Signals Museum
TRE July 1945
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T. R. E. JOURNAL
ISSUED BY THE
TELECOMMUNICATIONS RESEARCH ESTABLISHMENT
MALVERN
July 1945

THE RADAR BATTLE OF THE BAY OF BISCAY
By A.C.B. Lovell Ph.D.

This article was written in August 1944 but publication had to be delayed because of security reasons.

1. INTRODUCTION
The capture of the French West Coast ports by the Germans in 1940 enabled them to establish submarine bases on the edge of the Atlantic. The Bay of Biscay then became a"transit area", i.e. an area through which the U-boats passed to and from their bases but which was not primarily their operational area. For many reasons which need not concern us here, such an area is potentially a very good one for anti-submarine work; and in the case of the Bay, since it was within reasonable range of aircraft based in the U.K., a violent and lengthy aircraft versus submarine battle soon developed. Apparatus developed in TRE played a crucial part in this battle, and the purpose of thin note is to give a short review of events as seen from TRE.

2. JUNE 1942
Until the summer of 1942 the U-boats working from the Bay ports had matters largely their awn way. If they felt any danger of attack from aircraft in the day-time, they submerged and surfaced at night for recharging. Although 1½ metre ASV did exist in night flying aircraft, the U-boat was an extremely difficult object to see except under clear full moon conditions, and the U-boats were not seriously hampered in their passage to and from the Bay Ports. A considerable fleet of U-boats was therefore maintained in the Atlantic, menacing the convoy routes, and. the merchant shipping losses reached a very high figure.

In June 1942, however, an important development was put to operational use. A powerful searchlight, with a. flat topped beam and an azimuth spread of about 11° - the "Leigh Light" - was mounted in a retractable cupola underneath the fuselage of Wellington aircraft fitted with 1½ metre ASV. Instead of search in the dark for a U-boat which had been picked up and homed onto by the ASV, the Leigh Light operator was now able to switch on this searchlight when about 1 mile distance from the target. The operator could control the azimuth and elevation of the searchlight beam to facilitate the illumination of the U-boat. The attack was then carried out almost under daylight conditions. This sudden illumination and attack during what had previously been regarded as"safe" hours upset the U-boat crews so much that, although the total number of night-sightings during June and July were only about 20, by August the U-boats were no longer surfacing at night, but during the daytime, for recharging purposes. This enabled the day forces of anti-submarine aircraft to be deployed with such good effect that in September 1942 nearly 40 sightings occurred in daylight. Statistically this was equivalent to 100 sightings of U-boats known to be operating in the Bay area. Fig. 1 shows these facts graphically. Curve A gives the night sightings by Leigh Light aircraft. Curve B shows the rise in day sightings following the peak in A, and Curve C the total sightings expressed as a percentage of the estimated number of aircraft using the Bay. The importance of this curve is the demonstration that the effects in A and B were tactically real and not due to an increase in the number of U-boats operating.


Figure 1.

Unfortunately the U-boats quickly upset this favourable situation by the countermeasure of "listening" to the ASV transmissions. This enabled them to submerge when their receiver indicated that an ASV transmission was homing on to them. The density of the anti-submarine aircraft patrols was insufficient to make the U-boats submerge so frequently that it became a nuisance to them, and the pre-June 1942 conditions were rapidly restored. Thus during the winter of 1942 and early months of 1943 the sightings of U-boats by night and day were very few and after a decrease in shipping losses due to a seasonal lull and the North African operations, the losses took a sharp turn upwards in January and February 1943 and showed signs of reaching unprecedented proportions.

3. MARCH 1943
The incidence of the U-boat listening to the aircraft metre transmissions had one obvious technical solution, at least as a temporary measure - namely to make a major change in the wavelength of the radar transmissions. 10 cm. ASV had been under development in TRE even before the development of H2S started in late 1941, but by the autumn of 1942 it was obvious that due to the greater pressure on the production of H2S, this equipment would be available many months before the production of the ASV. Fortunately H2S had been designed so that it could be used as ASV - the project was known in TRE as H2S/ASVS - and although many people objected to the loss of certain refinements the main issue at stake - that of defeating the listening - was so great that a decision was made in the autumn of 1942 to divert some of the H2S equipment for fitting into Leigh Light Wellingtons for use as 10 cm. ASV. The main difference between the H2S as fitted in the Bomber Command heavies and the Leigh Light Wellingtons was in the scanner. In the Wellingtons the only possible position for the scanner was in a chin under the nose (see Fig.2).


