Patent GB-1330925-A (GB)
Title: Secondary radar system
IPC: G01S13/78
Filed: 1971-04-19
Abstract: 1330925 Pulse radar CHRISTIAAN HUYGENS LABORATORIUM NV 9 July 1971 [17 July 1970] 32371/71 Heading H4D [Also in Division G4] A secondary radar system comprises at least one interrogator sharing a common aerial with a primary radar wherein the interrogator calls a plurality of responders in a predetermined sequence by means of a transmission code different for each responder and a responder having recognized its code emits a response pulse at a different frequency to the primary radar when scanned thereby. In a harbour surveillance system (also stated to be applicable to aircraft) a portable transponder is carried by each pilot and installed on the incoming ships. At the primary radar station, Fig. 1, a calculator 8 predicts the position and determines the address code of the next transponder to be encountered during a sweep of the highly directional antenna 9. Information on newly arrived transponders may be entered manually or the interrogator may be used to search for the new transponder until its position is located. The address is encoded and transmitted as a series of pulses of carrier frequency f 2 . The responder receives the signals and after amplification (using A.G.C. initiated by the first part of the transmission) passes them to a decoder 15. Recognition of the code enables gate 16 to pass the primary radar pulse transmitted shortly after the calling code at a carrier frequency f 1 . This initiates a response pulse at a carrier frequency f 3  which is received by the primary radar receiver 7 and passed to PPI 23 and information extractor 8a which determines the exact position. The calculator 8 produces "lists" of address codes in which selected transponders are called for in the correct sequence over a single sweep of the antenna 9. Each "list" may represent a particular sector of the surveillance area. The responder decoder 15 shown in Fig. 5 utilizes an input sampling register 151 driven by clock generator 152. The transmission code is represented by a code 11 00 00 for a "0" in the address code and 11 11 00 for a "1". The address code comprises a series of bits of the form 000001 X 1  .. X n  where X 1  ... X n  represents the characteristic address of the responder. Unit 153 recognizes the code representing a "1" and 154 recognizes any digit. Unit 155 recognizes reset conditions indicating an error in transmission. At the recognition of the first five zeros in the address code five shift pulses are produced by unit 154 through AND gate 1512 by means of inverted clock pulses 152<SP>1</SP>. Register 156 initially set to all "1" thus has entered into it five zeros, since gate 1513 receives no input from "1" detector 153. After two zeros have been received recognition unit 158 produces a pulse 1581 which initiates the A.G.C. action. The other two outputs from 158 enable gates 1513 and 1514 when the register is not full and when it contains 000001 respectively. The decoding process continues until the whole address code is contained in registers 156, 157 thus producing a "1" at the input to gate 1519. Since the other input to this gate from register 151 also has a "1" an output 21 is formed which resets at least the third digit from the end of register 151 to zero. This changes the register 151 content such that the recognition unit 154 does not produce any further shift pulses for the registers 156, 157. Thus after two clock pulses from generator 152 output 21 resets to zero. This provides a gate pulse of precise length to gate 16 indicating completion of the decoding procedure. Logic 159 compares the address code with the responder address and if they are the same gate 16 is completely enabled and passes the primary radar pulse 20 which arrives shortly after the end of the address code.