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RADAR AND ARPA MANUAL
6.5.4 Transponder performance
monitors
The sensitive unit of this modern type of performance
monitor is an active (as opposed to passive) element
known as a transponder, which is located on the aerial
drive unit in such a way that it is irradiated by the beam
as it sweeps past. The transponder reacts to reception of
the radio frequency pulses by transmitting, after a short
delay, a low-level coded response at the same frequency.
A threshold set within the active element ensures that
the response will only be given if the received signal is
above a power level determined by the manufacturer.
The power level within th ...