Serrate was a World War II Allied radar detection and homing device used by night fighters to track Luftwaffe night fighters equipped with the earlier UHF-band BC and C-1 versions of the Lichtenstein radar. It allowed RAF night fighters to attack their German counterparts, disrupting their attempts to attack the RAF's bomber force.
The first Serrate systems were developed from the AI Mk. IV radar, which just happened to have antennas almost perfectly suited to receiving the Lichtenstein signals. Serrate simply disconnected the display from the radar's own transmitter and receiver and connected it to one tuned to the Lichtenstein. This produced a display indicating the direction to the German aircraft, but not the range. When the operator felt the range to be suitable, the display was reconnected to the Mk. IV's own electronics to provide both ranging and directional information during the last moments of the attack.
Serrate operations began by No. 141 Squadron RAF on the night of 7 September 1943. 179 operational sorties yielded 14 claimed fighters shot down, for 3 losses. After that point, the Luftwaffe realized what was happening and quickly introduced new versions of their radars working on different frequencies. Unlike the earlier sets, which just happened to operate on frequencies very close to the British radars, the new sets required entirely new detectors, which took some time to develop. New versions of Serrate were introduced, and moved from the Bristol Beaufighter to the faster de Havilland Mosquito, but later operations were never as successful as the original ones.
The AI Mk. IV radar was the first operational airborne interception radar, first used experimentally in April 1940 before entering widespread service in early 1941. These systems used a set of four receiver antennas that were arranged so they were most sensitive in different directions; two were sensitive above or below the aircraft, and the other two to the left and right. The output from these antennas was rapidly switched back and forth on the display, producing two blips for any given target, with the length of the blip indicating the strength of the signal in that direction. By comparing the length of the blips from, say, the upper and lower antennas, the operator could determine if the target was above or below their fighter.
For purely practical reasons, the antennas were shorter than ideal, about half the length they should be for maximum gain of the 1.5 m signals. However, the sensitivity was not critical given other limitations of the set, and the use of smaller antennas produced less drag on the aircraft. Ironically, their length just happened to be near perfect to receive the signals from the German radars, which worked on shorter wavelengths in the 50 cm range. The suggestion was made early on that the system could be modified to display the signals from the German radars simply by connecting the antennas and display to a receiver tuned to the German frequencies.
By late 1941 a new generation of radars was appearing that were based on the new cavity magnetron instead of the traditional tube-based electronics of the Mk. IV. These operated at much shorter wavelengths, about 9 cm, thus requiring much smaller antennas while also providing much higher angular resolution. The production AI Mk. VIII radar version became widespread by late 1942 and were installed primarily on the de Havilland Mosquito, leaving the problem of what to do with the older Bristol Beaufighters mounting Mk. IV. This difficulty arose at the time that the use of the H2S radar was being debated within Bomber Command, with the concern being expressed that an aircraft lost over Europe would reveal the secret of the magnetron to the Germans.
At this point the idea of using the Mk. IV equipped Beaufighters as intruder aircraft was once again raised. Lacking a magnetron, these presented no security risk, and offered a reasonable way to use these now obsolete night fighters. This was aided by the landing of a fully intact Junkers Ju 88R-1 night fighter in May 1943 in Scotland, by its defecting crew, revealing the latest operational frequencies of the German radars.
The technique described here is for the Monica tail warning radar: The technique developed was for the RAF nightfighters to fly slowly off the bomber stream, mimicking the characteristics of a heavy bomber, until the rearward-facing Serrate (Monica) detector picked up the emissions from a Luftwaffe night fighter approaching. The Radar Operator would then pass directions to the pilot until the fighter was 6,000 feet behind, at which point the Beaufighter would execute a swift turn onto the tail of the German night fighter, pick up the enemy aircraft on his forward radar and attempt to down it.
Serrate was also subsequently fitted to de Havilland Mosquito nightfighters.
No. 141 Squadron transferred to No. 100 Group Bomber Command in late 1943 and during the Battle of Berlin on the night of 16/17 December, a Mosquito crewed by Squadron Leader F. F. Lambert and Flying Officer K. Dear made Bomber Command's first successful Serrate-guided operational sortie when they damaged a Bf 110 with cannon fire. The Serrate night fighter offensive preceded far greater and wide-ranging support operations by the specialist 100 Group during 1944-45.