German Search Receivers and Radar
- Introduction to Allied radar and the first German search receiver
- The introduction of centimeter radar
- The "Magic Eye"
- Measures against infra-red detection and short-timed radar
- The Germans take stock
- The radiation specter
- The first non-radiating receivers
- Discovery of the use of centimeter wavelength radar
- Schnorchel and GSR
- German U-boat radar
1. SummaryIn the German navy's fight to preserve the "invisibility" of the U-boat, it's chief enemy in direct combat through most of the war proved to be Allied airborne radar. At the beginning of the war the U-boats had been able to range far at sea, often to intercept convoys without fear of advance detection by aircraft, and to sink their prey and get away before any defensive force could be effectively brought to bear against them. The equipping of aircraft engaged in anti-submarine operations with radar began early in 1941 and gradually initiated a contest between Allied radar and German countermeasures which became very acute by 1943. Among the counter-devices upon which the Germans pinned their hopes were search receivers designed to intercept radar transmissions and thus inform U-boat's in advance when they were in danger of attack by a radar equipped aircraft.
Several different models of the GSR were developed, with varying degrees of success. The first model, though poorly constructed and primitive, served its purpose. When the Allies resorted to centimeter wavelength radar, however, the tables were turned and a long period of desperate searching for the cause of Allied successes followed. For many months the German effort to determine that cause failed completely. The first step taken was to improve the GSR models currently in use. When that failed to produce results the U-boat Command turned to investigation of the possibility that infra-red detectors were being used by the Allies.
In the summer of 1943 the German navy became obsessed with the idea that the enemy was profiting by detection of radiations from the GSR. New models were developed to eliminated radiation, this at a great loss of valuable time. It was not until September 1943, after the Germans had discovered that Allied aircraft were using centimeter radar, that U-boats were equipped with GSR capable of dealing with all wavelengths in use by the Allies. Even this search receiver was inadequate, owing to its very short warning range and the fact that it never had a permanent aerial. Since the introduction of Schnorchel early in 1944 did not eliminate the possibility of detection by radar, it was necessary to fit the Schnorchel mast with an aerial, which,
however, was incapable of receiving centimeter wavelength transmission at distances more than 1,000 meters.
German radar, under development and in use concurrently with GSR, took a definite second place in the opinion of U-boat commanders. Confidence in its capabilities was expressed from time to time by Command, but fear that Allied aircraft were equipped with search receivers caused the commanders to neglect its use until the spring of 1944. Thereafter radar became more popular with the commanders and proved of value as a warning against aircraft and as an occasional aid in convoy shadowing, firing and navigation.
2. Introduction to Allied radar and the first German search receiverThe use by Allied aircraft of the Anti-submarine Visualizer (ASV) radar, operating on meter wave lengths, was instituted early in 1941 and soon proved its value in increased sinkings of U-boats, including not only those in the vicinity of convoys, but also those in other areas. Mounting Allied successes, together with the appearance of aircraft so frequently as to eliminated the possibility that their presence was due to visual sighting, aroused the Germans to the fact that some locating devise was being used against them. They were not backward in technical matters and were quite aware of the possibilities of meter wavelength radar. Aided by the capture of an ASV Mark II (150cm) in the spring of 1942, they concluded that radar was the cause of the U-boat's troubles and set about finding a means to counteract its effect.
The answer to their problem, proved by tests in the summer 1942, rested upon the fact that transmissions of a radar set could be received by means of a simple aerial and receiver; hence a U-boat could discover the presence in its vicinity of a user of radar and submerge before it could be successfully located. A makeshift search receiver, manufactured by the firm of Metox in Paris and designated the R600, was hastily put into production and designed to cover a range of 60 to 265 centimeters, including harmonics, and probably had a maximum effective detection range of about 6 miles. Hurriedly contrived as it was, it showed the varied and serious weaknesses which might have been expected under the circumstances. Its generally poor structure, particularly its primitive aerial, resulted in frequent breakdowns; its
usefulness was limited by its inability to distinguish between radar and non-radar disturbances, its inability to take a bearing and its inability to receive short transmissions. In addition, the set was a source of powerful radiations when in operation, a matter which was dismissed as of little consequence at the time, but which was later to be remembered, and exaggerated in the mind of the German technical expert to a significance out of all keeping with common sense, this at great cost to the U-boat.
Despite all its weaknesses, however, the Metox set was operationally successful for some time. It was improved in some respects as time went on, mainly in the substitution of a fixed basket aerial ("Runddipol") for the old cross type, addition of a second oscillator, and installation of a "Magic" mechanism; but it remained the only standard U-boat search receiver for more than 6 months. U-boat radio traffic for November and December 1942, the first read here, abounded in reports of "Ortung" (radar) and plainly showed the success of the German counter-device. That radar had been successfully counteracted for the moment was also shown by the general picture in the Atlantic war, for Allied shipping losses had continued high in the latter part of 1942 despite improved convoy arrangements. The U-boat still maintained its decisive advantage.
