Chain Home



Chain Home was the codename for the ring of coastal Early Warning radar stations built by the British before and during the Second World War. It was one of the first practical radar systems and the main component of the world's first integrated air defence system. The system otherwise known as AMES Type 1 (Air Ministry Experimental Station) consisted of fixed cable array antennae slung between three or four 360 ft steel towers. The radar transmitted on the HF 12 m frequency band between 22-25 MHz depending on operational requirements. The installations were known as 'stations' and were designed to provide long-range detection of aircraft. This system had shortcomings in not being able to detect aircraft at lower altitudes and thus was used in conjunction with the Chain Home Low system, or AMES Type 2, which could detect aircraft flying at minimum altitude level of 500 ft. This was further refined by the addition of Chain Home Extra Low, which gave cover down to 50 ft but at short ranges of only approximately 30 mi. CHEL was in operation by 1943 under the designation Type 13 / 14. CHEL was the first RAF centimetric radar system working on a frequency of 10 cm rather than the 12 m HF band Type 1.

Development
The concept of detection of objects by using radio waves had long been known, and the first 'radio location' patents were issued to Christian Hülsmeyer in 1904. However, there were major technical problems to overcome, and it was not until the 1930s that practical Radio Direction Finding (RDF) began to be developed simultaneously in several countries (Germany, Japan, the USA and USSR). The concept demonstration in Britain was in February 1935 outside the town of Daventry. This successful demonstration resulted in immediate funding for a chain of RDF stations around the British east coast from Orkney in the north to Weymouth in the south. The name given to the project was 'Chain Home'. By 1936 the first five stations were commissioned in the south east of England to protect the Thames estuary and London, and the reporting system for the stations was tested using aircraft from RAF Biggin Hill. The lessons learned and techniques of fighter control evolved were applied directly to operations in the 'Battle of Britain' in 1940. By the outbreak of war in September 1939 there were 21 operational Chain Home stations. After the Battle of France in 1940 the network was expanded to cover the west coast and Northern Ireland.

The Chain Home system was simple and basic by 1935 standards and 'primitive' by modern ones since it had been rushed into production by Sir Robert Watson-Watt's Air Ministry research station near Bawdsey on the Suffolk coast of eastern England. The system was developed using existing commercially available technology to speed introduction and to hasten battle readiness as Britain in general and the Royal Air Force (RAF) in particular was rearming in the face of a rising threat from Nazi Germany. The development team could not afford the time to develop and 'debug' new technology and they had to use what was available in Britain at the time. Watson-Watt, a pragmatic engineer, believed that "third-best" would do if "second-best" would not be available in time and "best" never available at all. Chain Home certainly suffered from glitches and errors in reporting, but skilful operation and acceptance of various limitations (poor low-level cover and a very low frequency data rate) produced an effective system that worked well.

Modern hindsight of seventy years is often dismissive of Chain Home, viewing it as 'obsolete and dead end technology', but it was a considerable technical achievement to produce an integrated air defence system from green field trial in 1935 to success in battle in 1940. This achievement was a world first, regardless of how scathing some modern authors are of the technology used. This derided HF frequency band is now exploited in extremely sophisticated modern ultra long range radars which can 'see' around the curvature of the earth.

Chain Home radar installations were normally composed of two sites. One compound containing the transmitter masts with associated structures and a second compound within a few hundred metres containing the receiver masts and receiver equipment block where the operators (principally WAAF: Women's Auxiliary Air Force) worked. Radar returns from the equipment were called 'plots' and were 'told' by voice telephone to the filter room at the Headquarters of Fighter Command at RAF Bentley Priory. This was the clever part of Chain Home; The radar was simple and crude and rushed into service but a lot of thought had gone into the entire system of command and control to produce an integrated air defence system. The filter room at Bentley Priory collated the various plots from the different radar stations and produced clean data which was passed up to the map plotters in the command operations room and down to the four regional RAF group controls, which in turn passed the data to their sector operation centres (SOC's), who controlled the fighter airfields. The data also went sideways to other defense units such as Royal Navy, Army anti-aircraft gun sites and RAF barrage balloon operations. There was also comprehensive liaison with the civil authorities, principally Air Raid Precautions.

