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Chain Home Low (CHL) was the name of a British
early warning radar An early-warning radar is any radar system used primarily for the long-range detection of its targets, i.e., allowing defences to be alerted as ''early'' as possible before the intruder reaches its target, giving the air defences the maximum ...
system operated by the RAF during
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...
. The name refers to CHL's ability to detect aircraft flying at altitudes below the capabilities of the original
Chain Home Chain Home, or CH for short, was the codename for the ring of coastal Early Warning radar stations built by the Royal Air Force (RAF) before and during the Second World War to detect and track aircraft. Initially known as RDF, and given the of ...
(CH) radars, where most CHL radars were co-located. CHL could reliably detect aircraft flying as low as . The official name was AMES Type 2, referring to the
Air Ministry Experimental Station AMES, short Air Ministry Experimental Station, was the name given to the British Air Ministry's radar development team at Bawdsey Manor (afterwards RAF Bawdsey) in the immediate pre-World War II era. The team was forced to move on three occasions ...
at
Bawdsey Manor Bawdsey Manor stands at a prominent position at the mouth of the River Deben close to the village of Bawdsey in Suffolk, England, about northeast of London. Built in 1886, it was enlarged in 1895 as the principal residence of Sir William C ...
where it was developed, but this name was almost never used in practice. The system had originally been developed by the
British Army The British Army is the principal land warfare force of the United Kingdom, a part of the British Armed Forces along with the Royal Navy and the Royal Air Force. , the British Army comprises 79,380 regular full-time personnel, 4,090 Gurkha ...
's research group, also based at Bawdsey, as a system for detecting enemy shipping in the
English Channel The English Channel, "The Sleeve"; nrf, la Maunche, "The Sleeve" ( Cotentinais) or ( Jèrriais), ( Guernésiais), "The Channel"; br, Mor Breizh, "Sea of Brittany"; cy, Môr Udd, "Lord's Sea"; kw, Mor Bretannek, "British Sea"; nl, Het Ka ...
. It was built using the electronics being developed for the Airborne Interception radar systems, which worked on the 1.5 m band. This high-frequency (200 MHz), for the era, allowed it to use smaller antennas that could be swung back and forth to look for returns, in contrast to the enormous fixed antennas of the 6.7 m
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
(45 MHz) Chain Home system. When the war began, the ''
Luftwaffe The ''Luftwaffe'' () was the aerial-warfare branch of the German '' Wehrmacht'' before and during World War II. Germany's military air arms during World War I, the '' Luftstreitkräfte'' of the Imperial Army and the '' Marine-Fliegerabt ...
'' began mine-laying missions where the
bomber aircraft A bomber is a military combat aircraft designed to attack ground and naval targets by dropping air-to-ground weaponry (such as bombs), launching torpedoes, or deploying air-launched cruise missiles. The first use of bombs dropped from an air ...
would fly almost all of their mission at low altitude. Chain Home could only see targets above 1.5 degrees over the horizon, so these aircraft only became visible at short range. Robert Watson Watt seized several dozen of the CD systems that were in final construction and installed them at CH stations and key locations along the seashore to fill this critical gap in the coverage. CHL remained an important part of the Chain network for the rest of the war, and was retained in the post-war era until it was replaced during the
ROTOR Rotor may refer to: Science and technology Engineering * Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator *Helicopter rotor, the rotary wing(s) of a rotorcraft ...
upgrades by the
AMES Type 80 The AMES Type 80, sometimes known by its development rainbow code Green Garlic, was a powerful early warning (EW) and ground-controlled interception (GCI) radar developed by the Telecommunications Research Establishment (TRE) and built by Decca ...
. The electronics, notably the high-power transmitter, was also re-used in a number of other systems, including the AMES Type 7.