3. The introduction of centimeter radarIn February and March 1943, a new refinement in the war of detection in the form of ASV Mark III (10cm), a set operating on centimeter wavelength, was introduced on Allied aircraft. The R600, with its 60 centimeter lower limit, was quite unable to detect transmissions on the new very short wavelengths, and the result was a gradual increase in the number of sightings of U-boats and attacks against them, accompanied by an increase in the proportion attacks carried out without prior appreciation by the U-boat that radar was being used. In his message 1631 of 30 May 1943, for instance, Kentrat (U-196) reported that he had been flown at twice during the hours of darkness by aircraft "without radar"; and in his message 1019 of 26 September, Schroeteler (U-667) reported: "So far 5 attacks at slight distance without any radar. Cruising at night is impossible, since there is no time for outmaneuvering." A more correct interpretation was recorded by Hartenstein (U-156), who reported in his 0105 of 7 March
1943: "A new kind of radar, Metox is useless. Very accurate attacks without searchlight." The deteriorating situation quite naturally became evident to the Germans before many months had passed, but they were totally unaware of the cause of it, and contrived counter-measures, most of which were aimed at the wrong objective.
4. The "Magic Eye"The first attempt to solve the new detection problem centered about improvements to already existing search receiver sets, the Metox R600 and the Grandin, a receiver which covered the same frequencies as the former. It was first assumed that Allied radar transmissions were now producing headphone vibrations outside the range of audibility. Working on this hypothesis, a visual tuning indicator of the "Magic Eye" type was introduced into the improved Metox R600A, a development mentioned in ENIGMA traffic in message 2105 of 10 March 1943. By the rarest good fortune, it happened that naval aircraft in the Mediterranean area were using supersonic modulation at the time, and the success of the "Magic Eye" in detecting these particular aircraft evidently led the German technical experts to believe, for as long a period as a couple of months, that they had solved the problem. Evidence for this appeared in message 1217 and 1304 of 23 May in which it was indirectly stated that the "Magic Eye" indicated inaudible impulse frequencies and that outfitting of U-boat's would continue. But the continued Allied successes, representing the worst situation for U-boats since the beginning of the war, could not permit continued reliance on the "Magic Eye."
5. Measures against infra-red detection and short-timed radarThe next line of attack taken by the technical authorities was in the field of counter-measures for infra-red detection. The suspicion that infra-red detection might be the secret of the new Allied success was based upon reports for U-boats at sea mentioning a dull red glow emanating from attacking aircraft. The German technical staff turned its attention with characteristic singleness of purpose to the discovery of a means of counteracting the suspected bogey. Infra-red detectors were carried by some U-boats for experimental purposes, but
difficulties in operations of the device precluded formation of any definite conclusions from the lack of successful experiments with it, and the Germans wasted the whole summer of 1943 clinging tenaciously to their suspicion that infra-red was the cause of their troubles. U-boats were covered with a special paint calculated to absorb infra-red rays and other precautions were taken, yet sinkings by Allied aircraft continued. Simultaneous research directed toward developing means of detecting the suspected use of short-timed radar as an anti-interception technique also produced no visible results in a decrease in U-boat sinkings.
6. The Germans take stockAfter several months of failure by the Germans to find the cause of their misfortunes, it was apparent that the U-boat Command and its commanders had lost faith in their search receiver. In the Bay of Biscay U-boats began to spend the nights underwater. Preferring to take a chance on visual sighting of an enemy during daylight for the period of time necessary for ventilating and charging the batteries. In July 1943 the Director of the Naval Communications Division, German Naval War Staff, admitted in long communication concerning the search receiver service that the "invisibility" of the U-boat was gone, that the element of surprise in U-boat attacks had been eliminate by new Allied location methods which had so far proved inaccessible to interception. The Director outlined plans for overcoming the German deficiency, including increased use of radar itself by naval and coastal defense forces. Taking note of many serious mistakes made in the past, he announced extensive changes in organization and procedure aimed at strengthening radar and radar interception research and practices, and stated that the Commander in Chief of the navy had promised his special support toward attaining that end, as indeed Admiral Dönitz had promised U-boats in May.
