Aircraft interception radar
Aircraft interception radar, or AI radar for short, is a historical British term for radar systems used to equip aircraft with the means to find and track other flying aircraft. These radars are used primarily by Royal Air Force and Fleet Air Arm night fighters and interceptors for locating and tracking other aircraft, although most AI radars could also be used in a number of secondary roles as well. The term was sometimes used generically for similar radars used in other countries, notably the US. AI radar stands in contrast with ASV radar, whose goal is to detect ships and other sea-surface vessels, rather than aircraft; both AI and ASV are often designed for airborne use.
The term was first used circa 1936, when a group at the Bawdsey Manor research center began considering how to fit a radar system into an aircraft. This work led to the AI Mk. IV radar, the first production air-to-air radar system. Mk. IV entered service in July 1940 and reached widespread availability on the Bristol Beaufighter by early 1941. The Mk. IV helped end the Blitz, the Luftwaffe's night bombing campaign of late 1940 and early 1941.
Starting with the AI Mk. VII, AI moved to microwave frequencies using the cavity magnetron, greatly improving performance while reducing size and weight. This gave the UK an enormous lead over their counterparts in the Luftwaffe, an advantage that was to exist for the remainder of World War II. By the end of the war, over a dozen AI models had been experimented with, and at least five units widely used in service. This included several US-built models, especially for the Fleet Air Arm.
The AI naming convention was used in the post-war era as well, but these generally dropped the "Mk." when written in short form and used numbers instead of Roman numerals. A good example is the AI.24 radar of the Tornado ADV. These radars were often given common names as well, and generally better known by these; the AI.24 is almost universally referred to as "Foxhunter". Other widely used post-war examples include the AI.18 used on the de Havilland Sea Vixen, and the AI.23 Airpass on the English Electric Lightning. This article will use Mk. or AI. depending on which is most commonly used in available references.
Development history
Early radar development
In order to provide the maximum possible warning time of an incoming raid, the RAF's Chain Home radar stations had been positioned as far forward as possible, right on the coastline. These systems could only see targets in front of them, over the English Channel. Tracking over land fell to the Royal Observer Corps using visual means. In testing it was found that the two different reporting systems provided information that varied enough to make tracking targets confusing and error prone, and the sheer volume of information could be overwhelming.Hugh Dowding addressed this through the creation of what is today known as the Dowding system, networking together the radars and observation centres by telephone to a central station. Here, in the Fighter Command's "filter room" at RAF Bentley Priory, operators would plot the map coordinates sent to them on a single large map, which allowed them to correlate multiple reports of the same target into a single track. Telephone operators, or "tellers", would then forward this information to group headquarters who would re-create the map, and then from group to the sector HQs who would give instructions to the fighter pilots.
Due to delays in the flow of information between the various centres, and inherent inaccuracies in the reports coming from multiple sources, this system was accurate to perhaps. Within 5 miles the fighters would normally be able to spot their targets visually and complete the interception on their own. Interception rates over 80% was common, and on several occasions the system succeeded in getting every fighter launched into position for an attack.
AI concept
While the Dowding system proved invaluable inputs during daylight attacks, it was essentially useless against night raids. Once the enemy aircraft passed the coastline they could not be seen by the radars, and the ROC could not see at night except under ideal conditions with bright moonlight, no cloud cover, and considerable luck. Even when tracks could be developed, the difficulty of spotting a target from the cockpit of an aircraft while flying it at night proved to be equally difficult. Henry Tizard wrote a memo on the topic in 1936, indicating that the Germans would likely begin a night campaign if the daylight campaign went as poorly as he believed it would due to Chain Home.The obvious solution would be to mount a small radar on the aircraft, one able to cover the range between the Dowding system's 5-mile accuracy and the average visual spotting range, about. As early as August 1936 "Taffy" Bowen, one of Robert Watson-Watt's hand-picked radar development team, personally requested that he be allowed to start research into an airborne radar set for this role. This was approved, and the small aircraft interception team set up shop in Bawdsey Manor's two towers.
At the time, radar development was in its infancy and the other teams were working with long-wavelength transmitters operating around 7 meters. An efficient antenna requires it to be about the wavelength or more, which demanded antennas at least long, impractical for an aircraft. Additionally, available transmitters were large, heavy and fragile. The first AI experiments thus used ground-based transmitters and a receiver fit to a Handley Page Heyford bomber, with an antenna consisting of a wire strung between the fixed landing gear. A working transmitter was first fit to the Heyford and flew in March 1937. In spite of this success, the system's antennas were still too large to be practical, and work continued on versions working at shorter wavelengths.
