Patent GB-989316-A (GB)
Title: Improvements in radar systems
IPC: G01S13/70
Filed: 1963-07-01
Abstract: 989,316. Secondary radar. COMPAGNIE FRANCAISE THOMSON-HOUSTON. July 1, 1963 [July 3, 1962], No. 26004/63. Heading H4D. In a radar system including at least one interrogator radar station adapted to transmit a train of recurrent radar signals towards a target responder and receive response signals therefrom, which target is provided with means for receiving the signals from the station and means for retransmitting response signals thereto said (or each) station further includes means for time-comparing the signals transmitted from it and the corresponding response signals received by it to derive an indication of current target distance, and means responsive to the target distance indication for varying the transmission timing of the recurrent signals from the station in proportion to the distance indication, so that the signals are received at the target at controlled, e.g. uniform, intervals substantially independent of variations in target distance. The system is such that interrogation signals from different stations cannot be received at the target responder simultaneously, or at such close intervals of time that the responder has only time to respond to the first received interrogation signal. The system of Fig. 1 comprises two interogator stations I and II and a common target responder III. A time base generator 1 feeds isochronic signals of a fixed repetition rate to both stations over distances such that the signals are delayed by amounts t 1  and t 2  on reception at the respective stations. The thus received time base signals are fed via first delay means 2 and 2<SP>1</SP>, the purpose of which will be explained later, to further delay means 6 and 6<SP>1</SP>, wherein each signal is delayed by a time of (T - d 1 /c) or (T - d 2 /c), where T is the time between consecutive time base signals, d 1  is the distance between station I and the target, and d 2  is the distance between station II and the target. The thus delayed signals pass to transmitters 7 and 7<SP>1</SP> to produce corresponding signals which are radiated towards the target. The signal from transmitter 7 reaches the target after a delay of d 1 /c, whilst the signal from transmitter 7<SP>1</SP> reaches the target after a delay of d 2 /c. Therefore, relative to the positions in time at the outputs of the first delay means 2 and 2<SP>1</SP>, the signals are delayed on reception by the target by constant amounts T. Due, however, to the delays t 1  and t 2 , and the delays #t 1  and #t 2  introduced by the first delay means, the signals at the target are spaced out at successive intervals of (t 1  + #t 1 ) - (t2 + #t 1 ) and [T- (t 1  + #t 1 ) + (t 2  + #t 2 )], irrespective of how d 1  and d 2  vary. The delays #t 1  and #t 2  introduced by the first delay means 2 and 2<SP>1</SP> are comprised of two parts, a constant delay set by controls 3 and 31 and a variable delay controlled by means 5 and 5<SP>1</SP>. Each of the signals received by the target cause the retransmission, after a time delay of t, of corresponding response signals. These are received back at the negative stations with delays of (2d 1 /c + t) and (2d 2 /c + t) relative to the corresponding transmitted signal. The received signals are compared for time in comparators 13 and 131 with the output signals of the further delay means 6 and 6<SP>1</SP>, via variable delay devices 14 and 14<SP>1</SP> whereby further delays of (2d 1 /c + t) and (2d 2 /c + t) are respectively introc c duced. Both the further delay means 6 and 61, and the variable delay devices 14 and 141 are controlled by a servo control 20<SP>1</SP>, which in turn is controlled by the output of the comparator 13. The servo/negative feedback arrangement is such that the delay means and devices always introduce delays using the correct station to target distances. By the use of two stations the position of the target can be determined. In a modification, Fig. 2 (not shown), the variable delay device 141 may operate on the pulses before the delay means 6<SP>1</SP>, and introduce a delay of (d 1 /c + t).