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Project POTEEN

The Blakehill AN/TPS71 Array that never was





Article by Chris Morshead - additional diagrams by Vincent Povey

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The Planned Relocatable Over The Horizon Radar

Most Radars operate in "line of sight" - in other words, the Radar signals are transmitted out, travel in a straight line, hit the target and then bounce back to the Radar in a straight line. When you are on a beach you "see" a visual horizon which, for someone 6ft tall, is about 5km away. Beyond that you can't see anything on the surface - anything below is simply hidden from view by your visual horizon. In just the same way, Radars have a “radar horizon” over which they can't see. Anything below the "radar horizon" will be invisible to the Radar. This limits how far a Radar can "see" a target on the ground or at low level – a Radar 6ft up would “see” things on the ground just out to 5km!


One special type of Radar differs from this - it is called Over The Horizon Radar (OTHR). The way it looks over the “horizon” is that it bounces Radar signals up to the Ionosphere (between 85 and 600 km above the earth) which then reflects the Radar signals back down to earth. Once they reach the ground (the "target area"), the signals bounce back up to the Ionosphere which then reflects the returning signal back to the Radar. This way, the range for ground targets of this type of Radar is well beyond line-of-sight of conventional Radars, often picking up ground targets out to 2500 - 3000 km or more. It should be noted that the famous long-range Ballistic Missile Early Warning System at RAF Fylingdales in Yorkshire is not an OTHR - it is a conventional Radar and so can only pick up targets 100’s of miles away once they have reached a very considerable altitude and have appeared over the "Radar horizon".


The RAF Fylingdales Radar cannot "see" ground or low-altitude targets and so cannot be used to track ships or even aircraft at long range - it picks up Ballistic Missiles but only once they have climbed to great heights and have appeared out from behind the "radar horizon". In some cases, the OTHR Transmitter and Receiver are at the same location (a Mono-static OTHR); sometimes they are located many km apart (a Bi-static OTHR). How they are set up depends on many factors which the designers will consider. However, whichever system is used - the principle is the same - they can see low level and ground targets many thousands of km away.

A diagram of a bi-static Over The Horizon Radar System


Cold War Radar Development

During the Cold War, OTHR systems were developed so that we could look deep into Soviet territory to track their aircraft, ships and even tanks and, of course, Nuclear missiles. One early example of such an OTHR system was called “SANDRA”, a system set up by the UK in Cyprus in 1961. This was designed to monitor what was happening in the Western Soviet Union and the Communist Bloc (including Poland, Czechoslovakia and East Germany) and, of particular interest, the Soviet Nuclear Missile Test Facility at Kapustin Yar, located at the Northern end of the Caspian Sea. “SANDRA” formed part of a comprehensive SIGINT (Signals Intelligence) network run jointly by the UK’s GCHQ (Government Communications Headquarters) and the US NSA (National Security Agency), the two organisations charged with monitoring and providing intelligence on such matters. Military SIGINT staff were intimately involved in this “Top Secret” work. Following on from highly successful ventures such as “SANDRA”, there were further plans to set up such “SIGINT” facilities in the UK and one famous American system called the "COBRA MIST" OTHR (using the AN/FPS-95 Radar) was set up at Orford Ness in Suffolk which ran from the mid 1960's to 1973. This system was to set up provide a different “view” into the Soviet Union from the West, covering the Warsaw Pact and the Western Soviet Union. In addition, both the Missile Test Sites at Kapustin Yar and, now, the Russian Nuclear Test Facilities at Novaya Zemlya in the Barents Sea would be covered. Novaya Zemlya was the site of the 1961 test of the “Tsar Bomba, the most powerful nuclear weapon ever detonated. In addition, “Sandra” in Cyprus was upgraded with another OTHR, called “COBRA SHOE”, to further enhance joint UK/US intelligence gathering from Cyprus.

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The Soviets also had their own versions of OTHR looking out over Europe. One famous one was dubbed the "Woodpecker" - a Duga OTHR.


