electronic warfare

electronic warfare, post-war term for what in the Second World War were known as radio counter-measures and radio counter-counter-measures. Radio counter-measures were those measures taken to counter an enemy's radar and radio systems. Radio counter-counter-measures were those measures taken to reduce the effectiveness of the enemy counter-measures. Although these techniques were used predominantly by and against aircraft, they were also employed on a much smaller scale in land and naval warfare.

The first occasion on which radar jamming was employed in action was during the battle of Britain when, in September 1940, the Luftwaffe attempted to neutralize the Chain Home early warning radars (20 to 52 MHz band) along the south coast of England. A ground station was set up at Mont Couple near Calais and employed a number of Breslau spot-noise jammers. The Breslau jamming caused some difficulties to the British radar operators, but at no time was it so serious that the plotting of incoming German formations was degraded to the point where they were able to avoid interception.

Meanwhile the RAF was building up its own jamming organization, No. 80 Wing, to counter the recently discovered Knickebein beam navigation system (see electronic navigational systems) employed by the Luftwaffe. The first jammers were hastily improvised by modifying hospital diathermy sets into low-powered transmitters. But soon afterwards a purpose-built jammer, code-named ASPIRIN, was designed and built to counter Knickebein.

ASPIRIN radiated Morse dashes on the German beam frequencies. The dashes were superimposed on the German signals, but not synchronized with them. Some accounts have stated that No. 80 Wing deliberately bent the German beams, making the bombers release their bombs almost to order. There were certainly wartime rumours to this effect, but it did not happen. The idea of beam-bending was seriously considered, and it was technically feasible, but such was the urgency of the programme to counter Knickebein that there was no time for such refinements. It is possible that on occasions the British dashes and the German dots did come together to produce some sort of bent beam, but there was never any deliberate re-aligning of the Knickebein beams. As a result of the ASPIRIN jamming, the German navigational system never achieved anything like its full potential.

In the autumn of 1940 a scientist in British intelligence, R. V. Jones, discovered that the Luftwaffe was using a new electronic navigation system, the X-Gerät (X-apparatus), which operated on a higher frequency than Knickebein (66 to 75 MHz) and was considerably more accurate (see Map 29). A suitable jammer was built, codenamed BROMIDE, and by mid- November 1940 the first few had been rushed into service. The third of the German beam navigational systems to be employed over the UK during the Blitz in 1940 and 1941, the Y-Gerät, was similarly countered by the DOMINO jammer. With the deployment of bomber units in preparation for the invasion of the USSR in June 1941 (see BARBAROSSA), large-scale night bomber attacks on Britain (the Blitz) came to an end.

The next major incident in the jamming contest occurred on 12 February 1942, when the German Navy sailed the battle-cruisers Scharnhorst and Gneisenau through the English Channel to Germany (see CERBERUS). As part of the elaborate preparations for the operation, one or more jammers were set up on the north coast of France to counter each radar on the south coast of England. Just before the warships came within range of the British radars, the jammers were switched on simultaneously. The jamming caused considerable confusion and slowed the British reaction, and as a result the German warships had passed through the strait of Dover before the first attacks were launched against them. The attacks were unsuccessful.

As part of the air defences for the German homeland, the Luftwaffe set up a chain of Himmelbett night fighter control stations, employing Freya (125 MHz) and Würzburg and Giant Würzburg (both on 570 MHz) radars (see Kammhuber Line). Night fighters were fitted with Lichtenstein radar (490 MHz). The main early warning radars were the Wassermann and Mammut (both of which operated in the 125 MHz band). In December 1942 RAF bombers began employing the MANDREL spot-noise jamming transmitter to counter Freya, Mammut, and Wassermann. The effect of the jamming was to slow the German defensive reaction during attacks, and it reduced the loss rate of bombers. The Luftwaffe replied by widening the frequency band used by the early warning sets, to dilute the effect of the jamming. The RAF replied by increasing the power and frequency coverage of MANDREL and, later, bombers were fitted with two or more jammers. The battle between MANDREL and the German early warning radars continued throughout the rest of the war.

In 1943 the RAF introduced a new type of counter-measure, WINDOW: strips of metal foil released from aircraft (called ‘Chaff’ in the USA, Düppel in Germany and Giman-shi in Japan) to jam the Würzburg, Giant Würzburg, and Lichtenstein radars. WINDOW was first used on a large scale on 25 July, when each of the 746 aircraft attacking Hamburg released large quantities of foil. The appearance of so many false targets on the radar screens saturated the defences and only 12 aircraft, 1.6% of the force involved, were lost. That was about one-quarter the loss rate usually suffered during attacks on this heavily defended target.

