Patent GB-1178167-A (GB)
Title: Improvements in Secondary Radar Echo Extractors for Pulse Code Trains
IPC: G01S13/78
Filed: 1967-03-21
Abstract: 1,178,167. Pulse radar. SOC. NOUVELLE D'ELECTRONIQUE ET DE LA RADIOINDUSTRIE. 21 March, 1967 [22 March, 1966], No. 13305/67. Heading H4D. In a secondary radar system a device 1, Fig. 1, termed an echo extractor is in circuit at the ground (interrogating) radar between the video output of the radar receiver 2 and a known automatic decoder 4, the echo extractor producing from coded craft responses code (e.g. identity and height), distance and azimuth data; according to the invention the echo extractor comprises a first system of circuits I, Fig. 2, for detecting the codes present in the video output, suppressing a synchronous signals and storing the codes detected, a processing unit 15 for extracting from the circuits I the code, distance and azimuth data distinctive of corresponding transponding craft for transmission (directly or via a telephone link) to the automatic decoder, and a second system of circuits II connected to the processing unit and storing the synchronous codes and the distance-azimuth data. The interrogating transmission is of pulse pairs of fixed duration and constant repetition rate; various modes are distinguished by the magnitude of the spacing between the pulses of a pair. The response is a number of pulses (maximum of thirteen) between two framing pulses; the code pulses are of a duration between 0À35 and 0À55 micro seconds, the spacing therebetween is 1À45 micro seconds and the spacing between the framing pulses is 20À3 Œ 0À1 micro seconds. General arrangement, Fig. 2.-In the embodiment the radar receiver video output V is amplitude-quatized to bring all signals above a threshold level to the same amplitude by top and bottom clipping in unit 10; the baseclipping level may be determined by a circuit 10 on the basis of integrated thermal noise amplitude. Response extractor 12 utilizes the fact that the secondary radar codes incorporate framing pulses of separation 20À3 Œ 0À1 micro seconds, a delay line being employed to delay the first framing pulse to enable possible coincidence with a later input pulse to be detected; all detected coincidences P are fed to a defruiter 13, defruiting utilizing comparison of the code position in consecutive repetition periods and synchronous coincidences PS being fed to processing unit 15. The response extractor also gives a signal K consisting of isolated codes and synchronous codes garbled with some asynchronous codes to a store 14 for subsequent use in the case of synchronous codes, asynchronous codes being rejected. The response extractor may be as described in Specification 1,111,701 or 1,136,442. Defruiter 13 includes ferrite devices storing the coincidences P for four consecutive repetition periods in each of three interrogation modes, Fig. 4 (not shown). (The conventional interrogation procedure is by three modes following one another in corresponding consecutive repetition periods.) A coincidence P is considered to be synchronous when e.g. three occur in the same time position (within a "tolerance window", Fig. 6 (not shown)), during four consecutive repetition periods of the same mode. The write-in and read-out operations of the stores are described in detail, Fig. 7 (not shown). Arithmetic processing unit and data stores.- Binary stores are provided for range/azimuth data; a mean target azimuth is obtained utilizing an "end-of-plot" decision, taken when in all modes there are three consecutive absences of responses.