Nightfighting in World War Two
By Lee Brimmicombe-Wood

Part 6-1. Confound and Destroy I: The Electronics War

THE STORY SO FAR: As the RAF adopts the bomber stream as a means of overwhelming the night defences, the Luftwaffe perfect Wild Boar and Tame Boar fighting and begin to inflict serious casualties on the British.

Air warfare in World War Two was revolutionized by radio. Radio provided communications, grids of radio beams to navigate by, and radar to find and fix approaching enemies. As radio began to dominate the night battles, a new form of warfare appeared. This black art, known as Radio Countermeasures (or RCM), had the aim of throwing a spanner into the works of the enemy's radio systems and of concealing friendly radio efforts.

Early attempts by the British in 1940 to negate German radio navigation beams such as Knickebein, X-Gerät and Y-Gerät had mixed success. They could not, for example, prevent the blitz of Coventry in November 1940. These systems worked by barrage jamming (broadcasting radio interference on the same frequency as the enemy beam in an effort to overwhelm it) or deception (rebroadcasting and repeating the signal, slightly out of phase with the original, to create false signals). German night bomber crews had little trouble determining the jammers from the real beams, but came to believe (on the basis of little evidence) that British knowledge of the beams permitted the defences to concentrate. So the Germans relied on their beams less and less, a morale effect out of proportion to the actual effects of jamming.

As the Night Blitz slackened and the British Bomber Command increasingly began to take the war to the Germans, the RCM war began to focus on the German radar chain. The first phase of any RCM campaign was the gathering of intelligence. In the 'Battle of the Beams' over Britain, equipment from shot-down bombers and the interrogation of aircrews gave the British insight into the beam systems. But over Germany there was no equivalent information source. The existence of the carefully hidden Freya early warning radar was not believed for many months and its discovery in early 1941 by the British was an mix of guesswork, spycraft and radio reconnaissance.

PHOTO: Taken in February 1941, this photo of two Freyas at Auderville in France was conclusive proof that the Germans had early warning radar. The existence of Freya was betrayed, in part, by its name--an oblique reference in Norse mythology to Heimdal, the watchmen of the gods who could see a hundred miles in all directions. The British took no chances with their codenames for the new radar and electronic warfare systems, picking monickers that gave no clues about their function. Thus the profusion of bizarre titles such as 'Airborne Cigar', 'Airborne Grocer', 'Mandrel' and 'Boozer'. The jamming system that became known as 'Window' was so named because the program director was looking at a window when he was asked what to call it.

Soon after Freya had been photographed for the first time by a reconnaissance Spitfire, the smaller Würzburg radar was discovered. This set in motion one of the most daring operations of the war, the Bruneval Raid of February 1942, in which a company of British paratroopers dropped onto a coastal Würzburg station in France, successfully stole the radar set and took its operator captive. Now armed with an understanding of the systems and their frequencies, Britain launched a 'Great Radar Hunt' to plot all the German radar sites. Radio listening, photo reconnaissance and secret agents identified many radar locations. The British even dropped caged carrier pigeons into Occupied Europe with labels on their legs asking the finder to write in details of any 'dish like' structures they saw before releasing the pigeon, a scheme that helped locate three previously unknown radars. The resulting radar map revealed the lines of controlled night fighting zones.

In July 1942, another radio threat was uncovered by the British. Radio calls by German nightfighter pilots referred to something called Emil-Emil, from which it was inferred that the Germans had started carrying AI radar in their aircraft. To confirm this theory another daring mission was launched, this time by No. 1473 (Wireless-Investigation) Flight, a unit of Wellington bombers trained to ferret out radar and radio transmissions. On 3 December 1942 one of these 'ferret' Wellingtons discovered the 490 MHz signal of a nightfighter's Lichtenstein AI radar. Just as they'd broadcast the frequency information back home they were attacked by the fighter and barely managed to escape, crash-landing in the sea off the coast of England. This act of bravery added a significant piece to the jigsaw picture of the Luftwaffe night defences. With this information the vulnerabilities of the system could be exploited to the full.

One of the most exposed elements of the Reich Air Defence was its communications. Fighters under ground control required clear communications. So a system named 'Tinsel' was fitted to every RAF bomber. This broadcast the noise from a microphone in the engine bay on a frequency set by the wireless operator. The operator would search for channels where he could hear German voice transmissions and then set the Tinsel transmitter for that frequency.

PHOTO: A wireless operator on a bomber tunes his set to the German HF fighter control frequencies, between 3 and 6 MHz. As soon as he's found a likely frequency he will switch on Tinsel to drown out the transmission. As Tinsel relied on the manual efforts of many wireless operators, not all of whom would find the correct frequency, it was somewhat inefficient. So in 1943 Bomber Command introduced 'Special Tinsel', whereby monitoring stations in England hunted down the Luftwaffe's broadcast commentary and then broadcast the wavelength to all the bombers so that the mass of bombers in the stream could spot-jam the frequency.

The other vulnerable part of the nightfighter system was German radar. One of the first countermeasures deployed was 'Mandrel', a jammer that transmitted a barrage of interference on the 120-130 MHz band used by Freya as well as the derivative Mammut and Wassermann long-ranged radars. A combination of standoff jammer aircraft and Mandrel-equipped bombers in the main force would try to blank out a section of the early warning cover.

The jamming offensive opened on 6 December 1942. As hoped, the twin-pronged attack of Mandrel and Tinsel so degraded the fighter control system that first night losses dropped to a tolerable 3.3%. However, in the subsequent weeks a truth about RCM became apparent: that victories were temporary and an enemy could learn to work around jamming. The effect of Mandrel was reduced by modifying the early warning radars to operate on a wider spread of frequencies. Also higher-powered radio transmitters were used to defeat Tinsel, while additional radios set to VHF day-fighter frequencies (around 38-42 MHz) were installed in aircraft as backups in case of jamming. Casualty rates began to rise again.

ILLUSTRATION: Short Stirling 'L' for 'Love', No. 199 Squadron, flew Mandrel patrols in support of the bombing campaign. Mandrel, like most barrage jamming systems, was most effective when the radar was pointed in the direction of the jammer, so the RAF set up Mandrel patrols on the approaches to raids and down the axes that the raids would withdraw.