During the autumn of 1943 the US strategic bomber forces in Europe also began employing radar counter-measures to reduce losses. The main target of their jamming was the Würzburg radars used for anti-aircraft fire control. The APT-2 CARPET, a spot-noise jammer, was first used in action in October 1943 and brought about an immediate reduction in losses. A couple of months later, the US bombers began to use ‘Chaff’ in addition to noise jamming, to counter German radars.

To reduce the vulnerability of Würzburg to ‘Chaff’ and electronic noise jamming, the Germans launched a crash programme to modify the radar. One anti-Chaff modification (NUREMBERG) provided an audio output so that the operator could hear the propeller modulation of the returning echo pulses and so track the target aircraft through ‘Chaff’. Another system (WÜRZLAUS), was a crude form of moving target indicator. To avoid the jamming from CARPET the band of frequencies used by WÜRZBURG was progressively widened, until by the end of the war it ran from 475 to 585 MHz. CARPET was modified to meet each change, as soon as it was discovered. It has been estimated that ‘Chaff’ and noise jamming used together reduced the effectiveness of 'WÜRZBURG-controlled Flak by about two-thirds.

During the winter of 1943–4 the German night air defences were reorganized and new and modified equipment was introduced. For a time it was possible to overcome the most serious effects of the jamming and regain the initiative. A new radar for night fighters was introduced, the SN-2 (90 MHz), which because of its lower frequency was unaffected by the type of WINDOW released by British bombers. With an effective airborne radar again, the German night fighter force achieved a dramatic run of successes culminating on the night of 30 March 1944, when 94 bombers were shot down out of a force of 795 attacking Nuremberg. But three months later the RAF captured a Junkers 88 night fighter complete with SN-2 radar. Flight tests of the captured fighter revealed that SN-2 was vulnerable to ‘Rope’ (long lengths of foil, which unreeled or unfolded during its fall) and, from the summer of 1944, it suffered the same fate as its predecessor.

The Normandy landings in June 1944 (see OVERLORD) represented the greatest seaborne invasion in history. For this giant set-piece battle the stakes were high, and a massive counter-measures operation was set in motion to neutralize the dense chain of radar stations built into the German Atlantic Wall. First, ground direction-finding stations in southern England pinpointed the positions of the radar stations. Then fighter-bombers flew some 2,000 sorties against these targets. Finally, on the night of the invasion, an enormously powerful barrage of ship-borne and airborne jamming was turned on the surviving radars. Two squadrons of modified bombers flew up and down the line of the River Somme jamming on the German fighter control frequencies; their aim was to prevent German night fighters west of the line receiving instructions from their control stations to the east of it.

While the invading fleets headed for the landing areas on the coast of Normandy, two ‘ghost’ fleets headed towards Le Havre and Boulogne (Operations TAXABLE and GLIMMER, respectively). In fact these ‘fleets’ contained no full-sized ships; the illusion was created by aircraft flying carefully-planned orbits and releasing large quantities of WINDOW. Each ‘ghost’ fleet involved eight aircraft divided into two waves, with 3 km. (1.8 mi.) between individual aircraft and 13 km. (8 mi.) between each wave of four. The two waves of aircraft flew a series of oblong patterns measuring 13 km. long and 3 km. wide, maintaining their formation. During the long legs of the orbits, when the planes were flying towards or away from the coast, they released one bundle of WINDOW every five seconds; that is to say, one bundle per 400 m. (438 yd.) flown. In this way the planes laid out a vast field of WINDOW measuring 25 km. by 22 km. (16 mi. by 14 mi.), with no gaps larger than the discrimination limits of the German Seetakt coast-watching radar. Each orbit took seven minutes and at the end of each the formation moved forward 1.6 km. (1 mi.). The whole vast field of WINDOW thus appeared to move towards the coast of France at 15 km/h (8.5 knots)—realistically like an actual invasion fleet. To add realism to the operation, other aircraft flew near the WINDOW droppers radiating noise jamming. But the positions of these aircraft had been carefully chosen so that German radars watching the area would just see the ‘ghost’ fleet through chinks in the jamming.