7. The radiation specterFor some reason, the fact that search receiver radiations were capable of being detected at a distance of several miles suddenly began to plague the Germans in the summer of 1943. No new technical knowledge was concerned in their
appreciation of that fact, for they had known of that possibility when the search receiver was first introduced and had discounted its importance as a threat to the U-boat. The Allies had also investigated the practicability of using search receiver radiations for homing purposes and had abandoned the idea because of the difficult operating conditions aboard aircraft. But now the Germans, still searching for the cause of Allied detection successes, fastened upon this specter as the object of their feverish activity during the next several months. Tests on the Metox receiver showed that it produced very powerful radiations, and in the middle of August use of both Metox and Grandin was prohibited because of the fear of benefit to the enemy. (PSRE 0410/15)
To make sure that the prohibition was observed, it was ordered that the U-boat commander should remove an essential part of the set, keep it under lock and key and enter that action in the war log. (PSRE 2022/17) The Metox was replaced by a new GSR called Wanz Gl or Hagenuk, details of which were passed to OP-20-S in a memorandum of 6 October 1943. This set, designed for frequencies between 120 and 150 centimeters, was originally produced for the specific purpose of detecting suspected short-timed radar transmissions, but by chance produced radiations only about one twenty-fifth as powerful as those emitted by the R600. Hence it was also an improvement over the old R600 in other ways, for scanning with the visual indicator was automatic, and only after a transmission had been picked up was manual scanning for sound reception necessary. In addition, the set had a greater range (about 30 miles for an aircraft at about 1,000 feet altitude, 60 miles at 6,500 feet), and greater sensitivity. Yet, in spite of all these improvements, especially the greatly reduced radiation, sinkings continued.
The German mind was still not convinced that radiation was not its great enemy. On 5 November 1943 messages were sent instructing U-boats not to use their Wanz Gl because freedom from radiation had not been established. (2049/5 November) Instead they were to be supplied with either Wanz G2 or Borkum search receivers. As further insurance against radiation ordinary radio programs were to be received by U-boats east of 18°W. while submerged, using VL/F.
8. The first non-radiating receiversThe Wanz G2 was a later version of the Gl, as its name implies. Covering the same frequency range, it had the advantage of practical elimination of radiation, though at considerable cost in warning range (only about 37 miles with aircraft at 6,500 feet altitude). Another of the stop-gap devices, it was subject to frequent breakdowns, but all its imperfections were accepted in favor of the release from fear of detection through radiation. The Borkum set, designed to receive all signals in the 75 to 300 centimeter band without tuning, had been first introduced in the summer of 1943. Out side the band mentioned its sensitivity decreased, but it is possible that it gave some indication even of 10 centimeter transmissions. An emergency installation, its efficiency was considerably below that of the Wanz G2 (about one fourth, according to 0246/16 June 1944) and was used in practice chiefly when the latter was out of order. The Borkum, too, produced radiations below evaluable magnitude, so the Germans had a long last conquered their imaginary enemy. The real enemies still remained.
9. Discovery of the use of centimeter wavelength radarIn some way U-boat Command became aware by September 1943 that centimeter wavelength radar was in use against them, a discovery which should have led to the speedy solution of the radar problem. Actually the search receiver developed for use against the newly recognized threat at first defeated its own purpose because of its lack of sensitivity. The Naxos set was designed for the 3 to 12 centimeter band, in which receiving would have been difficult enough with an excellent set. The Naxos was far from excellent. It was very delicate, therefore subject to continual breakdowns. A particular weakness was the antenna, which was subject to frequent breakage because of the use of porcelain rods in its construction, and which had to be mounted each time the U-boat surfaced and taken down again before submerging. The disadvantage of this was strikingly illustrated in the case of U-625, whose commander was lost at sea because of difficulties which developed in removing the antenna in time for a crash-dive to avoid an attacking aircraft. (0532/4 January 1944) The warning range of the set with its original antenna was
pitifully small, so small as to render it almost useless, but every effort was made to increase its efficiency, particularly by improvement of the antenna.
The first improvement of this nature was the introduction in early February of the Fliege, a reflector antenna giving approximate bearings and operating in the 8 to 12 centimeter range. Use of this antenna greatly increased the range of Naxos against 10-centimeter radar, making it an effective detection device. In addition the Fliege was used separately with some success for aiming anti-aircraft guns in advance of appearance of the target. Another antenna, the Miicke, was put into operational use in May 1944 as a complement to Fliege to cover wavelengths from 2 to 4 centimeters, for it had been discovered through capture of a crashed aircraft that 3-centimeter radar was in use by the Allies. Also a directional antenna, the Miicke covered approximately the same warning range as the Fliege. These two antennae were later incorporated into a single frame called "Tunis", and the Naxos set continued in use against centimeter wavelength radar until the end of the war.
