Rebecca/Eureka transponding radar

The Rebecca/Eureka transponding radar was a short-range radio navigation system used for the dropping of airborne forces and their supplies. It consisted of two parts, the Rebecca airborne transceiver and antenna system, and the Eureka ground-based transponder. Rebecca calculated the range to the Eureka based on the timing of the return signals, and its relative position using a highly directional antenna.

The system was developed in the UK at the Telecommunications Research Establishment by R. Hanbury-Brown and J.W.S. Pringle. Rebecca was essentially an ASV naval radar fit to a new broadcaster unit, while the Eureka system was all-new. Initial production began in 1943, and the system was used for dropping supplies to resistance fighters in occupied Europe, after delivery of the portable Eureka unit. The US Army Air Force started production in the US as well, and both examples could be used interchangeable. Over time, the Rebecca/Eureka found a number of other uses, including blind-bombing, airfield approach, and as a blind-landing aid in the BABS (Beam Approach Beacon Signal) form.

As many of the war-era systems used similar display units, the Lucero system was introduced to send the proper signals to interrogate any of these systems, allowing a single display unit of any type to be used for H2S, ASV, AI, Rebecca and BABS.

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.

History
The prototype Rebecca I was created by bolting a transmitter on to an A.S.V. (Air-to-Surface Vessel) Mark I receiver. An experimental version of Rebecca I and Eureka I built at T.R.E. was demonstrated at the C-in-C, Army Co-operation Command in the  summer of 1941. The Rebecca system interested the Special Operations Executive (S.O.E.) and a number were built for the use of partisans in the underground movement in Europe.

The Airborne Forces Equipment Committee took up development of the system in the summer of 1942, funding low-priority development of a Mark II system intended for use on glider tugs and paratroop aircraft. At the time, it was decided that each Eureka should be able to handle interrogation from up to 40 aircraft at a time. They also selected a design based on several sub-units that would allow the equipment to be changed simply by swapping sub-units out from a common chassis.

Both Rebecca II and Eureka II were developed by Murphy Radio with early pre-production of Rebecca II by Dynatron Radio. A system using a selection of tuned capacitors was used to select the operational frequency, from a set of four. Looking for a controller, Murphy selected a General Post Office 5-position electromechanical system used in their telephone exchange systems. A similar selection of four channels was available in the Eureka units, but these were selected manually. Rebecca was powered off the aircraft mains, while Eureka was battery powered with a lifetime of about six hours.

In testing, Eureka II proved to be too heavy for practical use, so A.C. Cossor was selected to build a Mk. III version. They used US miniature 9000-series tubes for this version and used a much smaller battery with a three hour life. This version sparked US interest, who started production of a number of versions of the Mk. III as the AN/PPN-1 (Eureka), AN/PPN-2 (Portable Eureka) and AN/TPN-1 (Transportable Eureka). The AN/APN-2 (Rebecca), also known as the SCR 729, used a display that saw use for a number of purposes.

When many British military gliders failed to reach their landing zones in Sicily even in excellent conditions, a rushed effort to develop and even smaller and lighter Rebecca III system started. Cossor was again selected for the development, using a super-regenerative receiver and batteries for only 30 minutes of operation. The Rebecca IIIN version was used for strike aircraft in the pacific theatre. These versions used capacitors in all five positions of the rotary switch.

The introduction of the miniature B7G tubes in 1944 led to a new round of development of the Rebecca/Eureka. Dozens of different variations were eventually developed.

Versions
There were large numbers of versions of the system, early models were limited to a single frequency - later ones could switch between 5 frequencies.

British
Eureka Mk VII was a rack mounted, non-mobile transponder used at RAF bases for aircraft to home onto.

A Mark X version of both Rebecca and Eureka that worked in the 1000 MHz range. This was developed for use during in-flight refueling, enabling the receiving aircraft to locate the tanker while maintaining radio silence. The Tanker aircraft carried the Eureka and the receiving aircraft carried the Rebecca. This equipment was trialled by 214 Squadron in the early 1960s.

American
The Rebecca code name was derived from the phrase "recognition of beacons".
 * AN/PPN-1
 * AN/PPN-1A
 * AN/PPN-2