Patent GB-1498413-A (GB)
Title: Pulse radar apparatus
IPC: G01S7/295
Filed: 1974-05-01
Abstract: 1498413 Radar DECCA Ltd 24 April 1975 [1 May 1974] 19160/74 Heading H4D In a marine radar multiple range scale display is provided for a given constant transmitted pulse repetition frequency. Received radar video signals are quantized, converted to binary digits and written into a store at the transmitted p.r.f. The stored data is extracted from the store, decoded and applied to the C.R.T., e.g. P.P.I., display at a rate dependent on the range scale selected. In Fig. 1, a first embodiment includes quantizer 16 which produces four level radar video signal quantization on lines 17 which are encoded at 18 and fed to digital processor 19 along with synchronizing pulses on line 13. Memory 20 comprises shift registers from which, under control of clock 21, the stored data is read out at a selected rate faster or slower than it is written in. The write speed for memory 20 corresponds to a display range of 6 miles and longer range scales of 12, 24, 48 nautical miles may be provided by sampling the memory at the appropriate rate. Gating unit 34 may perform the speeded up read out two or three times per Tx p.r.p. to increase the displayed brightness for longer displayed ranges. For shorter ranges, e.g. , ¢,¥,1¢ and 3 nautical miles the drive to read-out gate 34 is selected by operating switch 36 and the memory is read out at the selected low sample rate. Processor 37 may be included and compares the sampled signal on a scan-to-scan basis to suppress signals not coherent at the radar p.r.f., for reducing interference from other radars and may further perform scan-to-scan correlation on a jittered radar p.r.p. In a second embodiment, Fig. 2, radar video signals are fed from the receiver 50 on line 52 to unit 53 where other signals including a range marker signal may be added and the signal 3-level quantized and encoded in a 2 bit Gray code under control from store control 61 which is synchronized to the Tx on lead 63 and is controlled by range switch 64 to effect sampling, at a rate corresponding to the selected range scale, so that for range scales from 1¢ to 48 n.m., the sampling frequency is halved for each increase in range scale. The sampled signal corresponding to a TX p.r.p. is written into a free, i.e. one not being used for read out purposes, shift register store 82, 83, 84 as selected by unit 85. Simultaneously, another of the stores containing the previous p.r.p. signal is being read out at the display frequency of 3 kHz. For range scales below 1¢ n.m., the write frequency is maintained constant and the read clock frequency is halved, the read period being maintained constant. Hence at long ranges the number of range cells is chosen to match the required resolution, and at short ranges the range resolution is adjusted to match the scale length. In unit 87 the digital store read out signal is decoded and may be mixed with a signal giving a heading marker display and fed as brightness modulation depending on the quantized signal amplitude to C.R.T. 65. Display control 67 under control from clock 62 is connected to timebase 68 and 69 and to rotating coil 66 for P.P.I. display. On interscan periods an alphanumeric display of other information may be provided from character generator 78.