K8758. In October 1936, having learned of the formation of the
Air Ministry Experimental Station (AMES) at
Bawdsey Manor, the
British Army's scoured their ranks looking for radio experts that would be able to help develop
radars for Army use. Ironically, two of the researchers,
W. A. S. Butement and P. E. Pollard, had been the first to suggest developing radar to detect ships in the
English Channel in 1930, but the
War Office proved uninterested at the time. Now, with the news that the Air Ministry had succeeded in detecting aircraft, they sent Butement and Pollard to Bawdsey to form the Military Applications Section, referred to universally as the Army Cell. They were given the task of using radar to accurately measure the range to a selected aircraft, a major problem for existing
anti-aircraft artillery operations. Around the same time,
Henry Tizard grew concerned about the German reaction to the
Chain Home radar. He concluded they would turn to night bombing, where the radar would still provide early warning of their approach, but the fighter aircraft would not be able to see their targets. His memo led
Robert Watson-Watt to set up a new group within AMES to develop a radar that could be fit inside an aircraft to allow them to track down their targets. The resulting experimental system was being tested in 1937 when the operators noticed odd returns while flying near the shore of the Channel. They eventually realized these were the piers and cranes at the
Harwich docks miles south of them. Shipping also appeared, but the team was unable to test this very well as their
Handley Page Heyford was forbidden to fly over water. To further test the concept,
Robert Watson-Watt provided the team with two
Avro Ansons that were able to fly out over the
North Sea from nearby
RAF Martlesham Heath. The testing system was crude; a small
dipole antenna was held by hand outside one of the escape hatches and rotated looking for the peak signal, indicating the antenna was aligned with the target ship. This was not easy as the signal naturally fluctuated. The first successes were in August 1937.
Test systems The Army Cell had already been asked about ship detection in April 1938 and they responded in May that they believed they could have a set in operation within a year of getting more help. It was not until October 1938 that such help arrived and work began. At the time, they were exploring two concepts. The first was a set operating at 1.2 m wavelength that had initially been developed by
Edward George Bowen's team for airborne use, and was designed to use several
Yagi antennas ganged together to improve directionality. Experiments demonstrated it was difficult to keep the antennas tuned to the required degree. The second concept operated at the extremely short wavelength, for the time, of 18 cm and used
parabolic reflectors to produce very good directionality. Development of this model dragged on and it was still not ready for testing in February 1939 when the rapid development of a third concept suggested development of the other two be shut down. The third set was an adaptation of the Air Ministry's 1.5 m
sets for aircraft, developed directly from the hand-held sets in the Anson. To improve directionality, the system used a "split" antenna, today known as
lobe switching, in which two antennas are placed close together in front of a common reflector. A switch rapidly connects the transmitter to one antenna and then the other, while adding a small electrical delay to one of the two antennas on reception. The result is that two
blips appear on the display for every target, and the one that is more closely aligned with the antenna will be longer. By rotating the entire antenna until the two blips are the same length, the system can accurately track the angle of the target. Testing in May demonstrated that angles on the order of 15 minutes of arc, a quarter of a degree, were possible. A high mounting allowed ships of to be detected at . During these tests, the first examples of
anomalous propagation were seen, with detections on sailing boats at distances that would normally be below the
radar horizon. Another important result was that the sets detected low-flying aircraft on several occasions, including one seaplane flying at altitude at distance, well below the lowest angles that
Chain Home could achieve. The Air Ministry researchers immediately ordered a second set in mobile form that could be moved around the countryside to test its performance from various altitudes on sea-side cliffs. On 20 June 1939,
Winston Churchill visited Bawdsey, where he was shown the experimental CD unit, along with CH. In July, the experimental set at Bawdsey happened to be operating when a coast defence artillery station at
Brackenbury Battery protecting the Harwich docks was test firing their
BL 9.2-inch guns. To the radar operator's surprise, the splashes of the shells in the water produced strong blips on the displays and they lasted more than long enough to take measurements of their range and bearing. From this point the concept of Coast Artillery radar was developed in parallel with CD, mostly based on the same hardware. During a demonstration to Army artillery technical staff in August, they saw not only the splashes, but even the individual rounds on their way through the air. The major difference in the CA role was introduced by Butement around the same time, the spiral
time base. Most radars of the era used a
A-scope type display, where the blip was drawn horizontally across the face of the
cathode ray tube (CRT), with a maximum length of about . If the operator might be expected to measure the blip to an accuracy of a millimetre, and the display was showing range, that was an accuracy of about , too low for gunnery. Butement's system moved the blip in a spiral motion such that the range to the target was a combination of the distance from the centre of the tube as well as the angle around it. This produced a line that was up to long, dramatically improving range accuracy. If the same 1 mm accuracy was be measured on this display, that represented a distance of only 15 metres.
