Dr John Pringle was a Cambridge biologist who
worked at TRE Worth (Leeson House unit) with Hanbury
Brown and E.K. Williams. Early in 1940, they were
working on a project that involved an airborne
transmitter/receiver set and a ground beacon that
could recognize each other's signals 'automatically'.
This is not strictly 'radar' because it does not
rely upon reflected signals but upon a separate
'response' signal. The primary idea was for a
friendly aircraft to be able to drop supplies
to a beleaguered force on the ground with pinpoint
accuracy.
The airborne unit (Rebecca) sent out an 'interrogation
pulse' and when this was received by the ground
unit it caused a pulsed and identity-coded reply
(to be generated by a transponder) which 'sends'
on a different frequency. This new signal was
received by two aerials on the aircraft and displayed
on a Cathode Ray Tube (CRT) in a form which showed
range and direction. Later designs allowed 5 different
transmit / receive frequencies to be selected.
Some of the ground sets were disguised as biscuit
tins because they were bulky. Nevertheless, the
French partisans and other covert operators such
as the Special Operations Executive (SOE) found
the system invaluable when working in enemy occupied
territory because ground lights were no longer
necessary. The ground sets were usually fitted
with detonation devices to prevent capture
.
The system was regularly used throughout the
war and particularly during Operation Overlord
(the D-Day landings) Murphy Radio Ltd built the
beacon equipment and Bush Radio Ltd supplied the
aircraft display screens. The prototypes were
ready by June '42 and had a range of about 80
miles.
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