The main limitation in use was that Chain Home was a fixed system, non-rotational, which meant it could not see beyond its sixty-degree transmission arc or behind it once the targets had flown overhead, and so raid plotting over land was down to ground observers, principally the Observer Corps (from April 1941 known as the Royal Observer Corps). Ground observation was acceptable during the day but useless at night and in conditions of reduced visibility. This problem was reduced on introduction of more advanced surveillance radars with 360-degree tracking and height-finding capability and, more importantly, aircraft fitted with Airborne Intercept radar (AI), which had been developed in parallel with Chain Home from 1936 onwards. This new equipment began to appear in late 1940 fitted to Bristol Blenheim, Bristol Beaufighter and Boulton Paul Defiant aircraft.

An example of the newer ground radars introduced being the Type 7 Ground Control Intercept Radar (GCI) on a wavelength of 1.5 m, introduced in 1942.

Chain Home Type 1 came in two versions, either 'East Coast' or 'West Coast'. East Coast sites sites used three or four 360 ft steel lattice towers for transmission and four wooden 240 ft receiver towers. These sites had transmitter and receiver blocks protected with earth mounds and blast walls, along with separate reserve transmitter and receivers in small bunkers with attached 120 ft aerial masts. These reserves were in close proximity to the respective transmitter/receiver sites, often in a neighbouring field. Most transmitter/receiver blocks also contained duplicated equipment. 'West Coast' sites relied on site dispersal for protection with duplicated transmitter / receiver buildings. The west coast sites used 360 ft guyed masts rather than towers for supporting the antennae along with the same 240 ft receiver towers from the east coast installations. Chain Home was in many ways technically inferior to German radar developments, but the better German technology came at a cost. The simpler Chain Home stations provided comprehensive coverage by the start of the Battle of Britain, whereas the Germans had only commissioned around eight of their Freya stations by this time. Although simple, Chain Home could determine distance and direction of incoming aircraft formations. The method was called Radio Direction Finding (RDF), later called "radar" (RAdio Direction And Ranging) in the U.S. Most stations were also able to measure the angle of elevation of the formation, which, together with the range, gave the height; local geography prevented some stations from measuring elevation. Although not originally a design goal, the operators became very adept at estimating the size of detected formations from the shape of the displayed returns.

Chain Home looked nothing like later radar equipment. The antenna did not rotate: the transmitting array was formed of fixed wires strung between 110 m metal towers which sent out a "floodlight" beam of radio energy about 100° wide. The receiving array was on wooden towers about 73 m high, and consisted of two antennas at right angles to each other. The receiving antennas were directional, so the signal strength received by each depended on the angle between it and the target. An operator would manually adjust a comparator device to find what angle to the target best matched the relative strengths of the two received signals. The angle of elevation to the target was estimated by similar comparison of the signal strengths from a second pair of receiving antennas closer to the ground, which produced a different sensitivity in elevation. The time delay of the echo determined the range to the target.

The Chain Home stations were designed to operate at 20–50 MHz, the "boundary area" between high frequency and VHF bands at 30 MHz, although typical operations were at 20–30 MHz (the upper end of the HF band), or about a 12 m wavelength. The availability of multiple operating frequencies gave some protection from jamming. The detection range was typically 120 mi, but could be better. The transmitters were made by Metropolitan-Vickers and were based on the design of a short-wave transmitter at Rugby. The receivers were built by A. C. Cossor, Ltd expressly to a specification engineered by  the  Telecommunications Research Establishment.

The Chain Home Low stations operated at 200 MHz on the VHF band, or about a 1.5 m wavelength. Technically, they were not closely related to Chain Home, and they employed a rotating antenna.

Freya, by contrast, operated in the 2.5 to 2.3 m (120 to 130 MHz) band, with a maximum range of only 100 mi, and could not accurately determine altitude.

