Secondary radar system

Claim 1

A distance measuring system comprising a first measuring station which includes means for transmitting a first modulated carrier wave, means for receiving a second modulated carrier wave and means for comparing the phases of the modulations of the first and second waves to derive an indication of distance; and a second measuring station which comprises: means for receiving the first modulated carrier wave; means for retransmitting the second modulated carrier wave, the modulation of the second carrier wave being in phase with the modulation of the primary carrier wave; and means for preserving the phase of the modulation of said first carrier wave for a longer period than the reception of that modulation by the second station, which preserving means comprises: a zero crossing detector responsive to the primary carrier wave to derive time impulses; a source of clock pulses; a first digital counter for accumulating clock pulses under the control of the said time impulses; a bi-directional counter which is arranged to increment and decrement alternately with a count corresponding to that accumulated in an interval between said time impulses by the first counter; a bistable means which is arranged to change state at the times when the bi-directional counter reverses its direction of count so as to provide a signal in fixed phase relationship with the said zero crossings; and, means for converting the latter signal into the modulation for the secondary carrier wave.