Serrate Radar

Serrate Radar System

30 Jun 1943 – First use of Serrate radar homing equipment fitted in RAF nightfighting aircraft. The receivers allowed Beaufighters to home in on German night-fighter interception radars.

SERRATE was a British radar device designed to detect German night fighter radar transmissions from their Lichtenstein aircraft interception radar (AI).  At that time, Luftwaffe fighters were causing increasing number of losses amongst RAF bombers attacking targets in Germany and Occupied Europe. The German fighters were using their FuG 202/212 A.I. radar to locate their targets on an established frequency of 485-505 Mc/s.  On 6th January 1942 ACM Leigh- Mallory, AOC Fighter Command, wrote to the Under Secretary of State saying that since the frequency of the German AI radar had now become known, there should be a possibility that a homing radar device could be designed to lock onto the transmissions of the German fighter radar. The suggested device should enable his night-fighters more effectively to seek and destroy the enemy.

There was a group at the time called Air Interception Committee (AIC) who were charged with the responsibility of overseeing the development of devices to aid the interception of enemy aircraft.  They approved the proposal for research into ‘homer radar’ on 3rd April 1943.  As a result, Telecommunication Research Establishment (TRE) based at Malvern were requested to investigate the possibilities.  The result was a homing radar device with the code name of SERRATE. The German radar was in turn using transmissions from sets in the British bomber that were switched on during the whole trip to the target and return.  Later in the war, aircrews were instructed to reduce severely the time radars were activated to reduce the chance of detection.  The RAF bombers at that time did not carry radar that could detect the presence of the German fighters in the vicinity.  The SERRATE system was capable of detecting the German night fighter up to 100 miles away. The system could indicate bearing but not range with the result that a 2nd radar aid had to be used in conjunction with it. This was the Air Interception Radar AI Mk lV which itself had been in use since 1940.  The AI radar could detect an aircraft by bouncing radar signals off the mass of an aircraft at a much shorter range than SERRATE. Homing in on the radar transmission of an enemy aircraft was not the function of the AI Mk lV radar. The theory was that the SERRATE would detect the enemy fighter at long range and when the range was suitably reduced the AI Mk lV could be used for the final interception.  A common set of two Cathode Ray Tubes (CRT) was used for both radars and the operation of a switch would convert one to another.

 More advanced AI systems were available at the time but were generally used over UK Airspace.  Since the squadron was expected to serve over continental air space it was felt that it was more prudent to use the earlier version in case of the inevitable destruction and capture of some aircraft.  Under these requirements the shortcomings of the AI Mk lV were ignored.  The A.I.C. then required the SERRATE Radar system to consist of the following:
A SERRATE homing tuneable receiver operating over the 485-505 Mc/s radio band.
Twin cathode ray tubes (C.R.T) serving both AI and SERRATE devices through an interconnecting switch.
An aerial system giving both azimuth and elevation
An AI Mk lV interception radar.
GEE Mk 2 navigation system
Identification Friend or Foe (I.F.F) interrogator.  The I.F.F interrogator was a device to differentiate friendly aircraft from enemy and was in general use throughout Bomber Command.

The service trials of SERRATE took place using the Beaufighter Mk. VI(f) aircraft of 141 Squadron which was then in 12 Group of Fighter Command.  A captured Ju 88 was used to familiarise the observer/ operators with that of their forthcoming opponents.  The service trials took place between June 14/15 and September 6/7, 1943, which resulted in a total of 179 complete sorties being undertaken. Twenty enemy aircraft were either destroyed or damaged during these initial sorties.  In spite of these successes, experience quickly proved that the Beaufighter lacked both speed and manoeuvrability to catch the German aircraft. It was calculated that 1 combat resulted from every 11 sorties and that a small number of crews were much more successful than others. For example, Wing Commander Braham achieved 9 out of the 23 successes.  On 3rd of July 1943 a letter was addressed to the Chief of Air Staff by ‘Bert’ who was ACM Arthur Harris; later Sir Arthur. He said that SERRATE appeared to be highly successful and requested that Fighter Command assemble a force of 100 night-fighters fitted with SERRATE to destroy the German night-fighters.  This would encourage the morale of Bomber Command crews and the new force should be used to support every major raid. Portal replied that such an increase in the resources was not practical but he was prepared to authorise the formation of 3 night-fighter squadrons, which would be met by disbanding the Ranger operation flights to form the new squadrons. The new units were to be 141,157 and 169 Squadrons; 239 Squadron is also on record as having been involved.

