EMPLOYMENT OF SEARCH RADAR IN RELATIONS TO
Radar has been only one of the many weapons applied to defeat the German use of U-boats, but it has played an important role at certain critical times. Because the U-boat high command failed to anticipate the effectiveness of search radar, its use by the Allies caused especially grave concern to the U-boats. The moves and countermoves of the radar war offer an interesting example of the importance of quick and accurate evaluation of enemy measures. Although the events of World War II conform in general to the scheme of measures and countermeasure set forth in Volume 1 on Operations Research, a review of them shows that evaluation of the operational effectiveness of enemy countermeasures is of utmost importance. Even when a countermeasure has excellent theoretical performance, it is a rare event for it to be applied widely enough and with sufficient operational effectiveness to justify the extreme tactic of abandoning the weapon being countered. Usually the prompt (but not premature) application of counter-countermeasures will largely restore the effectiveness of the weapon. This has been particularly true of the radar versus search receiver competition which arose during the latter part of the U-boat war.
The following discussion will accordingly pay primary attention to the development of German search receivers and the counter-countermeasures employed to reduce the effectiveness of the German gear. In addition, the importance of Schnorchel and radar camouflage as a countermeasure to Allied search radar will be considered. There have also, however, been a number of less important German radar countermeasure developments, which are described briefly below.
1. Submergence. It is evident that the submarine can escape radar detection by staying submerged, and in the final phases of World War II German U-boats made a policy of keeping below the surface as much as possible when in dangerous waters. As pointed out in previous chapters, however, the loss in mobility and in offensive capabilities was very serious, and a normal type of U-boat (without Schnorchel) could not operate effectively using maximum submergence tactics. Such tactics as these must be considered a very unsatisfactory last resort from the submarine's point of view.37
2. Aircraft warning radar. Radar of this type (Hohentwiel) was developed and installed on a considerable number of U-boats. Its use was slight, however, and successes insignificant. This was due to two factors, the relatively low power and short range of this radar and the fear of Allied search receivers. In general the range on aircraft of the long-wave, low power U-boat radar was shorter than the range on surfaced U-boats of Allied microwave air-to-surface vessel [ASV] radar. This increased the lack of confidence in radar current among U-boat commanders which was caused by their basic objection to any radiation of energy which could be listened to and located by direction-finding devices. They were convinced of the danger of Allied listening gear long before it existed in adequate quantity to be of operational importance. This threat was sufficient to nullify the use of U-boat radar. This fact has been subsequently established from statements made by German personnel but was appreciated earlier on the basis of information obtained from ferret planes equipped with listening gear. They were dispatched to areas in which U-boats were known to be operating, but the sequence of negative reports showed that there was no significant use of the radar sets installed on the U-boats. Meanwhile laboratory developments and small-scale production of homing and direction finding search receivers anticipated a revival of U-boat radar. It is possible that submarine radar would
be a handicap rather than a help if opposed by adequate airborne listening gear.38
3. Radar decoy. Various types of radar decoys or false targets have been employed in the hopes of drawing the attention of Allied radar search craft away from the U-boats themselves. Metallized fabric-covered balloons [RDB] and arrays of reasonant dipoles on spar buoys (Thetis) have been used, especially in the Bay of Biscay. In sufficient numbers they might be expected to lead to a serious waste of search time and effort, and there has been at least one instance of a United States surface vessel being torpedoed while investigating a decoy target. In general, however, decoys have not been very successful in confusing Allied ASV radar search, largely because they proved to be too small as radar targets to be detected. In addition, careful study of target motion aids the radar operator in distinguishing stationary spar buoys or wind-blown balloons from true targets.
The most significant German countermeasures to Allied search radar have been twofold-first, the development of intercept receivers and, second, the development of Schnorchel and associated radar camouflage. These will now be discussed.
