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 antenna s 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 capacitor s 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 glider s 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 . The " Rebecca " code name was derived from the phrase " recognition of beacons " .