German records revealed that the defenders were forced to tie down large numbers of troops during the crucial initial hours of the invasion, and send reconnaissance aircraft and patrol boats to scour the coast to the east of the Somme, before the ‘ghosts’ were exorcized. The level of confusion on the night of the invasion was so great that the only German radar to pass plots on the real invading ships went unheeded. The first positive indication of them, which the Germans did believe, came not from radar but from observers on the eastern side of the Cherbourg peninsula who heard the rumble of ships' engines. No conceivable counter-measures effort could possibly have achieved more.

During 1944 the RAF formed a new unit, 100 Group, to support night bombing operations. The group operated five squadrons of specialized jamming aircraft: B17 Fortresses, B24 Liberators, Halifaxes, and Stirlings. It also operated six squadrons of Mosquito night fighters, which were to serve as escorts for the bombers, and eventually built up its strength to about 250 aircraft. Special equipment was necessary to enable the Mosquito crews to find German aircraft in the darkness, in a sky full of friendly ones. One such device (SERRATE) homed on emissions from the German fighters' radars. Another device (PERFECTOS) fitted to the Mosquitos transmitted interrogating pulses on the German IFF (Identification Friend or Foe) frequencies. When German aircraft replied, the signals appeared on a screen showing its range and bearing. Several German aircraft were shot down following PERFECTOS contacts.

In the Pacific war, as in Europe, the use of electronic counter-measures had become firmly established by 1945. Despite early research into the cavity magnetron, at that stage of the war Japanese radar technology was at roughly the same position as that of the Germans had been in 1941; airborne radar for night fighters was still at the trials stage and the early warning and fire control radars were relatively crude. Nevertheless, so long as the radars remained unjammed, Japanese anti-aircraft guns could have caused serious losses to B29 units attacking the Japanese homeland. To counter them, the US bomber force hurled a massive jamming capability built to 1945 standards of technology.

During daylight attacks the B29s flew in tight formations with each aircraft radiating jamming and dropping ‘Rope’ to counter the Japanese fire control radars; in this way there was a high degree of mutual screening between the aircraft in each group. During the night attacks the bombers radiated jamming and dropped ‘Rope’ in the same way but, because the aircraft were more scattered, there was far less mutual screening between bombers. To overcome this problem a few B29s were fitted out as specialized jamming aircraft, to cover the night raiders. These aircraft, nicknamed ‘Porcupines’, had their bomb racks removed and in their place each carried up to eighteen separate jamming transmitters and almost a ton of ‘Rope’. During attacks, the ‘Porcupines’ orbited the target area and jammed the defensive radars.

In the face of such a concentrated barrage of counter-measures, the ill-equipped Japanese radar system collapsed. During operations against Japan the B29s' average loss rate from the enemy defences was 0.8%, a remarkably low figure and the more remarkable considering that, in terms of anti-aircraft guns, the Japanese cities were by no means weakly protected. The great conurbation of Tokyo, Yokohama, and Kawasaki, for example, was defended by more than 500 anti-aircraft guns. The defenders' problem was not lack of fire-power, but lack of effective fire control due to the jamming.

What was the effect of the various electronic counter-measures on the course of the Second World War? The neutralization of the German radio beam systems, in 1940 and 1941, undoubtedly played an important part in enabling the UK to survive the heavy night attacks of the Luftwaffe. And without the successful anti-radar attacks and the jamming effort, the fight to secure the beachhead in Normandy in June 1944 would certainly have been far bloodier.

The value of the jamming support given to the RAF and the US strategic bombing forces cannot be assessed accurately, because it is impossible to calculate how many more bombers would have been shot down had there been no protection from jamming. Probably the jamming cover for the night attacks saved the RAF something of the order of 1,000 bombers and crews during the course of the war. The USAAF bombers made most of their attacks by day, when the enemy fighters and A-A gunners could usually engage visually and jamming was less effective; nevertheless, there are grounds for believing that the use of electronic counter-measures saved about 400 US heavy bombers and their crews over Europe, plus a further 200 of those attacking targets in Japan.

Only twice during the Second World War did the use of electronic counter-measures bring about the complete collapse of an air defence system. The first was when WINDOW was introduced with such effect during the attack on Hamburg in July 1943, though within a few months the Germans had recovered from its worst effects. The second was during the spring and summer of 1945, when American bombers effectively countered the primitive Japanese radars by the use of ‘Rope’ and noise jamming. For the rest of the war the effect of the counter-measures was a continual, if less spectacular, reduction in losses by something in the order of one-sixth. See also strategic air offensives and TINSEL.

Alfred Price

Bibliography

Price, A. , Blitz on Britain (London, 1977).
—— Instruments of Darkness (London, 1967).