14.2 THE PROBLEM OF GERMAN SEARCH RECEIVERS
For submarines, in particular, employment of an effective radar intercept receiver provides a very satisfactory countermeasure to search radar. Upon receiving a signal of sufficient intensity from the enemy radar, the submarine can dive and become completely hidden. So long as the intercept receiver can be depended on to outrange the enemy radar (as it should, in principle, usually be able to do), the protection offered is complete. Nevertheless, such complete protection is rarely achieved in practice. (See Chapter 5 in Volume 2B.) It is valuable to describe the various stages in the radar versus search receiver competition that took place in the U-boat war as an example of the problems involved in evaluating correctly the effectiveness of countermeasures of this type.
14.2.1 Meter Radar Is Compromised by Metox German Search Receiver
Even as early as August 1941 it had been suggested, since aircraft not employing ASV radar were more successful than aircraft using ASV in sighting U-boats, that U-boats were detecting ASV transmissions. To obtain evidence of this an experiment was carried out by Coastal Command, and aircraft operating off Brest observed ASV silence on alternate weeks for a brief period. The results of this experiment, given in Table 1, showed that there was no evidence of any disadvantage involved in the use of ASV, since aircraft using radar made more contacts than those not using radar in the same amount of flying.
TABLE 1. Contacts with and without radar.
(Biscay area, September 1941.)
Radar off Radar on Flying hours in area 528 541 Contacts: Visual 3 5½ Radar --- 2½ Total 3 8
Later intelligence showed that this conclusion was indeed correct. From the start of World War II, the Germans were fully aware of the possibilities of meter ASV radar and had developed their own airborne search equipment, but it was not until the summer of 1942 that they concluded that the Allies were using radar for U-boat search and initiated a hurried program for the development of search receivers to detect the radiations. The first equipment to be installed on U-boats was the R-600 or Metox with a low wavelength limit of 130 cm. It was of the heterodyne type, thought to be the only type capable of sufficient sensitivity, and so it radiated energy, a property which eventually caused its abandonment. Nevertheless, it was used with apparent success, and the conditions of its introduction and use are of considerable interest.
FIGURE 1. Fraction of U-boat transits of Bay of Biscay that were sighted.
The German development of search receivers was closely related to the progress of the aircraft anti-U-boat offensive in the Bay of Biscay. In June 1942 the introduction of night attacks by Leigh Light Wellingtons and the accompanying sharp rise in the number of attacks made on U-boats caused the Germans to take some action, and the Metox German search receiver [GSR] was the result. Its operational success against the British Mk II radar was considerable, as sightings in the Bay transit area were greatly reduced. Figure 1 shows that the fraction of U-boat transits sighted dropped markedly with the introduction of Metox after the summer of 1942, although the amount of flying done remained approximately constant.
During the period shown in Figure 1 data on the effectiveness of radar search were also obtained in other regions than the Bay of Biscay, for example, in the Trinidad area of the Caribbean Sea Frontier.
TABLE 2. Contacts with and without radar.
(Trinidad area, Aug 1942 - Jan 1943.)
Month Hours of flying Contacts Effective search
rate in sq. miles
No radar Radar No radar Radar No radar Radar Aug 1,511 851 11 6 526 507 Sept 3,326 1,650 8 15 93 351 Oct 1,878 3,164 6 21 120 248 Nov 1,016 3,714 0 9 0 149 Dec 3,229 1,200 6 2 106 95 Jan 3,648 1,544 4 2 97 114 Total 14,608 12,123 35 55 127 241
These data, like those obtained earlier by Coastal Command, showed little evidence of any effective use of radar listening gear by the U-boats, as shown in Table 2. Two conclusions were possible, either that the use of the gear was much less effective in the Trinidad area than the Bay of Biscay or that the change in the Bay was not due to Metox but was, for example, a seasonal change. The actual situation may have involved more psychological benefit from the Metox than physical benefit. The use of Leigh Light Wellingtons in the summer of 1942 may have scared U-boats into abandoning the policy of surfacing at night even though the number of planes was small and surfacing in the daytime was still much more dangerous. With the advent of Metox they may have returned to the safer policy of nighttime surfacing. In the Trinidad area, on the other hand, there was no radical change in U-boat tactics except for a general increase in caution.