Development


Accidental discovery

CHL traces its origins to early experiments with Airborne Interception radar systems in 1936. These were developed as a short-range radar that would be used to close the gap between
Chain Home Chain Home, or CH for short, was the codename for the ring of coastal Early Warning radar stations built by the Royal Air Force (RAF) before and during the Second World War to detect and track aircraft. Initially known as RDF, and given the of ...
's (CH) approximate accuracy and the visual range of a
night fighter A night fighter (also known as all-weather fighter or all-weather interceptor for a period of time after the Second World War) is a fighter aircraft adapted for use at night or in other times of bad visibility. Night fighters began to be used i ...
pilot at about . Developed by a team at Bawdsey Manor led by "Taffy" Bowen, the new radar had to operate at much shorter
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
s in order to limit the antenna sizes to something that could be practically fit on an aeroplane. After considerable experimentation, the team settled on a set working at 1.5 meter wavelength, about 193 MHz in the
VHF Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter. Frequencies immediately below VHF ...
band. In early experiments with the new set, the team found that detection of other aircraft was problematic due to their target's relatively small size, but especially due to reflections off the ground. The latter caused a very strong signal that appeared to be at a range equal to the aircraft's current altitude, and everything beyond that was invisible in the resulting clutter. This meant that a typical night bombing run by German aircraft at altitude would only become visible at that range, far less than the desired minimum of . These same experiments demonstrated an unexpected side-effect. As the aircraft flew around over Bawdsey, which is located on the coast of the
English Channel The English Channel, "The Sleeve"; nrf, la Maunche, "The Sleeve" ( Cotentinais) or ( Jèrriais), ( Guernésiais), "The Channel"; br, Mor Breizh, "Sea of Brittany"; cy, Môr Udd, "Lord's Sea"; kw, Mor Bretannek, "British Sea"; nl, Het Ka ...
, the team found strong constant returns that they later realised were the cranes at the Harwich docks, miles away. Other smaller returns were quickly identified as boats in the Channel. These were being detected at ranges far beyond the maximum range against aircraft, in spite of the antennas not being designed for this role. The potential of this discovery was not lost, and
Robert Watson-Watt Sir Robert Alexander Watson Watt (13 April 1892 – 5 December 1973) was a Scottish pioneer of radio direction finding and radar technology. Watt began his career in radio physics with a job at the Met Office, where he began looking for accura ...
asked the team to demonstrate the concept in a real-world setting. A series of
military exercise A military exercise or war game is the employment of military resources in training for military operations, either exploring the effects of warfare or testing strategies without actual combat. This also serves the purpose of ensuring the co ...
s in the Channel in September 1937 provided a perfect test. On 3 September the team's test aircraft,
Avro Anson The Avro Anson is a British twin-engined, multi-role aircraft built by the aircraft manufacturer Avro. Large numbers of the type served in a variety of roles for the Royal Air Force (RAF), Fleet Air Arm (FAA), Royal Canadian Air Force (RCAF) ...
''K6260'', detected several
Royal Navy The Royal Navy (RN) is the United Kingdom's naval warfare force. Although warships were used by English and Scottish kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years' War against Fr ...
ships in the Channel, and the next day repeated this performance in spite of almost complete overcast.
Albert Percival Rowe Albert Percival Rowe, CBE (23 March 1898 – 25 May 1976), often known as Jimmy Rowe or A. P. Rowe, was a radar pioneer and university vice-chancellor. A British physicist and senior research administrator, he played a major role in the developme ...
of the Tizard Committee later commented that "This, had they known, was the writing on the wall for the German Submarine Service."