From May to August 1939 the German Zeppelin LZ130 Graf Zeppelin II made flights along Britain's North Sea coast to investigate the 100-metre-high radio towers the British had erected from Portsmouth to Scapa Flow. LZ130 performed a series of radiometric tests and took photographs. German sources report the 12 m Chain Home signals were detected and suspected to be radar; however, the chief investigator was not able to prove his suspicions, so Germany went to war uncertain of British radar defences. Other sources are said to report different results.

The Germans observed 12 m pulse signals at the western front without being able to recognize their origin and purpose. In mid-June 1940, the German Aeronautic Research Institute (DVL) set up a special group under the direction of Professor von Handel and found out that the signals originated from radar installations on the coast of the English Channel. They pinpointed the location of the 12 m CH installations that up to then had been thought to be coastal radio stations. The observation that the transmissions of the individual stations were pulse-modulated, in order to avoid mutual interference, was exploited in July 1940 to put the CH out of service for some time.

Operations
The Chain Home stations were arranged around the British coast, initially in the South and East but later the entire coastline, including the Shetland Islands. They were first tested in the Battle of Britain in 1940, when they were able to provide adequate early warning of incoming Luftwaffe raids. Their early deployment had allowed the UK time to develop a well-integrated communication system to direct responses to enemy formations detected.

Chain Home had many limitations. With fixed antennas facing the sea, the Observer Corps had to be employed to report aircraft movements once the coast was reached. With detection poor below 5000 ft, Chain Home Low stations were placed between Chain Home stations to detect aircraft down to 2000 ft but only out to 35 mi from the coast, about one-third the range of Chain Home.

Calibration of the system was carried out initially using a flight of mostly civilian-flown, impressed Avro Rota autogyros flying over a known landmark, the radar then being calibrated so that the position of a target relative to the ground could be read from the position on the display CRT. The Rota was used because of its ability to maintain a relatively stationary position over the ground, the pilots learning to fly in small circles while remaining a constant ground position despite a headwind.

During the battle, Chain Home stations — most notably the one at Ventnor, Isle of Wight — were attacked several times between 12 and 18 August 1940. On one occasion a section of the radar chain in Kent, including the Dover CH, was put out of action by a lucky hit on the power grid. However, though the wooden huts housing the radar equipment were damaged, the towers survived owing to their open steel girder construction. Because the towers survived intact and the signals were soon restored, the Luftwaffe concluded the stations were too difficult to damage by bombing and left them alone for the remainder of the war. Had the Luftwaffe realised just how essential the radar stations were to British air defences, it is likely that they would have expended great effort to destroy them.

The last success of wartime Chain Home Type 1 was as the first ballistic missile early warning radar used to track German V-2 missiles in their launch phase of attacks on London. This was known as operation 'Big Ben', lasting from September 1944 until May 1945. Triangulation between radar sites was used to locate the launch sites and to calculate the probable impact area for civil defence purposes. Fighter bombers were then tasked to attack the launch area once it had been located. Success in this task was aided by the missile profile, which acted as an excellent quarter-wave reflector for 12 M band HF radar. However, the German launch convoys were motorised, well camouflaged and highly mobile, and there are no verified reports of any launch convoy actually being destroyed in this way, despite the precision of the radar tracking. The V-2 was a supersonic missile and thus impervious to attack by guns or aircraft while in flight, unlike the V-1 flying bomb, which was a small pilotless aircraft, similar in concept to a modern cruise missile. The V-1 could be successfully engaged by fighter aircraft and anti-aircraft guns, but the only defence against the V-2 was for the Allied land forces to overrun the launch areas, which pushed the missiles out of range.

The British radar defences were rapidly run down during the last years of the war, with many sites closed and others on 'care and maintenance'. However, immediate postwar tensions with the Soviet Union resulted in recommissioning of some wartime radars as a 'stopgap'. Specific radars were remanufactured to peacetime standards of quality and reliability, which gave significant increases in range and accuracy. These 'rebuilt' systems were the first phase of Chain Home's replacement system, ROTOR, which progressed through three phases from 1949 to 1958. The very last Chain Home Type 1 systems were retired in 1955 along with the wholesale demolition of most of the steel and timber towers.