If this last explanation is taken as the correct one it is still, in a sense, fair to say that Metox caused the change in Bay of Biscay results in the fall of 1942. It would not be safe, however, to conclude that Metox was, therefore, a very effective search receiver. The actual mechanism linking cause and effect was a more subtle one. Whatever the explanation of the apparent success of Metox, it was short-lived, for the Allies soon introduced S-band (10-cm) radar and the effectiveness of air search reached even higher levels than before.
14.2.2 Germans Baffled by S-Band Radar
Meanwhile, Allied development of airborne S-band radar was proceeding, based on the magnetron transmitter tube, and it was put into operational service early in 1943 as the U.S. SCR-717 and ASG, and the British Mk III types. This new wave band not only provided immunity from detection by Metox but also gave increased ranges of detection on U-boats.39 The operational success of this type of gear was considerable. Aided by the seasonal upswing in aircraft effectiveness normally occurring in the spring months because of better weather and more daylight hours, S-band radar had much to do with the peak of aircraft achievements reached in the summer of 1943.
Evidences of the success of the air war against
FIGURE 2. Fraction of U-boat transits of Bay of Biscay sighted.
U-boats at that time are shown in Figures 2 and 3. Figure 2 presents data on the sighting of U-boats passing through the Bay of Biscay. The sharp rise in the spring months is very evident. In Figure 3, which gives the overall results of the aircraft offensive in terms of U-boats sunk per month, there is also a sharp rise at this time. S-band radar was by no means the only cause of this increase in air effectiveness, but there is no doubt that it made a significant contribution. The other major factor was the introduction of aircraft based on escort carriers for mid-ocean offensive operations.
With this upswing in Allied aircraft success, the Germans became convinced that Allied aircraft were using some new detection device and started a frantic activity to identify and counter it. For a time they occupied themselves with the idea that it was an infrared detector, since they had tried to develop one of their own, and experimented with special paints intended to give no infrared reflections. They also considered the possibility of a frequency-scanning radar and developed a scanning receiver with a cathode-ray tube presentation. This was of definite advantage to the operator, but it still covered only the same meter-wave band. The sinkings of U-boats continued.
FIGURE 3. U-boats sunk per month by aircraft.
In desperation they jumped to the conclusion that their GSR radiations were being homed on. The Metox receiver was outlawed and the Wanz G-1 introduced. This was of an improved design and radiated much less power. However, the almost pathological fear of radiation which had been bred in the minds of U-boat captains prevented them from trusting it. Continued sinkings and skepticism of the technical advantages kept it from being used.
Next the German scientists turned to the much less sensitive crystal detector-receiver, which was entirely free from radiation, and produced the "Borkum." This was a broad-band intercept receiver which covered the 7-300 cm band. Neither it nor the Wanz was effective against the new Allied radar, however.
Finally, in September 1943, the U-boat command realized that 10-cm radar was being used against them. The "Rotterdam Gerat," a British H2S radar working in the 10-cm band, had been captured intact at Rotterdam by the German Air Force in March 1943, and German scientists had soon determined its characteristics. How the 6-month delay from March to September occurred is unexplained. It was a significant time factor in the U-boat war. A further delay of about 6 months intervened before the first really effective S-band receivers became operational in April 1944. During this interval the frantic experimentings of the German Technical Service became evident in such incidents as the patrol of the U-406 carrying one of their best GSR experts, Dr. Greven, and his staff, with a full complement of experimental
search receivers. The U-406 was sunk, and other experimental patrols also had short careers.
The drop in results of the Allied air effort at the end of the summer of 1943, which is shown in both Figure 2 and Figure 3, was only in small part due to GSR developments. The U-boats simply adopted an ultraconservative policy of maximum submergence and rarely exposed themselves to air attack. In passing through the Bay of Biscay they crept along the Spanish coast, in regions inaccessible from Britain, and surfaced as little as they could. With such tactics U-boat effectiveness was very low, but they gained respite from air attack.