The Bomber Support Group No. 100 under command of Air Cdre E.B. Addison was formed on 3rd December 1943. It had 2 objectives, to blind the enemy electronic eyes and ears and 2nd to destroy the defending Luftwaffe night-fighters. 141 Squadron with its Beaufighter aircraft Mk. IV(f) was transferred to 100 Group Bomber Command on December 3rd 1943. As the year ended it was equipped with a mixture of Beaufighter and Mosquito II aircraft. The 2 other night-fighter squadrons followed 141 into 100 Group and were fitted with the SERRATE Mk. II systems. These too were scheduled for re-equipping with Mosquito II aircraft.  The SERRATE Mk. II radar was fitted in the nose of the Mosquito Mk. II in place of the more usual nose mounted 4 machine guns.  The RAF was scoured in the search for Mosquito Mk II aircraft; 60 Operational Training Unit (OTU) for example had all its Mosquito Mk II aircraft transferred to 100 Group, and replaced with other Marks of Mosquitos.

1692 Flight had its early history in the latter half of 1942 as the Radar Development Flight.  In June 1943 it was designated 1692 Flight dedicated to training aircrews in the use of SERRATE. It too was transferred to 100 Group in 7th December 1943. For training purposes Defiants and Beaufighters were fitted with radar sets of similar characteristics to that of the FuG202/212 Lichtenstein radar sets used by the German night-fighters. As the Group’s Mosquito units increased in numbers the flight undertook training in other areas.  141 Squadron’s first operation with the new group took place on December 16/17th 1943. The Mosquito proved to be little improved upon the Beaufighter, being old and fitted with now unreliable engines. In February – March 1944 the 100 Group replaced all the engines of their Mosquitos with Merlin 22 version

An experiment was tried whereby SERRATE equipped aircraft undertook close escort of the bomber stream. This proved to be a failure since signals from the nearby bombers swamped the CRT screens of the SERRATE fighters. It proved to be much more productive to fly parallel to the bomber stream, 30 miles distant or to patrol the assembly beacons in France around which German fighters would gather awaiting instructions.  In the second period commencing 8th December 1943 the number of SERRATE contacts began to decrease.   The widespread use of ‘WINDOW’ and SERRATE by the RAF, had rendered the German AI radar almost useless.  Unbeknown to the RAF, in about June 1944, the Luftwaffe introduced gradually a new version of their radar set the Lichtenstein SN2 into their twin engine fighters. This change commenced somewhere between August and October 1943.  It was February 1944 when No. 80 Wing noticed a decline in Lichtenstein transmissions, which raised the possibility that the Germans had developed a new Lichtenstein radar set. In a crash programme the Germans had produce 200 sets by February 1944 and 1,000 sets by May 1944.  This new radar employed a frequency range of 80-85 M/cs.  The use of the new German radar led to an important change in tactics against the bomber streams and this in turn led to the Luftwaffe’s greatest victories against Bomber Command. The use by the Luftwaffe of their new Zahme Sau tactics and their Lichtenstein SN2 radar meant that comparatively few of the newly equipped fighters were effective.  It was some time before the RAF fully understood the new tactics.

