Patent GB-1473973-A (GB)
Title: Secondary radar system
IPC: G01S13/78
Filed: 1975-05-23
Abstract: 1473973 Secondary radar INTERNATIONAL STANDARD ELECTRIC CORP 23 May 1975 [13 June 1974] 22905/75 Heading H4D A secondary radar system comprises a plurality of transponders A to E, some of which are standard, e.g. A and D and some of which are complex, together with an interrogator station transmitting cyclically a sequence (Mode 3, Fig. 2) of different pulse combinations (e.g. P1, P2, P3, A1), each complex transponder responding to a set of said different combinations which is unique to said transponder (e.g. transponder B responds to set, 2000, 0100, 0200, 0020, 0001) enabling its identification at the interrogator. The interrogator transmits in a cyclic sequence of modes as shown in Fig. 2; Mode 1 being for the location in standard radar fashion of the standard transponders A and D; Mode 2, in which the spacing t2 between the P1 and P3 pulses is greater than that (t1) for Mode 1, being for the location of the complex transponders; and Mode 3, in which a coding pulse A1 &c., is transmitted at various timings with respect to the pulse group P1, P2, P3, being for the identification of the complex transponders. The interval after the pulse group of the coding pulse becomes progressively larger as the cycle proceeds, with each group of four pulses being assigned a code number such that by adding the codes of those MODE 3 interrogations to which a transponder replies, the identifying code of the transponder is obtained, e.g. transponder B is coded 2321. Since each transponder can respond or not to any of the p=10 combinations, then there are 2<SP>10</SP> different sets of combinations and a possibility of uniquely identifying 2<SP>10</SP> different transponders. At a complex transponder, Fig. 3, the MODE 2 and 3 combinations are detected at 14, with the P2 pulses only being passed if they are of sufficiently high amplitude to indicate reception on a side lobe. Thus assuming such reception, the P1 pulse is clocked through tapped register 12 to appear at tap t 0  simultaneously with the arrival of P2 at the register input. Gate 15 therefore produces an output inhibiting operation of gates 17 and 20 and stopping response. When not on side lobe reception, MODE 2 or 3 is identified in dependence on whether the P1 pulse is at the t2 or the t3 tapping when the P3 pulse appears at the register input. MODE 2 gate 17 or MODE 3 gate 20 are then respectively enabled. The output of gate 17 causes calibrator 19 of encoder 7 to generate a response from transmitter 8. The output of gate 18 causes monostable 21 to generate a pulse enabling the coding pulses A1, &c. to pass through gate 19 to the X gates 221-22X. The register has 'coding pulse' tappings at points corresponding to the combinations (coding pulse positions) to which the transponder is to be active whereby pulse P1 is at one of these tappings to enable a corresponding gate 22 when the coding pulse is detected. The enabled gate 22 then causes a response from generator 8. At the analyser section of the interrogator, Fig. 4, the transponder to be identified is selected by means of range/azimuth gate pulses from a primary radar. The Mode 1 replies as identified by interrogator sequencer 3 at all ranges, from receiver 9 are fed to a defruiter circuit 25 where a 2 out of 3 correlations is effected for successive responses before an indicator 40 is enabled, giving localization of the standard transponders. The Mode 2 responses are likewise defruited at 32 for conditional display on indicator 40, giving localization of the complex transponders. The Mode 2 sequencer indication of the transponder to be identified is passed through gate 34, enabled by the output of the defruiter 32 and the range gate pulse, to a delay register 35, giving enablement of Mode 3 gate 36 for the responses corresponding to the four Mode 3 combinations between successive Mode 2 responses and readying of ten flip-flops 39, each corresponding to a particular combination. The combination being transmitted is identified by sequencer 3 and if a response to the combination is received, gate 36 is fully enabled to set the corresponding flip-flop 39. At the end of a cycle of interrogations, the flip-flop states are converted in indicator 40 to the responder code.