Little Heidelberg

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Klein Heidelberg (KH) was a passive radar system used by the Germans during World War II. As a transmitter, it used the signals from the British Chain Home System and a series of six stations along the west coast of continental Europe as passive receivers. In modern terminology, the system was a bistatic radar. Since the system did not send out any signals of its own, the Allies knew nothing of its presence and did not find out about the system until long after the invasion on D-Day. The system is referred to in some references as the Klein Heidelberg Parasit.

history

The large antennas from Chain Home (CH) could be seen from the French coast, which meant the Germans were aware of their exact location. The fixed direction of the stations towards the continent made it easy to determine which signal was being broadcast by which station. This was supported by the way in which the broadcasters spread their broadcasts in a series of time slots, the so-called "running rabbits", which made it possible to track a signal pulse back to a specific station through the timing.

From 1942 onwards, Wächter von Telefunken, in collaboration with the radio engineers of the Reichspost, built a passive radar system with the CH transmitters and their own receivers from this information. This was not unlike the Daventry experimental set-up first used in early 1935 to demonstrate the radar concept in Great Britain. In both cases, transmissions from a remote transmitter were used as the signal, and when an aircraft entered the signal it reflected some of it back to the receiver and produced a distinct "blip" on the display.

To this basic concept, Wächter added the ability to measure the rough azimuth of the target by rotating the entire antenna and looking for the maximum signal. The relatively long wavelength of CH, around 6 m, required very large receiving antennas and quite 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 rotary piston shift system was considered, it apparently never got into service.

Several test systems were tried out in Cherbourg in 1942 and 1943. The first operational KH system was built at the moth station on Mont de Couple between Boulogne and Calais, across the English Channel from Dover, which was put into operation at the end of 1943. A second station in Oostvoorne in the Netherlands (beaver) followed in spring 1944. Four more stations were completed in 1944: Vaudricourt (scorpion), Oostende (brake), Cap d'Antifer (capercaillie) and Cherbourg (millipede). These were built on Wassermann radar antennas and some of them used Wassermann's own signals to determine altitude.

System description

The system used two antennas, a very large one on a rotating platform that was used to receive the signal reflected from the aircraft, and a much smaller antenna that was placed about 60 meters away and received the signal directly from the CH station. The signals from both antennas were sent to two CRTs on the Würzburg radar, which were used as J-scopes in typical German fashion. This means that the distances are measured as angles around the face of the pipe and not as a linear distance. The angles of the various "blips" in mils (0 to 400) were measured using a scale around the outside of the CRT.

The operator first selects a single CH station by changing a timer so that he only receives the signals from that station. CH stations broadcast in time slots one after the other so it was easy to select a single station with a simple timer. After the station was selected, the signal was examined on the left side of the two displays, which showed the entire 1⁄ of a second assigned to that CH station. With the help of a handwheel, the operator set the time setting until the main pointer of the directly received signal was at the 12 o'clock position. Then the right CRT, which only shows the first 1⁄ ​​of the time window, was used for finer measurements. The maximum time for this area was 300 kilometers (190 miles), but delaying the sweep allowed the operator to continue tracking while the aircraft flew over Germany.

The measurement of the angle around the front of the display revealed the difference between the time of arrival of the signal and the time of arrival of the direct signal as elapsed time. There are an infinite number of locations corresponding to each measured time difference, and when they are recorded they create an ellipse with the CH station at one focus and the KH receiver at the other. Any set of ten numbers on the circular scale, e.g. B. 100 to 110, was assigned to a pre-calculated ellipse, in this case ellipse 10. The operators could then look up this ellipse on a number of the charts provided.

To measure the angle to the target, the larger antenna was rotated around its vertical axis until the selected target view was maximized, or alternately disappeared or "nulled". The angle can then be read on another scale. The diagrams showed the ellipse and the position of the KH station; a line was drawn out from the station's position at the measured angle, where it would eventually intersect the ellipse to locate the aircraft.

Estimates vary in terms of the effectiveness of the system. Prichard cites a range accuracy of 1 to 2 km with a bearing accuracy of 1 degree at 400 km, while Price suggests 6 miles (9.7 km) at 280 miles (450 km). Due to the lack of switching of the rotary lobes, the angular accuracy was more in the order of 10 degrees (+ or −5 degrees).