The Germans were well aware that the British were homing onto their early Lichtenstein AI systems.  In their new radar they selected a frequency from within the band already used by their ground plotting stations. This subterfuge proved to be very successful since a British operator would pick up the more numerous and powerful signals put out by the Freyas. This made the ordinary British receiver of little practical use in trying to find and DF on to relatively weak SN2 signals. In addition, the new German radar sometimes emitted a direction finding note thus making it easier for other German night-fighters to locate the bomber stream.

Fitting the new radar to the German fighters had the effect of steadily decreasing the number of contacts with Luftwaffe night-fighters that were registered to 141 Squadron aircraft. In addition, apparent jamming of the AI Mk. IV radar sets took place. This effect was found to be by enemy jamming but by a defect inherent in the design of the Mk IV AI radar. This fact was confirmed after the war by a German historian. In an attempt to alleviate the problem, all AI Mk. IV radars were retuned to 188 MHz from 193 MHz but this did nothing to alleviate the situation.  For a period, the RAF night-fighters could no longer depend upon their SERRATE radar to locate the enemy and had to depend upon their short range AI Mk. IV systems.

The Bomber Support Development Unit (BSDU) was formed at Foulsham during April 1944 as a trials and development unit for 100 Group.  It carried out a wide range of functions for 100 Group including their Radio Counter Measures (RCM) activities such as jamming as well as supporting the SERRATE programme.  It had in addition a small production facility to make short runs of radar sets or components for use in modifications. As an example, PERFECTOS was another type of ‘Homer’ radar set which was said to have been developed and manufactured at BSDU Foulsham.  The use of the Lichtenstein SN2 led to the urgent development of SERRATE Mk IV as a method of homing onto this new German radar. The signals were presented to the Mosquito navigator aurally through a set of normal headphones rather than through a cathode ray tube (CRT). The DF (direction finding) is conveyed to the operator by coding the signal through ‘dots and dashes’ as with a Lorenz beam. This was necessary because of the number of CRTs already in use in group aircraft for the navigator to read, had almost reached their maximum.

When the Germans came to design their SN-2 radar, they were aware that the British were able to lock their AI radars on to their earlier design. To make their new radar more secure, they selected a frequency within the band already used by some of their ground plotting stations – the Freyas. This simple approach met with considerable success because if a receiver was used to pick up the weaker SN-2 signals it would also pick up the more numerous and much stronger signals of the FREYAS. They also made their new radar to have facilities for front and rear scanning.  In the SERRATE Mk. IV, using audio-frequency filtering, the FREYAS signals were almost completely removed while the SN2 signals came through the headphones as a characteristic high pitched note with a pronounced tremor due to the ‘Split’ transmitter aerial. This became operational in January 1945.  The establishment of the frequency of the SN2 became an urgent problem to solve. It was not completely established until a Ju 88 landed in error at Woodbridge at July 13th 1944.  The Lichtenstein SN-2 was impervious to the original British ‘WINDOW’ but when during a raid on Stuttgart in July 1944 the lengths of RAF ‘WINDOW’ had been increased the German night fighter operators decided their radar has developed a fault and, with other night-fighters returned to base.

SERRATE Mk. IV and SERRATE Mk. IV(a) became available at about the same time. The Mk. IV was a modification of the TR1143 receiver while the TR1430 was used as a basis of the SERRATE Mk. IV(a).    SERRATE Mk. IV appears to have been fitted to 157 and 169 Squadrons only, with 141 being omitted.    Later model Mosquito’s eventually replaced the older aircraft but for a time they were diverted to ‘Operation Diver’ which was the destruction of Flying Bombs (FZG 76).  A SERRATE Mk. VI appears to have been developed but little is known about its development at this time.  A number of authors have expressed the opinion that SERRATE was an operational failure but this is not the view of the author. The reduction in contacts during one period was due to the introduction of ‘WINDOW’ by the British and it had an unfortunate effect upon SERRATE Mk. II. This was remedied relatively quickly by the introduction of the clever design of SERRATE Mk. IV.