14.2.3 Naxos Search Receiver Covers S-Band
Out of this confusion finally came the "Naxos" intercept receiver covering the 8-12 cm band. The first models introduced in the fall of 1943 were crude portable units mounted on a stick and carried up through the conning tower upon surfacing. The range was short, because of the crystal detector principle, the broad-band coverage, and the small, non-directional antenna: estimates of range from prisoner-of-war reports were 8 to 10 miles.
Allied reaction to intelligence reports about Naxos as early as December 1943 brought the fear that S-band radar was compromised. Even earlier than this (November 1942) "disappearing contacts" had led many to assume compromise long before 10-cm search receivers were thought of by the Germans. A serious morale problem developed among Allied ASV flyers with this news and the drop in U-boat contacts. Radar was turned off completely in several squadrons where its use could only have resulted in more numerous contacts. Tactics were improvised to salvage some usefulness for the radars on the assumption that the GSR would outrange the radar (an assumption that was largely false). Some of them are listed below.
- Prohibition of special radar procedure during the approach, such as "searchlighting" the target, sector scan, or change of scan rate, since such changes would indicate to a GSR operator that radar contact had been made, and the U-boat could then take evasive action. (It was considered unlikely that U-boats would dive immediately on receiving a signal on GSR.)
- Attenuators, such as "Vixen," were initiated to cause a slow and steady decrease in transmitted power as range closed and so to confuse the GSR operator. In order to use Vixen successfully, the contact must be made at a range of 15 miles or greater and the cycle started soon after. Since this is greater than the average radar range under many conditions, it could only be used for less than half the contacts. Production was slow and installation slower, with the result that Vixen had no operational opportunity for justifying the effort spent in its development.
- An interim tactic of "tilt-beam" approach was proposed to reduce signal intensity as range was closed by tilting the radar beam up off the target. This requires unusual skill and cooperation between pilot and radar operator to be effective, and its value has never been adequately proved.
- Almost in desperation the tactic of turning the spinner aft (for the 360-degree scanning radars) and approaching by dead reckoning was suggested. The chances of a successful navigational approach are small, however, as compared with radar homing on the target, and this proposal was not very promising.
A serious error in some Allied thinking at this phase consisted of overestimating the capabilities and efficiency of the Naxos GSR. It was felt that such a search receiver would completely nullify the effectiveness of S-band radar, but such did not prove to be the case. The data presented in Table 3 show the effectiveness of night flying during the period October 1943 to January 1944 in the Bay of Biscay (the region in which use of GSR might be expected to cause the largest reduction).
If we consider that the discrepancy between expectation and results obtained is entirely due to
TABLE 3. Results of S-band radar night flying.
(Bay of Biscay, October 1943-January 1944.)
* This poor showing probably due to lack of experience.
Type of aircraft
on basis of
Wellingtons, British Mk III 62 15 23 Halifaxes, British Mk III 32 18 15 Liberators, ASG, searchlights 30 12 14 Liberators, ASG, no searchlights 25 1* 13
149 46 65
GSR, we would conclude that it enabled 25 per cent of the U-boats that would normally have been contacted to escape detection altogether, 44 per cent to dive after being contacted by radar but before being sighted visually, leaving 31 per cent that were still sighted, even with GSR. At the most, then, it can be concluded that a reduction of 69 per cent was caused. As an overall average, day and night, including other areas, the reduction would be much less, possibly 25 to 50 per cent. Since the sweep rate in S-band radar search is under most conditions several times that for visual, the use of radar was still imperative.
Efforts were made, therefore, to revive the confidence in radar and keep it in operation. Contacts by S-band radar continued to be made, and it became evident that the Germans did not have much confidence in the effectiveness of their search receivers, nor did they use them consistently. In the coming months they were to become more and more committed to the use of Schnorchel rather than search receivers to defeat Allied radar.
