Patent GB-1216406-A (GB)
Title: Improvements in or relating to radar installation
Inventor: MOLHO FRED
IPC: G01S7/292
Filed: 1968-04-11
Abstract: 1,216,406. Radar. THOMSON INFORMATIQUE ET VISUALISATION. 11 April, 1968 [12 April, 1967], No. 17644/68. Heading H4D. The Specification describes a pulse radar system using statistical detection methods for rejecting echoes from extended objects both in range and in azimuth. The video echo signal is time quantized into quanta of duration # equal to the transmitted pulse width by means of timer 2 and the amplitude of each quantum is converted into an n bit digital code by means of coder 1. The thus digitally coded video signal is fed into a set of n parallel multistage registers, 3, having 2<SP>P+1</SP> stages and stepped by the interval pulses from timer 2. The register 3 thus represents a range gate of width (2<SP>P+1</SP>)# and the video signal content within said gate is determined in a "computer" 13 by subtracting the output of the register from the input to the register. Thus subtraction is performed by subtractor 54, and said content is registered in accumulator 72. By dividing said content by a factor proportional to the range gate interval, divider 8 produces an output representing the average amplitude of a quantum of the gated video signal. For a discrete target echo, this output will be low but for an extended "clutter" echo S, Fig. 3(a), the output A, Fig. 3 (c) will be high. In Fig. 3, the "clutter" echo S has a duration to - t 1   >  2<SP>P</SP>. The output of accumulator 72 will start to rise as soon as the echo enters the register and will level off for the period (t o +2<SP>P</SP>)# t 1  when the trailing part of the echo is still entering the register and the leading edge is leaving. As the echo leaves the register, the output falls. A subtracter 51 determines the difference between the inputs to the first and second stages of the register, whilst subtractor 52 determines the difference between the outputs of the penultimate and last stage. These subtractors thus detect the leading and trailing edges respectively of changes in the video signal. The outputs of the subtractors are further subtracted at 53 and the resultant fed to an accumulator 71. The output of the accumulator 71 is added at 9 to the output of divider 8 whereby the basic output A at Fig. 3(c) is supplemented to become output A<SP>1</SP>. This output may be further multiplied at 91 to become output A<SP>11</SP>. Output A<SP>11</SP> is compared at 11 with the video output S<SP>1</SP>, Fig. 3 (b) for register stage 2<SP>P-1</SP>+1, i.e. the video output delayed to occur half-way in said range gate, whereby video output is passed to output 12 only if it is found to be above the "threshold" level of output A<SP>11</SP>. Thus all of echo S<SP>1</SP> will be suppressed, but a discrete echo on top of echo S<SP>1</SP>, e.g. that from an aircraft flying in cloud, will be above said threshold, and will be passed. A discrete echo from a point target will have a width T but if occurring simultaneously with one of the stepping pulses from timer 2, may be broadened to occupy two stages in the register 3 and have a width of 2#. To overcome this the output of coder 1 is also fed to a register 4 having 2<SP>P-1</SP> stages but stepped by timer 2 after register 3. A switch 10 alternates between the output of registers 3 and 4 in time with said delayed stepping whereby the output of the switch for a discrete echo, is always equal to #. The "rise time" of the output A<SP>11</SP> may be shortened from 2<SP>P</SP> to 2<SP>P-1</SP> by determining the difference between the input to and output from  2 P-1 stages 32<SP>1</SP>, Fig. 4, whereby "computer" 13 produces the output A 1 , Fig. 3 (d). This is delayed in register 33<SP>1</SP> by period (2<SP>P-1</SP>)# to give output A 2 , Fig. 3 (e), and a composite output A 3 , Fig. 3 (f), comprising the higher sections of each output, is determined by means 14, 15, 15<SP>11</SP> and 20. This composite output more nearly follows the shape of echo S<SP>1</SP> and so blocks it more accurately. The range gate thus described may be extended over consecutive repetition periods to give a range/ azimuth gate. Thus a plurality of memories 16(1) ... 16 (2N), Fig. 5, are provided, each storing consecutive digitized video from complete repetition periods. A selector 17 sweeps through all the memories in synchronism and determines the video signal having the highest amplitude in the swept quantum. This peak amplitude is fed to register 3 and computer 13 for processing as described. The peak amplitude may be detected, Fig. 6 (not shown), for memories 16(1) to 15(N) and for memories 16(N-1) to 16(2N) separately, in a manner corresponding to the arrangement of Fig. 4. Instead of using the outputs A<SP>11</SP> or A3 for comparison (threshold) purposes, said outputs can be used to control the amplifications of the video signal by enabling a selected number of video amplifiers 181-184 in series, Fig. 8.