It was known informally as the "Woodpecker" due to the annoying "clicking" interference it caused on many radio stations as many amateur Radio "Hams" will vividly remember!    The Duga radar system overlooking Europe and the UK was given the code "5Н32-West" by the Soviets, and was set up in two closed towns in the Ukraine; Liubech-1 held the two transmitters and Chernobyl-2 the receivers.


The Duga OTHRs operated for over a decade between 1976 and 1989.

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While we don't really know how successful the Duga OTHR systems were, rather surprisingly, "COBRA MIST" suffered from various technical issues - hence it's very short life. The received signal contained a lot of background noise, called "clutter", which made it very difficult to see anything - even though the "clutter" should have been filtered out. It was sometimes possible to detect aircraft and ships - even trains - within the Soviet Union but the technical problems encountered meant it could not provide a workable OTHR system and, after testing to try and isolate the cause of the issues, it was decided to abandon the Project. “COBRA SHOE” in Cyprus was far more successful and was last reported as still being operational in 2014 after a number of upgrades.

The Birth of the Blakehill Farm ROTHR Plan

By 1991, the US Navy had a more modern Relocatable Over the Horizon Radar (ROTHR) system, the AN/TPS-71, which had been installed on the isolated Aleutian Island of Amchitka, Alaska where it looked into the Eastern Soviet Union. Amchitka used a pre-production ROTHR system and, while not perfect, it had already been decided to roll out a number of production AN/TPS-71 ROTHR systems in other locations around the world to cover both the Soviet Union and, by now, a growing threat, China. Plans were made to base 3 systems in the Western Pacific, with another system in Japan and another in Hampton Roads, Virginia in the USA. A further system was planned for installation here in the UK. The UK system was called Project “POTEEN” and was due to form part of the NADS, the North Atlantic Defence System.

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On the 20 April 1990, a formal agreement between the US and UK Governments was signed. The plan under “POTEEN” was for the Transmitters to be located at RAF St Davids, Pembrokeshire. Since GCHQ already operated the site at RAF Blakehill Farm as a Research Station, it was decided to locate the Receivers here at RAF Blakehill Farm. The UK ROTHR was planned to look North to track ships and aircraft operating in the North Atlantic and the Norwegian Sea between Greenland and Northern Siberia and as far South as the Iceland-Faroes gap. Initially planned to be a 2-year Operational Evaluation, a decision probably influenced by the earlier lack of success with “COBRA MIST” 20 years before, the idea was to further refine the Radar signal processing based on how it performed here in Europe before making a final decision on turning RAF Blakehill Farm into a permanent Radar site. As Alaska had already proved, the system worked quite well and was quite capable of tracking aircraft over several thousand miles away though the detection of shipping needed a bit more work to improve that aspect. And, after “COBRA MIST” 20 years before, POTEEN just needed to prove itself to be suitable for use from the UK.

Project POTEEN - What might have been


Had it had been built, RAF Blakehill Farm would have been home to one of the world's largest OTHR radar arrays. The transmitter station at RAF St David’s would have had an 85m long array with the required restricted areas encompassing 45 acres. The Blakehill Farm receiver station would have had an array of aerials in a row some 2.6km long consisting of no less than 372 pairs of antenna, each some 4m in height. Indeed, the array would have been wider (East to West) than the old airfield site and so it would have meant obtaining additional land adjacent to Blakehill Farm to house the complete array. All the equipment would have been housed in 33 ISO vans at Blakehill Farm with a brand new permanent Operations Building on the site and a staff of over 100 US and UK personnel based at Blakehill Farm with about 20 further staff based at RAF St Davids. An RAF Wing Commander was to be Officer-in-Charge of the RAF Signals Unit running the ROTHR on behalf of NATO.