14.2.4 X-Band Radar and Tunis GSR
Since an S-band search receiver could, in principle, be highly effective, the use of radar of an even higher frequency was an obvious next step as a countermeasure to Naxos. Development and allocations of X-band (3-cm) equipments even preceded the advent of Naxos and were further stimulated by the problem it presented. However, the Germans were not caught napping this time. An H2X blind-bombing aircraft was lost over Berlin in January 1944, and from the damaged remains the Germans learned of the new frequency band. It was assumed that this frequency would also be applied to ASV radar, and the development of X-band intercept receivers was started before use of the X-band radar by the Allies in U-boat search had produced many results. A well-designed receiver was developed, known as Tunis, which consisted of two antennas, the Mucke horn for X-band, and the Cuba la (Fliege) dipole and parabolic reflector for S-band, and installations started in the late spring of 1944. Installations seem to have been completed during the period of inactivity following the withdrawal to Norwegian and German bases, but the operational use of Tunis was not great, because of the reliance placed on Schnorchel from that time on.
The chief feature of Tunis was the directional antennas which gave increased sensitivity and range. To obtain full coverage, the antennas were rotated manually at about 2 rpm. Allied tests on captured specimens of this gear showed good performance with a range of about 15 miles on an aircraft at 100 ft increasing to 40 miles or more at altitudes in excess of 1000 ft. Ranges obtained in operational use were undoubtedly somewhat shorter, but Tunis was apparently an effective search receiver.
The outstanding tactic proposed for Allied use against Tunis was intermittent radar operation. A schedule of two or three radar scans at intervals of 1 to 2 minutes for a narrow-beam radar will point the beam on target for only a small fraction of the time. Since the sweeping GSR beam is only occasionally pointed at the aircraft, and since a coincidence of these events is necessary for the GSR to achieve a detection, the probability of detection by GSR can be made rather small. Discussion of this procedure is given in Volume 2B, Chapter 5. Since it was not used to a significant extent in operations, it will not be discussed further here.
14.3 USE OF SCHNORCHEL AND CAMOUFLAGE
Apparently dissatisfied with search receivers as a means of achieving immunity from radar detection, the U-boat Command turned to more drastic measures during the last year of the war. The development of Schnorchel was carried out during the latter months of 1943 and fitting of the equipment to U-boats began early in 1944. Using Schnorchel for Diesel intake and exhaust, the U-boat could run at periscope depth with only a small Schnorchel head showing above the surface, yet accomplish the ventilation of the boat and recharge of batteries that previously had required surfaced operations. Starting in the spring of 1944, U-boats spent very little time on the surface, employing Schnorchel or electric propulsion almost exclusively.
Original Schnorchel gave reduced radar echoes merely because of their small size. As a result the average range of detection is reduced to about a third of that on a surfaced U-boat. In addition, the echo is frequently too small for detection until it has entered the area of sea returns which mask it quite effectively, and many potential Schnorchel contacts are missed completely for this reason. Even without camouflage, a Schnorchel is a difficult target to detect by radar. Not content with this state of affairs,
the Germans developed non-reflective coatings for application to Schnorchel which still further reduced the echo from it. Fortunately this camouflage reached operational use only a few months before the end of World War II.
14.3.1 Radar Detection of Schnorchel
In order to evaluate the performance of radar in detecting Schnorchel numerous trials have been conducted with dummy or mockup Schnorchel targets. It is not intended to present a summary of them here. The values quoted in Table 4 are typical. The figures show a serious reduction in both range and reliability of detection as compared with that on a surfaced U-boat. Depending on the radar set involved and the sea state, the reduction in sweep rate varies from 75 per cent to 95 per cent, even though the position of the Schnorchel was approximately known in these trials, which would make detection unrealistically easy.
TABLE 4. Test results on Schnorchel detection.