Figure 2.

This entailed a re-design to an aperture of 28" instead of 36" and certain other adjustments necessitated by operations at 2,000 feet instead of 20,000 feet. (The chin position prevented complete all round looking and caused a blackout in a 40° sector directly behind the aircraft).

There were many unnecessary digressions during these months. For example, the insistence of the RAF on the inclusion of Lucero and Babs facilities diverted much technical effort at a difficult period and was not comparable in importance with the main task of getting some cm. ASV flying over the Bay. Similarly the insistence on fitting the equipment in Wellington XI's caused a very considerable turmoil and eventually fizzled out. However, in spite of this two prototype "ASV Mark III" installations in Wellington VIII aircraft (LB129 and LB135) arrived at Defford in December. A series of technical troubles then emerged mainly due to the essentially different layout from the heavy bomber installations. The worst of these was caused by pick-up in unscreened crystal leads which ran from the T2R unit to a crystal current meter in the Indicator Unit.

These troubles were largely surmounted, however, early in 1943, and by January 30th 1943 there were 2 Wellington XII aircraft and one of the prototype VIII's at Chivenor. As the Wellingtons arrived at Chivenor an urgent modification to the aerial system was made - substitution of the original
dipole feed to a waveguide feed - and this considerably improved the performance. By February 27th there were 12 ASV Mark III's at Chivenor and probably one of the largest concentrations of TRE technical effort ever seen on one squadron to keep them going. In the evening of March 1st, just one month after the first H2S operation by Bomber Command, two ASV Mark III Wellingtons took off from Chivenor for the first Bay Patrol with 10 can. ASV Mark III. One of our greatest doubts about the apparatus - that of serviceability - was soon dispelled. The pulse transformer which caused so much trouble in Bomber Command reacted more favourably to the lower altitude treatment in Coastal and the serviceability was far better than expected. On the night of March 17th 1943 the first submarine was picked up on the ASV at a range of 9 miles by H538, but the Leigh Light jammed
and no attack was made. The next night H538 obtained another sighting at 7 miles range and this time succeeded in attacking with 6 depth charges - "both times the submarine was fully surfaced and under way, showing no signs of suspecting attack".

From then on a most extraordinary change occurred in the Bay battle. The ASV Mark III's obtained 13 sightings in March and 24 in April and this had precisely the same tactical effect as the successful night operations in June 1942 - namely the U-boats were so scared by the night attacks that they submerged during darkness and surfaced in the daytime. This time, however, the day forces of Coastal Command were much superior in strength and possessed more lethal weapons than in the summer of 1942. Consequently, in May 1943 an absolute slaughter of the U-boats occurred. Nearly 100 daylight sightings were made during that month which represented a 204 sighting of U-boats known to be operating in the Bay area. The graphs in Fig. 1 show the remarkable similarity between the events of June/September 1942 and march May 1943 - the peak in day sightings following two months after the peak in night sightings. In 1943, however, the Germans had no immediate technical countermeasure - they were taken completely by surprise and were forced to fight the battle in daylight while the ASV Mark III Wellingtons kept them submerged at night. By June 1943 our merchant shipping losses had fallen suddenly to a very low level - and stayed there. In fact a month or two later, Hitler announced "- - - the temporary setback to our U-boats is due to one single technical invention of our enemies - - ".

Installations of ASV Mark III were later carried out in Halifaxes and Sunderlands. The Sunderland installation was, in fact, the main requirement against which the original ASVS was developed. It uses double scanners, one underneath each wing, in order to obtain all round looking; these rotate in synchronism and the r.f. power is switched from one to the other just before the beam becomes obstructed by the fuselage. When the H2S type of apparatus superseded the ASVS development the scanners and synchronising system were, with slight alterations, maintained as originally planned.
The r.f. unit is situated In the hull and the power carried through the wings to the scanners by waveguides, otherwise the apparatus is similar to the normal ASV Mark III.