Some of the steel transmitter towers still remain, although the wooden receiver towers have all been demolished. The remaining towers have various new uses and in some cases are now protected as a Listed building by order of English Heritage. One such 360 ft transmitter tower can now be found at the BAE Systems facility at Great Baddow in Essex. It originally stood at RAF Canewden in Essex. This is the only surviving Chain Home tower still in its original, unmodified form with cantilever platforms at 50 ft, 200 ft & 360 ft. Swingate transmitting station in Kent (originally AMES 04 Dover) has 2 original towers (3 up until 2010) which are used for microwave relay although the towers lost their platforms in the 1970s. RAF Stenigot (picture below) in Lincolnshire has another, almost complete tower, less its top platforms and used for training aerial erectors.

The only original Chain Home site which is still used as a military radar station is RAF Staxton Wold in Yorkshire although there are no remains on site of the original 1937 equipment as it was completely cleared and remodelled for the Rotor replacement: Linesman/Mediator system in 1964.

The 240-foot timber receiver towers were some of the tallest wooden structures ever built in Britain. Two of these wooden towers were still standing in 1955, at Hayscastle Cross. Unlike the transmitter tower pictured here, those at Hayscastle Cross were guyed.

The wooden reception towers at Stoke Holy Cross were demolished in 1960.

Use by the Germans
The Germans developed a radar system to use British generated radar signals to assist them in tracking British aircraft. 'Heidelberg-Gerät' was an early bistatic radar erected in 6 locations from Oostvoorne in Holland to Cherbourg in France. A bistatic radar consists of widely separated transmitter and receiver for technical or operational reasons. One unique feature of the bistatic system is that the primary radio source need not be friendly or connected in any way to the passive receiver. Any high-power radio wave will do, and the bistatic concept lends itself to 'covert' system development. This idea is illustrated by the original British Daventry experiments in 1935, where the initial radar trials used a high-power BBC commercial transmitter and received that signal by reflection from the target aircraft. In modern times bistatic systems have specific uses in combating 'stealth' vehicles and as a control radar for semi-active missile equipment, where the missile uses a reflected radar pulse from the target for guidance. In this case the German system 'piggybacked' or 'hitchhiked' onto the 25 MHz HF signal generated by the British Chain Home system as a passive receiver. This had two principal advantages: The system did not transmit and was thus less of a target and was also very hard to jam as any jamming would affect the primary British radar. This particular bistatic system only worked because the Chain Home radars were fixed in position with a floodlight beam and did not rotate. If they had rotated then the technical requirement to synchronise the passive receiver to revolving 'enemy' radar would have been impossible for the technology of the day. This is perhaps illustrated as the German system was only designed to work with the Type 1 HF radars with no attempt to use the multiple, and more sophisticated, rotating radars progressively introduced in Britain from 1940 onwards.

The equipment was developed in 1942 to 1944 and was commissioned as extra receiving antennae attached to existing German search radars. This system gave very long ranges and allowed German air defence to track British aeroplanes all the way from England to Germany. The "floodlight" nature of the Chain Home transmissions provided a pair of signals which could be used to locate aircraft. The primary signal was received directly by the German receiver from the Chain Home transmitter; the second, weaker, signal was that reflected from the aircraft. The time delay between these two signals established how much longer was the reflected path compared to the direct path, and from geometry this longer path described an ellipse on which the aircraft must lie. The focal points of this ellipse were the transmitting and receiving antennas, and the Germans knew the location of both. A direction-finding antenna searching for the echo could be used to establish where on the ellipse the aircraft was. This system gave the Germans a radar with a range of up to 400 km, and an accuracy in range of 1 - 2 km and in bearing of about 1°. The Heidelberg Parasit was not affected by Window. In this case the frequencies of German radar (VHF / UHF) at 250–300 MHz against 25 MHz Chain Home (HF) are so far apart that jamming one would have little effect on the other.