Wartime systems
AI Mk. IV
A new system working at 1.25 m was ready by August 1937 and fitted to Avro Anson K6260 at RAF Martlesham Heath. This unit demonstrated the ability to detect aircraft at the range of about in the air-to-air mode, but also demonstrated the ability to detect ships on the ocean at ranges up to. This ability led to the split between AI and air-to-surface-vessel radar systems, both of which would be widely used during the war. Practical ASV radars were operational in 1940, but the AI developments proved much more difficult.It was not until 1939, with the war obviously looming, that the team was once again moved back to AI development full-time. A lingering problem was that the minimum range remained around 1,000 feet, too long to allow easy interception. This was due to the transmitter signal not turning off sharply, leaking through to the receiver causing it to oscillate or ring for a period. While this powerful signal was dying down, reflections from nearby aircraft were lost in the noise. Numerous solutions had been attempted, but were of limited use.
Starting in late 1939 the development team was asked to fit the existing Mk. III design, of limited use, to aircraft. This ended further attempts to address the minimum range issue while they worked on installations. While their development effort ended, the headquarters staff at the University of Dundee attempted to develop their own solutions to the problem. This led to considerable strife and in-fighting between the two groups. The AI group was eventually broken up at the end of March 1940, leaving Bowen out of the AI effort.
A solution was eventually provided by EMI who had developed a new type of transmitter that was not based on the common self exciting principle. Instead, a separate squegging oscillator was used to produce pulses of the carrier signal using a timer. This timer also muted down the receiver, solving the ringing issue. Minimum range was reduced to about 400 feet. The resulting AI Mk. IV went into production in July 1940 and all units were sent to newly arriving Bristol Beaufighters. The Beaufighter/AI Mk. IV achieved its first victory on the night of 15/16 November 1940, when an aircraft from No. 604 destroyed a Junkers Ju 88A-5 near Chichester.
Several advanced versions of the Mk. IV were also produced, which offered direct readings for the pilot and options to allow use in single seat aircraft. However, these developments were overtaken by the rapid improvements in microwave systems, and both the Mark V and Mark VI saw only limited production and service.
Mk. VIII
In February 1940, John Randall and Harry Boot at Birmingham University successfully ran the first cavity magnetron, eventually generating 1 kW at 9.8 cm. Supported by GEC, the device quickly developed into a practical 10 kW system, and several test units were available by May 1940. Microwave wavelengths are so much shorter than the Mk. IV's 1.5 m, fifteen times, that the dipole antennas required for reasonable gain were only a few inches long. This dramatically reduced the size of the system, allowing it to fit entirely in the nose of the aircraft.While a team under Herbert Skinner developed the electronics, Bernard Lovell was put in charge of examining the use of a parabolic dish to improve the directionality of the signal. The resulting beam was so sharply focussed, spanning about 10 degrees, that it easily avoided ground reflections at even low altitudes. The narrow beam also meant that the radar could only see targets directly in front of the antenna, unlike the Mk. IV which could see anything in the entire volume in front of the aircraft. To solve this problem, the dish was mounted on a bearing system from Nash & Thompson that allowed it to be rotated in a spiral pattern.
The cockpit display was modified to spin the timebase at the same speed as the antenna, 17 times a second. The display still produced blips similar to those on the Mk. IV, but as the timebase now spun, they drew short arcs on the display during the period the antenna was pointed in that direction. Like the Mk. IV, the distance from the center of the CRT indicated the range. As the target moved closer to the centreline of the aircraft, the beam spent more time painting the target, and the arc spread out, becoming a ring when dead ahead.
First introduced in March 1941, it was found that the ground reflection created a sort of artificial horizon on the bottom of the display, a surprising side-effect which proved very useful. However, the limited power of the magnetron, about 5 kW, provided range of about, not a great improvement over the Mk. IV. Performance of the system at low altitude was so improved over the Mk. IV that it was decided to make an initial run of 100 units out of what were essentially prototype systems as the Mk. VII, requiring very large amount of aircraft space for the install. Conversions on the Beaufighter began in December 1941.
This run was followed by the production Mark VIII that included the new "strapped magnetron" of 25 kW, improving range to about. This version also had several major clean-ups in the electronics, support for IFF Mark III which caused a sunrise pattern to appear when aimed at friendly aircraft, and beacon tracking allowing it to home in on ground-based transmitters emplaced by friendly units. In September 1942 a Mosquito NF.II was upgraded to the Mk. VIII, serving as the pattern for the Mosquito NF.XII. Starting in December, Beaufighter units were upgraded to the similar Mk. VIIIA, an interim type awaiting production quantities of the VIII.