4. WINTER 1943-44
During the inception of ASV Mark III there were many pessimistic estimates of how long it would be "safe" from the German countermeasure of listening. There was certainly justification for this pessimism because of the use of H2S over enemy territory and hence the certainty that the enemy would quickly recover apparatus from lost aircraft and know what frequencies to listen on. In the Spring of 1943 steps were therefore taken in TRE to develop new ASV systems which would defeat the enemy's listening countermeasure should it ever be employed. It was clear that a further jump in frequency, say to X band, might succeed in getting outside the range of the search receiver, but that the solution would be very temporary and would only last until the enemy extended the scope of his search receiver to cover the new frequency. As cover for any acute emergency, however, the X band H2S Mark III system was developed so as to be capable of use as ASV and a trial installation known as ASV Mark VII was carried out in a Leigh Light Wellington. The differences between this ASV Mark VII and H2S Mark III in the heavies were essentially the same as the differences between ASV Mark III and H2S Mark II, i.e. the main change being in the scanner design.

The main effort was, however, put into an attempt to find a more permanent solution to the listening problem and from this development ASV Mark VI emerged containing two principal new features:-

(a). Increase of transmitter power from 50 kW to 200 kW
(b) Use of an attenuator to enable the radiated power to be controlled by the operator.

The increase of power (a) together with various other improvements was expected to increase the range of pick-up by 1½ to 2 times. The effect of this increased range in a transit area is that a given number of search aircraft will got more sightings and homings on to U-boats and hence it becomes possible to create a situation where it is very unprofitable for the U-boat to dive on receipt of every warning of approach given by his listening receiver. The use of the attenuator (b) by the ASV operator, when the homing run is commenced, enables the signal strength of the transmission as heard in the U-boat's listening receiver, to be kept constant. Under these conditions, listening gives the enemy no warning of actual approach by the aircraft and he must either assume that the aircraft is not homing or else dive at extreme range of pick-up.

During September and October 1943 the number of"disappearing contacts" on ASV Mark III showed an appreciable increase, and evidence accumulated that the enemy was beginning to make efficient use of a listening receiver ("Naxos"). The conversion of ASV Mark III to ASV Mark VII was easier than to ASV Mark VI, but the Air Staff ruled that, since ASV Mark VI would soon be available, no X band equipments could be spared from Bomber Command for this purpose, unless the U-boat situation became much more serious. (H2S Mark III was at that time coming into use for the bombing of Berlin). Therefore, although plans had been laid for 50 conversions of ASV Mark III to ASV Mark VII, the programme never proceeded beyond the trial installation in one aircraft. The conversion of ASV Mark III to ASV Mark VI proved a very difficult installation problem, and the first machines were not delivered to Coastal Command until the end of January 1944.

Fortunately the morale of the U-boats had not recovered sufficiently from the summer battle to enable them to stage a major come-back and the delay in providing ASV Mark VI and the cancellation of ASV Mark VII did not have the serious consequences which might have been anticipated from the trend of events in September and October 1943.

5. CONCLUSION
The use in night operations of a small number of Wellington Leigh Light aircraft fitted with a converted H2S Mark II equipment (ASV Mark III) caused a major change in the U—boat battle in the Bay of Biscay during the first half of 1943. In fact, the Minister of Aircraft Production's remark during a recent visit to TRE that this was the most significant single event which had happened in the war would appear to be well justified. The re—occupation of France by the Allies has now ended that particular battle. The development of new ASV's based largely on the original H2S units has also reached the reasonable limit of usefulness in Mark VI and VIE. There are, however, serious ASV problems still to be faced, not only in the Pacific, but also in countering, for example, the increasing use of Schnorkel by the U-boats in the European war. The new ASV developments thick are required for this and which will no doubt eventually supersede ASV Mark VIB do not directly belong to the H2S chain of development, and therefore have not beet dealt with here.


Page last updated on 15th February 2018 by Colin Hinson.