CD Mark I In July 1939 a development contract was sent out to build prototypes of production versions based on the results of these tests, with the goal being to release the final design in October. Both the Air Ministry and Army were part of the contract, as the RAF planned to use the system under the name
Chain Home Low (CHL). The only major difference between CHL and CD was that the former later included a
plan position indicator (PPI) which made it easier to track fast-moving targets. This was not as useful in the CD role where the targets were much slower moving. The declaration of war on 3 September resulted in pre-war plans being put into immediate effect, which included the movement of the radar teams away from their exposed location on the Channel coast. The Army Cell was ordered to move to the
Christchurch area, which had been heavily fortified due to its location near
Bournemouth and
Southampton. The CD team was sent to a shoreside location with practically no infrastructure other than a few pre-fab buildings. The team spent the next days involved in everything from running electrical wires to digging sewer lines. Despite this, they were able to restart development of the production sets soon after arriving. Within days another demonstration was arranged, this time for Admiral
James Somerville, with the specific intent of testing the system for use against
submarines. Butement hastily returned to Bawdsey and got the original system operational in time for to pass by in two days. For this demonstration,
John Cockcroft was invited to attend as well. As they were demonstrating the system to track the submarine, an RAF aircraft, apparently thinking it was a German
U-boat, began attacking it. Somerville personally warned them off with "effective naval phraseology" and the test went on to successfully prove the radar could detect submarines as well as ships. As Cockroft described it: Somerville's own recording of the event is somewhat more circumspect: Work was aided by the arrival of a team of researchers from Cockcroft's Cavendish Laboratory at
Cambridge University. They took it upon themselves to disassemble the Bawdsey unit and bring it to Cambridge, where
Pye Radio engineers were brought in to begin producing copies.
Immediate deployment On 14 October the Royal Navy suffered an enormously embarrassing setback when
U-47 snuck into the harbour at
Scapa Flow and sank the battleship
HMS Royal Oak. This led to demands for an immediate deployment of the CD systems in the anti-submarine role, under the name CDU. Three sets, one of these possibly being the Bawdsey prototype, were shipped north on 27 October 1939 in an emergency-speed deployment to
Fair Isle and
Sumburgh in the
Shetland Islands. Days later it was realized that a required part had not made the voyage, and a Navy message asked for it to be delivered simply to "Cockcroft, Edinburgh". Hours of phone calls eventually solved the mystery of who this might be, and he quickly left with the needed part. Sumburgh was operational in the first week in December, but the conditions on Fair Isle were significantly more difficult. The first set was operational there on 16 February, and the second on the 23rd. All of these stations soon proved themselves highly valuable in detecting aircraft attacking the fleet. In November 1939, the Air Ministry placed an emergency order for more CHL sets to counter German aircraft laying
magnetic mines that were causing havoc on the east coast. The first two stations were installed at
Foreness Point and
Walton-on-Naze, which came online on 1 December and the 14th, respectively. Three more stations followed in early 1940, at
Shotton,
Happisburgh and
Spurn Head. Cockcroft introduced a new method of attacking enemy aircraft at Foreness. The CHL station would track a target while an air traffic controller would guide a
night fighter toward the station's area such that it would fly into the radar beam. As the targets were flying up the
Thames Estuary, this normally meant the fighter was directed to a point several miles north of Foreness. When the fighter entered the beam it became visible on the display along with the target, and from then a controller at the radar site took over radio control and directed the fighter left or right to stay centred in the beam while they continued to track the target. These experiments were successful, with Squadron Leader Pretty guiding the first interception, and soon led to the concept of dedicated
ground controlled interception radars. Real production models, as opposed to hand-built units, began to appear in December 1940. These were initially deployed mostly to the southeastern approaches, from
Broadstairs in the north to
Worthing in the southwest. A total of 120 units were built before production shut down in September 1941 in favour of new designs. As the chain expanded, the information being provided by the CD sites became highly valuable to the Navy as well, and reports from the CD sites were sent to both the Army and Navy plotting centres. By the start of 1941, the two systems were highly integrated.
Replacement In March 1941, a delegation of Army visitors was invited to the Navy's research centre at
Eastney Barracks to view the Navy's new
Type 271 radar. Based on the
cavity magnetron, it operated at a wavelength of only 9.1 cm, allowing it to be much more tightly focussed from much smaller antennas. The Navy had the problem that their ships rolled in the waves, so a tightly focused
pencil beam was not appropriate, at least not without using a
stabilization system. Instead, they took advantage of the fact that radio waves reflected forward off the water, and only returned a signal if something rose vertically off the surface. Instead of scanning close to the horizon, they used a "cheese" antenna that deliberately spread out the signal vertically, with most of it either going into the sky or reflecting away off the water. Only the small portion near the horizon could be properly reflected back to the antenna, and the large vertical sides of their targets gave them good returns even from the spread out signal. The Army did not have to worry about rolling in the waves, so they selected a parabolic reflector instead. The tight pencil beam allowed it to be scanned much more closely to the horizon than Mark I without seeing reflections off local landforms and shoals, which had rendered CD Mark I useless in some locations. These antennas also greatly increased gain, allowing them to see small boats as far away as right against the French coastline. Two such reflectors, one each for transmission and reception, were attached to the top of a vertical pole which could be rotated to scan. In contrast to the earlier CD system, rotation of this system was powered by a motor, reducing the number of people needed to run the station. The system offered much higher resolution and range accuracy than the existing VHF sets could achieve, and those Mark I sets that had been set aside for CA use were instead rebuilt using the new equipment. None of the original CA sets were ever released to service. Those stations with the original equipment were given station numbers with a leading "M", whilst those with the newer systems were labelled with a "K".
Channel dash The network of Mark I stations was well developed and the first microwave sets just arriving on 12 February 1942 when the
Channel Dash occurred. The Germans implemented an extremely sophisticated
radar jamming operation that rendered all of the Mark I sets almost useless, leaving the job to the few microwave sets. The K station at
Ventnor was out of action due to a small fire the night before. The K.7 station at
Fairlight picked them up, but the telephone line to CA plotting HQ had not been installed. They managed to forward their observations via Newhaven, but this reached the plotting centre at the same time as the first indications from microwave sets in Dover. In the end, the Mark I sets contributed nothing to the effort. ==Description==