Patent GB-1192436-A (GB)
Title: Improvements in Coherent Pulse-Doppler Radars
IPC: G01S13/528
Filed: 1967-09-14
Abstract: 1,192,436. Doppler radar. INTERNATIONAL STANDARD ELECTRIC CORP. 14 Sept., 1967 [21 Sept., 1966], No. 41993/67. Heading H4D. The specification describes a pulsed Doppler radar of the type wherein successive P.R.P. outputs of the coherent phase detector are written into corresponding successive lines of a storage matrix, and Doppler frequency variations in said output for discrete successive range increments are determined by reading out successive columns of the matrix. The invention lies in the variation of the P.R.P. according to a random law whereby blind speeds and ranges are avoided. Specifications 1,113,022, 1,132,284, 1,144,456, may be referred to for development. Trigger signals from clock 15, Fig. 8, having a Gaussian random P.R.P. variation are fed to transmitter 14. A coherent oscillator 8 is phase locked to the I.F. of the resulting transmitted pulses and the output thereof is applied to phase detectors 10 and 11 for detection with the received echo I.F. pulses. To enable the sense of the target's velocity to be determined the coherent oscillator output is fed to detector 10 via a 90 degrees phase shifter 9. Detector 10 produces a "cosine" output and detector 11 a "sine" output. Both outputs are treated similarly and only the treatment of the "sine" output will be described. Each repetition period output of the detector 11 is range quantized into 64 3.3Á sec. samples by sampling circuit 17 and a 0À3 mc/s. pulse signal from clock 15. The amplitude of each sample is converted into a sine digit code group in coder 18 and the 64 code groups are written into an appropriate line of a matrix memory bank 33 having 64 columns and 256 lines. Codes from 256 connective repetition periods can then be stored. The memory bank comprises 9 planes, i.e. one plane for each digit and is divided according to the rank of the line into four sections M1, M2, M3, M4, Fig. 11, having columns 1 to 64 read out in parallel. Section M1 stores in its S line the codes from the 4S-3 repetition period, section M2 stores in its S line the codes from the 48-2 repetition period and so on. The sections are thus read into successively at varying times corresponding to the random variation of the corresponding transmitted pulses but are read out simultaneously in parallel column by column, each read out taking 250 Ás. Since during any repetition period 64 codes are written in and 256 codes are destructively read out of the memory, 192 codes must be rewritten into the memory to stop depletion of its contents. Thus during the first repetition period line 1 (of section M1) is written into and columns 1 of all four sections are read out. The last 48 read out codes from section M1 are fed to a store. The first 16 of these are rewritten into the sections during the next repetition period, the second 16 during the third repetition period and the last 16 during the fourth repetition period. A similar process occurs for the other sections. The read-out codes are fed to a decoder 19 which yields the Doppler variations in each range quantum over 256 repetition periods. The variations are weighted according to a Gaussian function (see Specification 1,132,284) to eliminate interference due to the reading process. Doppler frequency signals are extracted by means of a band filter 21. The thus extracted Doppler signal, Fig. 9a, is distorted since the readout of the memory was at regular intervals whereas the write-ins were at the random times of the transmitted pulses. A sampler 22 thus extracts samples of the Doppler signal at times indicated by Fig. 9b and shifts these samples to times indicated at Fig. 9c corresponding to the times or the transmitted pulses. The samples, Fig. 9d, are fed to a bank of Doppler filters 29 whereby target responses are analysed as to speed and range. Threshold circuits 30 detect a high velocity and pass it via coder 31 to a register. Bearing and range information is also fed to the register.