Klein Heidelberg

Klein Heidelberg (KH) was a passive radar system deployed by the Germans during World War II. It used the signals broadcast by the British Chain Home system as its transmitter, and a series of six stations along the western coast of continental Europe as passive receivers. In modern terminology, the system was a bistatic radar. Because the system sent no signals of its own, the allies were unaware of its presence, and did not learn of the system until well after the D-Day invasion. The system is referred to as Klein Heidelberg Parasit in some references.

Chain Home's (CH) large antennas could be seen from the French coast, which meant the Germans were aware of their precise location. Their fixed-direction broadcasts towards the continent meant it was easy to determine which signal was broadcast by which station. This was aided by the way the stations spread out their broadcasts in a series of time slots, known as "running rabbits", which allowed a signal pulse to be traced back to a particular station through its timing.

Beginning in 1942, Dr. Wächter of Telefunken, working with radio engineers at the Reichspost, used this information to build a passive radar system using the CH broadcasts and their own receivers. This was not unlike the Daventry experiment setup that had first been used to demonstrate the radar concept in the UK in early 1935. In both cases, broadcasts from a distant station were used as a signal, and when an aircraft flew into the signal it reflected some of it to the receiver, creating a distinct "blip" on the display.

To this basic concept, Wächter added the ability to measure the rough azimuth of the target by turning the entire antenna and looking for the maximum signal. CH's relatively long wavelength, around 6 m, demanded very large receiver antennas and fairly complex antenna systems to support this rotation. Another side-effect of the long wavelength was that the angular resolution was relatively low, and although a lobe switching system was considered, it was apparently never used in service.

Several test systems were trialled at Cherbourg in 1942 and 1943. The first operational KH system was built at the Nachtfalter (Moth) jamming station at Mont de Couple between Boulogne and Calais, across the English Channel from Dover, which became operational near the end of 1943. A second station at Oostvoorne in the Netherlands (Biber) followed in the spring of 1944. Four more stations were completed in 1944, Vaudricourt (Skorpion), Oostende (Bremse), Cap d'Antifer (Auerhahn) and Cherbourg (Tausendfüssler). These were built on Wassermann radar antennas, and sometimes used the Wassermann's own signals for height-finding duties.

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