Cobra Mist
Encyclopedia
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.
radar, which was able to reliably detect aircraft at ranges up to 2000 nautical miles (3,704 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 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.
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.
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.
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 (926 km) due to the maximum elevation of the beams, while the maximum range was about 2000 nmi (3,704 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.
Over-the-horizon radar
Over-the-horizon radar, or OTH , is a design concept for radar systems to allow them to detect targets at very long ranges, typically up to thousands of kilometers...
station at Orford Ness
Orford Ness
Orford Ness is a cuspate foreland shingle spit on the Suffolk coast in Great Britain, linked to the mainland at Aldeburgh and stretching along the coast to Orford and down to North Wier Point, opposite Shingle Street. It is divided from the mainland by the River Alde, and was formed by longshore...
, Suffolk
Suffolk
Suffolk is a non-metropolitan county of historic origin in East Anglia, England. It has borders with Norfolk to the north, Cambridgeshire to the west and Essex to the south. The North Sea lies to the east...
, England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
. 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
Turkey
Turkey , known officially as the Republic of Turkey , is a Eurasian country located in Western Asia and in East Thrace in Southeastern Europe...
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
BBC World Service
The BBC World Service is the world's largest international broadcaster, broadcasting in 27 languages to many parts of the world via analogue and digital shortwave, internet streaming and podcasting, satellite, FM and MW relays...
until 2011.
Background
Cobra Mist was based on the Naval Research Laboratory's experimental MADREMadré
Madré is a commune in the Mayenne department in north-western France.The maternal great grandfather of Saint Thérèse de Lisieux, Louis Macé, was baptised in the village church on 16th March 1778....
radar, which was able to reliably detect aircraft at ranges up to 2000 nautical miles (3,704 km) from its base in Chesapeake Bay
Chesapeake Bay
The Chesapeake Bay is the largest estuary in the United States. It lies off the Atlantic Ocean, surrounded by Maryland and Virginia. The Chesapeake Bay's drainage basin covers in the District of Columbia and parts of six states: New York, Pennsylvania, Delaware, Maryland, Virginia, and West...
. With prior setup, MADRE was even able to detect rocket launches at Cape Canaveral
Cape Canaveral
Cape Canaveral, from the Spanish Cabo Cañaveral, is a headland in Brevard County, Florida, United States, near the center of the state's Atlantic coast. Known as Cape Kennedy from 1963 to 1973, it lies east of Merritt Island, separated from it by the Banana River.It is part of a region known as the...
and atomic tests in Nevada
Nevada
Nevada is a state in the western, mountain west, and southwestern regions of the United States. With an area of and a population of about 2.7 million, it is the 7th-largest and 35th-most populous state. Over two-thirds of Nevada's people live in the Las Vegas metropolitan area, which contains its...
.
With this successful demonstration, the US Air Force started plans to deploy a similar system in Turkey
Turkey
Turkey , known officially as the Republic of Turkey , is a Eurasian country located in Western Asia and in East Thrace in Southeastern Europe...
; providing coverage of much of the western part of the Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....
. 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
Plesetsk
Plesetsk is an urban locality Plesetsk is an urban locality Plesetsk is an urban locality (a work settlement and the administrative center of Plesetsky District, Arkhangelsk Oblast, Russia, situated about northeast of Moscow and south of Arkhangelsk. Municipally, it is the administrative...
. Although not as useful as the original site in Turkey, the UK site was nevertheless acceptable and the USAF accepted the new location.
Construction
RCA
RCA Corporation, founded as the Radio Corporation of America, was an American electronics company in existence from 1919 to 1986. The RCA trademark is currently owned by the French conglomerate Technicolor SA through RCA Trademark Management S.A., a company owned by Technicolor...
. 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
SRI International
SRI International , founded as Stanford Research Institute, is one of the world's largest contract research institutes. Based in Menlo Park, California, the trustees of Stanford University established it in 1946 as a center of innovation to support economic development in the region. It was later...
. 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
Electronic warfare
Electronic warfare refers to any action involving the use of the electromagnetic spectrum or directed energy to control the spectrum, attack an enemy, or impede enemy assaults via the spectrum. The purpose of electronic warfare is to deny the opponent the advantage of, and ensure friendly...
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 2040 feet (621.8 m) long, supported on masts from 42 feet (12.8 m) to 195 feet (59.4 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 (926 km) due to the maximum elevation of the beams, while the maximum range was about 2000 nmi (3,704 km) using one-hop off the ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
'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
Pulse repetition frequency
Pulse repetition frequency or Pulse repetition rate is the number of pulses per time unit . It is a measure or specification mostly used within various technical disciplines Pulse repetition frequency (PRF) or Pulse repetition rate (PRR) is the number of pulses per time unit (e.g. Seconds). It...
. 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
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
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.