Monday, May 11, 2015

Instruments of Darkness




LORENZ
A prewar, civilian radio beam navigation system adapted as a bomber aid by both the RAF and Luftwaffe. It comprised a radio signal broadcast by the airfield and received passively by approaching aircraft. It had a limited range of about 20 miles. Its main importance was to aid wartime development of the Knickebein and X-Gerät beam systems.

KNICKEBEIN
A Luftwaffe electronic navigation aid for night bombers in which two directional radio beams were broadcast to intersect over a target in Britain. The bombers followed one beam—guided by Morse dots and dashes—until it met the second, then released their bomb load. It was an advance in both range and accuracy on the prewar Lorenz blind-landing system used by civil aviation. By July 1940, the RAF developed a counter, code-named “Aspirin,” which imposed a British beam atop the German beam in a process called “bending the beam,” although the German beam was never actually “bent.” The Luftwaffe next moved to the X-Gerät system, as the “battle of the beams” continued.

ASPIRIN
The centrepiece of the British Electronic Counter Measures (ECM) effort was however ‘Aspirin’, a high power transmitter which emulated the signals from the dot modulated lobe of the Knickebein. This had the effect of confusing German pilots, who even when centred in the Knickbein beam would hear the dots emanating from the Aspirin jammer. By October 1940 the British had deployed fifteen Aspirin jammers to frustrate the Knickebein system. The stakes were high. Estimates of the accuracy of the Knickebein suggested that it was capable of putting a 300 m x 300 m box around an intended target, which when saturated with bombs from a mere 40 aircraft would put the bombs down on average 17 metres apart.

X-GERÄT
A sophisticated Luftwaffe beam navigation system. It was derived from the prewar Lorenz and early wartime Knickebein systems, which the RAF successfully countered by July 1940. X-Gerät used four beams and a bomb-release mechanical computer that was triggered by passing over the three cross beams. It was deployed by a special target-marking unit of the Luftwaffe, which then dropped flares and incendiaries to mark the target for follow-on bombers.

BROMIDE
The British built the ‘Bromide’ jammer, using radar hardware, to defeat the X- Gerät system. While the Bromide equipment was being developed, KG.100 conducted no less than forty raids using X- Gerät. These raids were essentially an ‘Opeval’ to determine the limits of the system and to develop tactics. KG.100 started dropping incendiaries, a tactic to mark targets for a larger bomber force. This was the origin of the pathfinder technique later used by the RAF to obliterate German cities.

X- GERÄT
By early 1941, the X- Gerät was losing effectiveness, along with Luftwaffe command confidence in the system. The Luftwaffe was, however, deploying a third radio bombing aid also designed by Dr Plendl, named Wotan II or Y- Gerät. Y- Gerät used a similar but automated scheme for bomber heading tracking, but used a beacon transponder arrangement for measuring range between the Y- Gerät station and the bomber. Not unlike a ‘back to front DME’, the Y- Gerät station operators could track the bomber’s position and send by radio course corrections to the pilot.

DOMINO
The Luftwaffe was less fortunate with Y-Gerät than with the previous two systems. The British requisitioned a mothballed experimental BBC television transmitter at Alexandra Palace in North London, and adapted it to rebroadcast the Y-Gerät rangefinding signal. Labelled the Domino, the Y-Gerät jammer was soon followed by a second installation at Beacon Hill near Salisbury. The RAF was effectively performing ‘range gate stealing’ jamming attacks on the Y-Gerät, to completely compromise the range measurement achieved by the Germans. The German ground station receiver would lock on to the Domino jammer instead of the Y-Gerät transponder on the bomber, and the British could then manipulate its range measurement. The Domino jammer proved effective, and in the first two weeks of March 1941 only 20 per cent of Y-Gerät raids resulted in commanded bomb releases. Three Heinkels were shot down in early May and the RAF recovered the Y-Gerät receivers. They quickly determined that the automated mechanism for measuring the bearing error in the beam was susceptible to continuous wave jamming, which crippled the bearing analyser circuit.

Time had run out for the Luftwaffe, and with the buildup for Barbarossa, the invasion of the USSR, the KampfGruppes were redeployed east and the Battle of the Beams was won by the British.

The Battle of the Beams is generally acknowledged to have been the first modern effort at electronic warfare, and was characterised by the Luftwaffe not attempting to seriously improve the jam resistance of their systems, but rather by deploying newer and more advanced systems. It remains an excellent case study of how the game of technical intelligence gathering and analysis is played, and how pivotal it is to success in modern warfare. 

Further Reading: Price A, Instruments of Darkness, Peninsula Publishing, 1987.

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