CD

The
British Army The British Army is the principal land warfare force of the United Kingdom, a part of the British Armed Forces along with the Royal Navy and the Royal Air Force. , the British Army comprises 79,380 regular full-time personnel, 4,090 Gurkha ...
was actually the first to consider radar, when
W. A. S. Butement William Alan Stewart Butement (18 August 1904 – 25 January 1990) was a New Zealand-born British-Australian defence scientist and public servant. A native of New Zealand, he made extensive contributions to radar development in Great Britain dur ...
and
P. E. Pollard P. is an abbreviation or acronym that may refer to: * Page (paper), where the abbreviation comes from Latin ''pagina'' * Paris Herbarium, at the ''Muséum national d'histoire naturelle'' * ''Pani'' (Polish), translating as Mrs. * The ''Pacific Repo ...
submitted a paper in 1931 suggesting using pulses of radio signal to measure the distance to ships. The Army was uninterested until they heard about Watt's work at Bawdsey, when they suddenly became very interested. In October 1936 a liaison team led by E. T. Paris and Albert Beaumont Wood was set up at Bawdsey, officially known as the Military Applications Section, but universally referred to as the "Army Cell". Ironically, the only two technicians with the required experience available were Butement and Pollard. The two quickly began development of two projects, the Mobile Radar Unit (MRU) which was a mobile version of Chain Home, and
Gun Laying radar A fire-control radar (FCR) is a radar that is designed specifically to provide information (mainly target azimuth, elevation, range and range rate) to a fire-control system in order to direct weapons such that they hit a target. They are somet ...
, a much smaller unit designed to provide range measurements against aircraft as an aid to aiming their
anti-aircraft artillery Anti-aircraft warfare, counter-air or air defence forces is the battlespace response to aerial warfare, defined by NATO as "all measures designed to nullify or reduce the effectiveness of hostile air action".AAP-6 It includes surface based ...
. Both operated at the longer wavelengths typical of the RAF radars of the era, which led to them being relatively large. The teams had made considerable progress on both projects by the summer of 1937, with Gun Laying radar, Mk. I (GL) about to enter initial production, and the MRUs later taken over in 1938 by the RAF as the AMES Type 9. With this work starting to move from development to production, coincident with Bowen's astonishing anti-shipping demonstration, Butement began looking for ways to adapt Bowen's 1.5 m set for new roles. They revisited their original concept to develop Coast Defence radar (CD), allowing the Army's
coastal artillery Coastal artillery is the branch of the armed forces concerned with operating anti-ship artillery or fixed gun batteries in coastal fortifications. From the Middle Ages until World War II, coastal artillery and naval artillery in the form o ...
to aim their guns at night or in fog. The CD set was in most respects a version of the GL working at the shorter 1.5 m wavelength, and like GL, used separate transmit and receive antennas that had to be rotated together to be aimed at a target. The earlier GL system operated at just over 6 m, which meant the antenna was very large. The GL array had only four horizontal elements in it, which offered resolution on the order of 20 degrees. This allowed the operator to pick out a single aircraft as long as they weren't in formation, but could not be used to directly guide the guns. In contrast, the 1.5 m wavelength of the new sets allowed an antenna of about the same size to feature eight dipoles, reducing the angle to about 1.5 degrees. Although this was of marginal capability in terms of directly aiming the guns, in July 1939 it was noticed that when the Army's 9.2-inch guns missed their targets, the splash of water caused by the shell would cause a brief but obvious return on the radar sets. This meant that any inaccuracy in the radar antenna's measurements could be eliminated by comparing the target and the splashes on the screen, as these would both have exactly the same error. The gunners could then correct their fire ("walk") onto the targets in the same fashion that they would when being given corrections by remote observers.


CD becomes CHL

Metrovick Type T3026 transmitter During early tests against Chain Home in 1938, RAF pilots had noticed they could escape detection by flying at low altitudes. This was due to the minimum angle of the CH being about 1.5 degrees above the horizon, which meant aircraft were below the radar's sight until they approached within a few miles. They could escape detection entirely by flying between two CH stations at altitudes around . At first this was not considered to be a serious limitation, as bombers typically flew at altitudes of 15,000 feet or greater, and at that altitude they could be detected over France. But as the magnitude of the problem became clear, Watt became concerned. In July 1939 he placed an order for twenty-four CD sets under the name AMES Type 2 (Type 1 being Chain Home), intending to place one at each Chain Home station to allow coverage at lower altitudes, as low as . These differed from the CD sets primarily in the antenna: instead of a single horizontal array, CHL used four stacked arrays of five dipoles, reducing the azimuth accuracy but allowing altitude to be estimated by comparing the returns from the different sets of vertical arrays. In keeping with its rapid introduction, CHL was a relatively simple manually-directed system that required the operator to hunt for targets by swinging the antenna back and forth looking for returns. The antenna was originally powered by WAAFs mounted on wheel-less bicycles whose chains were connected to a gear system. It was not long after the start of the war that the Germans accidentally noticed the effect when flying low. In this case, aircraft sent on minelaying sorties almost always returned while those on other intruder missions were almost always intercepted. These aircraft had to drop their payloads from very low altitudes, so they generally followed routes over water, including rivers, flying at low altitudes for most of the mission. At first, it was not obvious why they were surviving, as there could be many reasons; fighters might not be able to see them against the ground, AA guns positioned inland might not be able to aim at them, etc. But it quickly became apparent that low altitude flight meant they were not being detected on the radars. The ''
Luftwaffe The ''Luftwaffe'' () was the aerial-warfare branch of the German '' Wehrmacht'' before and during World War II. Germany's military air arms during World War I, the '' Luftstreitkräfte'' of the Imperial Army and the '' Marine-Fliegerabt ...
'' soon began a series of low-level attacks that proved almost impossible to defend against. Additional CHL sets were ordered and set up to fill the gaps between the sets co-located at the CH stations. In April 1941 all of the CHL sets were upgraded with a new antenna that was motorized to spin at 1, 1.5, 2 or 3.33 rpm, and used a single transmit/receive antenna instead of separate ones.