POTEEN Transmitter and Receiver Location
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The Radar (equipment and arrays) were being built in the USA and were to be shipped over to the UK by the US Government once complete with installation to start in late 1992 at both UK sites. The Radar was due to become operational in 1993/94. At the end of two years (sometime in 1996), it was planned to make a formal decision as to whether the Blakehill Farm ROTHR would remain as a permanent Cold War long-range Radar. Sadly, the system was never installed for one reason - the fall of the Soviet Union. General Secretary Mikhail Gorbachev resigned on the 25th Dec 1991, and by 31st December the formal dissolution of the USSR had taken place. The Cold War was over and the perceived Soviet threat had ended and attention had turned to such matters as Gulf War I. Projects such as ”POTEEN” were cancelled before any construction work had even started at Blakehill Farm. Shortly afterwards, and reflecting the fast-changing post-Cold War priorities, other GCHQ work also started moving away and the Cold War days of RAF Blakehill Farm were drawing to a closer. RAF St David’s saw its own progression to become a wildlife haven and is now owned by the National Trust and Blakehill Farm itself is owned (mostly) by the Wiltshire Wildlife Trust.

BELOW - A gallery of newspaper clippings relating to the project

The UK ROTHR was planned to look North to track ships and aircraft operating in the North Atlantic and the Norwegian Sea between Greenland and Northern Siberia and as far South as the Iceland-Faroes gap.

Where in the world? - the ROTHR Today

While the AN/TPS-71 ROTHR was never delivered to Blakehill Farm (and RAF St Davids), the system which was due to be sent to the UK still remains operational today nearly 30 years later. 


The equipment destined for Blakehill Farm and RAF St Davids was actually finally set up near (in US terms!) Corpus Christie, Texas along with the system already planned for Chesapeake Bay, Virginia, with the Radars becoming operational in 1993.   A further AN/TPS-71 ROTHR system was set up in Puerto Rico in the late 1990's.



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Today, those Radars are still fully operational. However, rather than looking for Cold War enemy missiles, ships and aircraft, today, the three ROTHRs are used in a counter-narcotics role in support of U.S. Atlantic Command against a very different "Enemy" - the Drug Runners.


The three ROTHRs cover some 3.5 million square miles of the Caribbean and South America, extending north-south from southern Florida to the middle of Brazil and east-west from the western coast of Central and South America to the Eastern edge of South America at Rio Grande do Norte. Aircraft and ships used by the Drug Barons to ship their illegal narcotics to the USA are tracked and intercepted thus helping reduce the supply of “street drugs”.

So, today, if you were to stand in the middle of Blakehill Farm looking out across the old airfield, now a scene of peace and tranquillity and home to many hundreds of species of plants and wildlife, you can only imagine what the countryside would have looked like had the ROTHR been installed.  Oh, and something else to consider - you'd probably be trying to explain to an Armed Guard why they had caught you standing in the middle of a Top Secret NATO Radar Station!   

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Two views of the 2.6km long ROTHR Receiver Array like the one which would have been at Blakehill Farm.

 Photo Credits:   Raytheon (Left) and the US Navy (Right)

RAF St Davids - the other side of the missing array

St David's Airfield, Solva consisted of thirty diamond-shaped hard-standings which were placed in five clusters around the perimeter, the most extensive clusters being to the north. The bomb store and fuel store were also placed on the northern perimeter, with the bomb dump further to the north. The south side of the airfield was the area determined for a watch office to pattern 12779/41, living quarters, and various maintenance buildings formed from Nissen or Maycrete huts. Provision for four T type hangars was also made on the southern perimeter, but only three were erected. Three Blister hangars were also built. The airfield had three runways 1801m (5910ft) long, 975m (3200ft) long, and 1088m (3570ft) long, established in a triangular pattern. The majority of buildings have now been demolished, but the runways and taxi-areas remain.   Source

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The ROTHR Transmitter Array at Viques, Puerto Rico like the one would have been at RAF St Davids linking back to RAF Blakehill. Photo Credit:   GeoJournal

The 1.6 mile long “Blakehill” ROTHR Receiver array at its current home in Texas, some 80 miles West of Corpus Christie (Credit: Google Earth)

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