On the radar question as a whole very extensive experimental work was done in the spring of 1944. In February and April, respectively. U-406 and U-473 left port fully equipped to investigate every type of Allied radar. Both boats were sunk, but others carried on constant investigation of Allied frequencies. "Feldwache," a combination of several different sets, was used for experimental searching of all wave bands from 320cm down to 5cm. A summary of results of experiences presented as a Current Order on the use of Tunis (0602/22 June 1944) indicated that the Naxos-Tunis combination would remain the chief instrument for search receiving, but that experimentation would meanwhile continue.
10. Schnorchel and GSRWhile the introduction of Schnorchel in 1944 proved a blessing to the U-boats, it did not eliminate entirely their susceptibility to location by radar when the Schnorchel was extended, although the radar target was considerably smaller than that of the whole U-boat. It was therefore necessary to devise a means of search receiving for the schnorcheling U-boat. This was brought about by including a drum type aerial,
called the Schnorchel round dipole, in the Schnorchel mast. The round dipole was capable of receiving radar transmissions on meter wavelengths, but according to message 2003/10 of July 1944, it could only register Fliege range radar at the short distance of 1,000 meters. The Germans apparently believed it practically impossible for a Schnorchel to be picked up by centimeter radar because of its rubber camouflage protections and considered search reception over the round dipole adequate protection from meter wavelength radar. At the end of the war they were trying to develop a permanent centimeter aerial for the Schnorchel mast. That they had not succeeded was shown by a reply from Germany to Surabaya's inquiry concerning the installation of such an aerial. Berlin replied:
At present there is still no gear available for observing centimeter radar during Schnorchel cruise.(PPB 86, 3 April 1945)
Type XXI U-boats were to have carried such aerials.
11. German U-boat radarIt was known from the first two months of U-boat traffic read here that at least some U-boats were fitted with radar and that its wavelength was 80cm. There was no indication that U-boats made any real use of radar. Reference in traffic were very few until the summer of 1943, when U-boat Command started a campaign to persuade U-boat captains that their own radar should be used for aircraft detection in lieu of the forbidden GSR. Among the significant developments in the radar picture, easily followed by reading of German traffic, was the continuing question of U-boat radar's merit relative to the GSR. There was great reluctance to use radar because of the fear that Allied aircraft were fitted with search receivers. Owing to the fact that GSR ranges were greater than those of radar, it was not surprising that the U-boats preferred to rely on GSR for warning against enemy aircraft, especially in view of the radiation scare of 1943.
There appeared to be a split between command on the one hand and the operating U-boats on the other as to the advisability of using the radar sets provided. This was chiefly illustrated by the periodic appearance of instructional messages from the COMSUBs emphasizing that the German
radar was an effective instrument and should be used. Occasionally there was direct reprimand for failure to use radar under circumstances favorable to its use. In June 1943, for instance, there were two messages (2343/5 and 1252/25) which directed that radar should be used as much as possible, one of them stating:
"Our own radar sets are well suited to establish the presence of airplanes independently of their use of radar, for example during surface cruise altitudes of 500 meters and more at a distance of 10 kilometers at the least. For the time being, enemy search reception of our radar is not to be presumed."
A slight shift in sentiment on the latter point was demonstrated in PSRE 0257/9 September, in which, while use of radar was still stated to be the only means for locating the enemy under certain conditions, it was pointed out that
"it is necessary to be prepared at all times for enemy search receiving activity."
A memorandum to COMINCH sent on 22 October 1943 recorded the fact that German radar seemed destined to occupy a role secondary to GSR an opinion which was borne out by future developments. Nevertheless, COMSUBs continued to exhort his boats to use radar, and provided them with improved sets to make its use more effective. The original set supplied, the Gema, had been a cumbersome, unsatisfactory set operating on 80 centimeters. As technical development progressed, improvements were made to the Gema and a new set, the Hohentwiel, operating on 50-55 centimeters, was introduced. Both the improved Gema and the Hohentwiel represented advances both in case of operation and in increased effective range.
There were continual reports of sets out of order, however, leading to a statement by COMSUBs that failures were often due to insufficient tending and lack of training. (1840/1 April 1944) Meanwhile the effort of Command to popularize the use of radar had continued, with three separate messages devoted to the subject to February alone (2141/4, 1752/16, and 1622 ff.) There were indications in March that the reluctance of U-boat commanders had been overcome to some extent, for several instances of successful use of radar were
reported. As time went on that reluctance was further broken down, with many reports of aircraft successfully detected by radar before they were able to attack. There was also increased use of radar for locating the shadowing of surface vessels. In addition, examples were recorded of U-boat's using radar as the means of aiming at both attacking aircraft (1947/16 October 1944) and surface targets (0308/15 December 1944) It was also pointed out that Hohentwiel could be successfully used as a navigation aid in hazy weather. (0018/5 December 1944)
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