500 nmi (930 km) due to the maximum elevation of the beams, while the maximum range was about 2,000 nmi (3,700 km) 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.
caption: Radiating lines at the site
exif: {"Image Make": "Canon", "Image Model": "Canon PowerShot S2 IS", "Image Orientation": "Horizontal (normal)", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image DateTime": "2010:12:12 22:10:50", "Image YCbCrPositioning": "Centered", "Image ExifOffset": "186", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "180", "Thumbnail YResolution": "180", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "2296", "Thumbnail JPEGInterchangeFormatLength": "6706", "EXIF ExposureTime": "1/250", "EXIF FNumber": "4", "EXIF ExifVersion": "0220", "EXIF DateTimeOriginal": "2010:12:11 12:08:34", "EXIF DateTimeDigitized": "2010:12:11 12:08:34", "EXIF ComponentsConfiguration": "YCbCr", "EXIF CompressedBitsPerPixel": "5", "EXIF ShutterSpeedValue": "255/32", "EXIF ApertureValue": "4", "EXIF ExposureBiasValue": "0", "EXIF MaxApertureValue": "107/32", "EXIF MeteringMode": "Pattern", "EXIF Flash": "Flash did not fire, compulsory flash mode", "EXIF FocalLength": "62/5", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "sRGB", "EXIF ExifImageWidth": "2592", "EXIF ExifImageLength": "1944", "Interoperability InteroperabilityIndex": "R98", "Interoperability InteroperabilityVersion": "[48, 49, 48, 48]", "Interoperability RelatedImageWidth": "2592", "Interoperability RelatedImageLength": "1944", "EXIF InteroperabilityOffset": "2148", "EXIF FocalPlaneXResolution": "11520", "EXIF FocalPlaneYResolution": "81000/7", "EXIF FocalPlaneResolutionUnit": "2", "EXIF SensingMethod": "One-chip color area", "EXIF FileSource": "Digital Camera", "EXIF CustomRendered": "Normal", "EXIF ExposureMode": "Auto Exposure", "EXIF WhiteBalance": "Auto", "EXIF DigitalZoomRatio": "1", "EXIF SceneCaptureType": "Standard"}