CHL and GCI

When first deployed, CHL was used both for early detection of low-level targets, as well as a system for tracking individual aircraft over land; unlike CH which was permanently facing over water, CHL could be turned to look in any direction. This latter role became outdated with the introduction of the AMES Type 7 in 1942. Electronically, the Type 7 was essentially a larger and more powerful version of the CHL, with a larger ground-level antenna. However, the antenna was continuously spun through a complete circle, and returns were plotted in a map-like form known as a
plan-position indicator A plan position indicator (PPI) is a type of radar display that represents the radar antenna in the center of the display, with the distance from it and height above ground drawn as concentric circles. As the radar antenna rotates, a radial tra ...
(PPI). Whereas CHL operators had to calculate a single target's position from the range and bearing, Type 7 operators saw all of the aircraft in their area simultaneously and could determine their map location directly. CHL was increasingly used purely for early warning, calling in the rough location of targets to Type 7 stations who would then know where to look. Later, 3 GHz-frequency Chain Home Extra Low (CHEL) radar were often co-sited with CHL sites, further extending detection as low as . Several adaptations of the CHL were made during the war. Such systems could be mobile in which units were placed on trucks for movement matching the enemy's, extending the RAF's options in engaging the enemy.


AMES Type 11

Although GCI began to supplant CHL during 1941 and especially 1942, CHL continued to provide an important early warning role. In February 1942 the Germans mounted the Channel Dash, moving two of their battleships to harbours in Germany by sailing them right up the English Channel. This major embarrassment was due largely to supremely effective jamming on the part of the Germans, who managed to render the CH and CHL radars covering the coast entirely ineffective without the operators even noticing. To address this, the RAF began development of the AMES Type 11, a truck-mounted CHL system operating at 500–600 MHz. This frequency was chosen to match that of German anti-aircraft radars, in the hopes that the signals would be more difficult to notice, and that jamming would have negative effects on the German's own radars. Type 11's were deliberately used only in times of jamming in order to avoid giving the Germans
signals intelligence Signals intelligence (SIGINT) is intelligence-gathering by interception of '' signals'', whether communications between people (communications intelligence—abbreviated to COMINT) or from electronic signals not directly used in communication ...
about them, and in the end were little used.


List of Chain Home Low sites


See also

*
Chain Home Chain Home, or CH for short, was the codename for the ring of coastal Early Warning radar stations built by the Royal Air Force (RAF) before and during the Second World War to detect and track aircraft. Initially known as RDF, and given the of ...
*
Air Ministry Experimental Station AMES, short Air Ministry Experimental Station, was the name given to the British Air Ministry's radar development team at Bawdsey Manor (afterwards RAF Bawdsey) in the immediate pre-World War II era. The team was forced to move on three occasions ...
*
ROTOR Rotor may refer to: Science and technology Engineering * Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator *Helicopter rotor, the rotary wing(s) of a rotorcraft ...
* Linesman/Mediator


Notes


References

* * * The history of ground radar in the UK during World War II. * * A history of radar in the UK during World War II told by the men and women who worked on it. * * {{cite book , last=Zimmerman , first=David , title=Britain's Shield Radar and the Defeat of the Luftwaffe , publisher=Sutton Publishing Ltd , location=Stroud , year=2001 , isbn=0-7509-1799-7


External links


Early radar development in the UK
World War II sites in the United Kingdom Battle of Britain Ground radars World War II British electronics World War II radars Military radars of the United Kingdom Air defence radar networks Military equipment introduced in the 1930s