Type Radar AN/APS-15 ASG Average range on surfaced sub (miles) 32 19 Average range on Schnorchel (miles) 10.5 4.1 % runs on which contact made on Schnorchel
Sea states 1 and 2
Sea states 3 and 4
Operational results have been even more discouraging. For the period November 1944 to March 1945 an analysis was made of a region near the British Isles which contained about 0.25 U-boat per 1000 sq miles. Assuming that they had been Schnorcheling about a quarter of the time, the expected number of sightings made by the 19,360 flying hours done in the area would be:
Number of sightings = area swept X density of Schnorchels
The total number of sightings and disappearing radar contacts made at night was 16, so that
The sweep width is then about 1/10 of a mile, only about I per cent of the value on a surfaced U-boat. In operations, therefore, Schnorchel has been very successful in countering radar detection. In the daytime, when visual sightings are also possible, its immunity from contact is not quite so great. The sweep width has been estimated at 0.6 mile.
14.3.2 Countermeasures to Schnorchel
A number of countermeasures to Schnorchel have been employed, among them the following.
- Modifications in tactical doctrine to match the decreased radar sweep width (see Volume 2B). This is not a universal scale factor, but must be analyzed for each search plan. For example, the shorter aircraft radar range is partially compensated for by the decreased submarine speed.
- Radar modifications to improve the efficiency of contact, such as sea return discriminator circuits.
- New radar developments of high-power, narrow-beam, short-pulse equipments designed to provide better resolution in search for small targets.
These have, however, by no means solved the problem, and Schnorchel remains a very difficult object to detect. In the overall picture of ASW, surface craft have played a very important part in combatting Schnorchel operations. The decreased speed and mobility of a Schnorcheling submarine require it to operate in relatively restricted waters and focal areas. In such conditions search and counteroffensive operations by surface craft have good chances of success. As mentioned in Chapter 13, the use of sono-buoys for detection of Schnorchel has promise.
14.3.3 German Camouflage Developments
The general problem of radar camouflage had been under intensive study in Germany since June 1943, but the decision to apply absorptive coatings to Schnorchels was not made until the fall of 1944. Plans were made and put into effect at this time to provide microwave protection for all U-boats, and it is estimated that 100 to 150 craft were actually fitted with coated Schnorchels before the end of March 1945.
Two types of coatings were employed, the Jaumann and Wesch absorbers. The Jaumann absorber was made up of spaced graduated layers of semiconducting
paper. The Wesch absorber was made up of rubber mat containing a high percentage of iron powder.
The Jaumann absorber was the more effective, and German reports appear to be reliable in that the ran--c of detection on a Schnorchel coated with it was only 15 per cent of the range on the uncoated Schnorchel. One of its disadvantages, however, was that it had to be preformed and could be used only on flat and cylindrical surfaces. The absorptive properties of the Wesch absorber were not so good.
It may be concluded, therefore, that the use of such absorbers would make Schnorchel practically safe from detection by 10-cm (and also 3-cm) radar. The Germans were concerned with the possibility of danger from radar of longer wavelengths, but it is doubtful that radar of this type would be very effective against such small targets. Use of a variety of frequencies would counter the effectiveness of these absorbers to some extent, until absorbers are developed to cover the whole microwave band.
It can thus be seen that this succession of measures and countermeasures, tactical and otherwise, represented, in the main, a series of concessions by the U-boats to the effectiveness of radar-equipped aircraft. The result has been a great reduction in the mobility and attack potential of the U-boats, which, however, has gained them considerable immunity from airborne radar search. This situation was recognized very clearly by the Germans, and their large-scale program for the development and introduction of new types of U-boats of increased underwater speed and endurance was an attempt to overcome these limitations. Whether future submarines, which will probably be forced to operate almost totally submerged (or on a Schnorchel), can achieve the success which submarines had in World War II is a problem for speculative analysis beyond the scope of this discussion. New technical developments make it seem quite possible, however.
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