Operation
The airborne Rebecca interrogator transmitted a 4-5 μs long pulse at a rate of 300 pulses per second on a frequency between 170 and 234 MHz. Upon receiving this signal, the Eureka rebroadcast the pulses on a different frequency. The Eureka unit also included a keying system that periodically lengthened the pulses over a period of seconds, allowing a morse code signal to be sent for station identification.
This rebroadcast signal was received by two directional yagi antennas on the aircraft carrying the Rebecca unit. The signal was then sent to a conventional ASV radar display, with the vertical axis measuring time (and thus distance) and the horizontal showing the strength of the signal. If the aircraft was approaching the Eureka from the side, the horizontal pulse would extend further on one side of the display than the other, indicating the need for the aircraft to turn toward the shorter blip in order to fly directly toward the Eureka.
There was a slight delay in the Eureka between signal reception and the return pulse. As the Rebecca units approached the Eureka the return signal would eventually overlap the interrogation pulse, and render the system ineffective. This occurred at a range of about two miles. At this time the crew had to switch to visual means of locating the drop zone. Reliance on Eureka without visual confirmation invariably resulted in premature drops, as during the American airborne landings in Normandy.
Read more about this topic: Rebecca/Eureka Transponding Radar
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