Much research work has been done in the last few years on Heidelberg-Gerät, and a very technical article in PDF is here which includes its own source references.

Chain Home sites


These links are to British Ordnance survey maps showing the locations of all 'Mainland' UK Chain Home Type 1 / Type 2 sites. Note that Northern Ireland had comprehensive Type 1 / Type 2 cover but these stations are not shown on the maps.

A link here shows modern aerial photographs of the locations of AMES Type 1 Chain Home. This link shows modern aerial photographs of the locations of AMES Type 2 Chain Home Low.

This link shows modern aerial photographs of the locations of Chain Home Extra Low. Radar site locations in this period are complicated due to the rapid growth in technology 1936-45 and the changing operational requirements. By 1945 there were 100+ radar sites in the UK. One of the primary objectives of post war ROTOR was to streamline and manage an unwieldy network that grew exponentially 'as required' in the war years.

Individual sites are listed below:
 * Bawdsey: Suffolk
 * Branscombe: Devon
 * Brenish: Western Isles
 * Bride: Isle of Man
 * Broadbay: Western Isles
 * Canewdon: Essex
 * Castell Mawr: Near Llanrhystud, Ceredigion, AMES No. 67
 * Dalby: Isle of Man
 * Danby Beacon: Lealholm, North Yorkshire
 * Darsham: Suffolk
 * Douglas Wood: Monikie, Angus
 * Dover (Swingate): Kent
 * Downderry: Cornwall
 * Drone Hill: Near Coldingham, Borders
 * Drytree: Goonhilly Downs, Cornwall
 * Dunkirk, Kent
 * Folly: Nolton, Pembrokeshire
 * Great Bromley: Essex
 * Greystone: County Down, Northern Ireland, AMES No. 61
 * Hawks Tor: Plymouth, Devon
 * Hayscastle Cross: Pembrokeshire
 * High Street, Darsham
 * Hillhead: Memsie, Aberdeenshire
 * Kilkeel: County Down, Northern Ireland AMES No. 78
 * Kilkenneth: Tiree, Argyll and Bute
 * Loth: Helmsdale, Sutherland
 * Netherbutton: Holm, Orkney
 * Nefyn: Gwynedd, AMES No. 66
 * Newchurch: Kent
 * North Cairn: Near Stranraer, Dumfries, AMES No. 60
 * Northam: Devon
 * Noss Hill: Shetland Islands
 * Ottercops Moss: Otterburn, Northumberland
 * Pevensey: East Sussex
 * Poling: West Sussex
 * Port Mor: Tiree, Argyll and Bute
 * Rhuddlan: Denbighshire, AMES No. 65
 * Ringstead: Ringstead Bay, Dorset
 * Rye: East Sussex
 * St. Lawrence, Isle of Wight: Isle of Wight
 * Saligo Bay: Islay, Argyll and Bute
 * Sango: Durness, Sutherland
 * Saxmundham, Suffolk, IP17 3QD
 * Scarlett: Isle of Man
 * Schoolhill: Porthlethen, Aberdeenshire :
 * Sennen: Cornwall
 * Skaw: Unst, Shetland Islands
 * Southbourne: Dorset
 * Staxton Wold: North Yorkshire
 * Stenigot: Louth, Lincolnshire
 * Stoke Holy Cross: Norfolk
 * Tannach: Wick, Caithness
 * Tower: Blackpool, Lancashire, AMES No. 64
 * Trelanvean: Goonhilly Downs, Cornwall
 * Trerew: Newquay, Cornwall
 * Ventnor: Isle of Wight :
 * Warren: Pembrokeshire
 * West Beckham, Norfolk
 * West Prawle: Devon
 * Whale Head: Sanday, Orkney Islands
 * Worth Matravers: Swanage, Dorset
 * Wylfa: Isle of Anglesey, AMES No. 76