Patent GB-1097968-A (GB)
Title: Radar systems having improved doppler shift responsive means
IPC: G01S7/36
Filed: 1965-08-20
Abstract: 1,097,968. Radar. HOTCHKISS BRANDT. Aug. 20, 1965 [Aug. 28, 1964], No. 35925/65. Heading H4D. The invention concerns a Doppler radar provided with means for counteracting interference from a hostile target having a responder beacon which shifts the frequency of the received radar signal before retransmitting it, such that the radar detects a false Doppler shift, and indicates a false target velocity. The responder may also transmit with a delay, to produce a false range indication. The invention uses the principle that if the radar transmits on two frequencies F and F<SP>1</SP>, and corresponding frequency shifts off and #<SP>1</SP> are detected in the echo signals then, if the frequency shifts are F f due solely to Doppler shifts, - = -. Two or F<SP>1</SP> f<SP>1</SP> more frequencies are therefore transmitted, the resultant frequency shifts tested for the indicated proportionality, and the frequency shifts only used for velocity determination if the said proportionality holds. In Fig. 1, transmitters 2 and 2<SP>1</SP>, pulsed by keying pulse generator 3, transmit pulses at the frequencies F and F<SP>1</SP>. Return signals are received by receivers 6 and 6<SP>1</SP> at frequencies of F+f and F<SP>1</SP>+#<SP>1</SP> respectively. The F+f return signals are fed to a range tracking arrangement, wherein each return pulse is time compared in comparator 14 with a keying pulse delayed by a variable pulse shifter 16. The output of the comparator operates a servomotor 20, which adjusts the pulse shifter 16, and controls a range indicator 28. The motor 20 also controls a rate of change of range, on velocity indicator 21. The return signals are synchronously detected in demodulators 24 and 24<SP>1</SP> with the aid of portions of the transmitted signals, to give the frequency shift signals f and f<SP>1</SP>. The ratio f/f<SP>1</SP> is determined by computer 25 and comparator 31 with the ratio F/F<SP>1</SP> determined by ratio generator 27. On equality of the ratios being detected, the comparator feeds a signal bore input of AND gate 35. The velocity equivalent to the shift signal f is determined by computer 32, and compared in comparator 34 with the velocity given by indicator 21. On equality of the velocities being detected, the comparator feeds a second signal to the AND gate 35. On simultaneously receiving signals from comparators 31 and 34, gate 35 produces a signal keeping open gates 33, 33<SP>1</SP> and 10, such that the range indication at 28 and the velocity indication at 29 remain. If, on the other hand comparator 31 and/or comparator 34 fail to give a signal, indicating that the return signals are from a target equipped with a responder beacon of the type described, the AND gate fails to produce a signal and gates 33, 33<SP>1</SP> and 10 are closed, removing the false range and velocity indications. The arrangement of Fig. 2 is a modification of the described arrangement, wherein the ratio computer, ratio generator and ratio comparator are removed, and the shift signals are fed to two banks of filters 130 and 130<SP>1</SP>. AND gates 140 are each connected to a filter in each bank, the centre frequencies of the pass bands of the two filters connected to any F gate having a ratio equal to-. An AND gate F<SP>1</SP> in bank 140 thus only gives an output if the desired shift frequency ratio is met (i.e. the frequency shifts are not false). The AND gates feed AND gate 135 (equivalent to gate 35 of Fig. 1) via OR gate 160 and a frequency selection circuit 150, which determines which AND gate produced and output and feeds the corresponding velocity to indicator 129 (29) and velocity comparator 134 (34). In the arrangement of Fig. 3, a single transmitter and receiver are used, the transmitter producing pulses at both frequencies F and F<SP>1</SP>, which are gated to the antenna, such that groups of pulses at the two frequencies are radiated sequentially. The receiver output is corresponding gated in sequence to the correct synchronous demodulator and filter bank. The outputs of the filters in one filter bank are stored for a period so that a shift signal therefrom will appear simultaneously at the AND gate with the shift signal from the other filter bank. Whereas the arrangements of Figs. 1 to 3 are range tracking radars, the arrangement of Fig. 4 is a search radar, displaying the target on a P.P.I, indicator 388. The range of the radar is broken up into plurality of range increments and each shift signal is fed in parallel to a corresponding plurality of range gates 382A, 382B, 382C, 382<SP>1</SP>A, 382<SP>1</SP>B, 382<SP>1</SP>C opened in sequence by the sweep circuit of the indicator. Each gate feeds a filter bank 330A, 330B &c., and corresponding filter outputs are connected to AND gates in gate banks 340A, 340B and 340C. The three AND gates connected to corresponding pairs of filters are connected to OR gates 360, feeding frequency selector circuit 350. An output from the selector circuit, indicating a true target velocity is fed for comparison in comparator 334 with a velocity derived from the rate of change of range of the target to produce a signal when said velocities are equal, which is fed to one input of AND gate 335. When a signal is simultaneously received by the gate 335 from frequency selector 350, the gate 335 feeds a signal to the indicator 388, such that the target can be displayed. Since each filter in the filter bank has an appreciable bandwidth, both shift frequencies may be passed by corresponding filters, even though the target has introduced a small frequency shift, one frequency being for example near the lower limit of the lower frequency filter's bandwidth, and the other being near the upper limit of the other filter's bandwidth. In Fig. 6, each rectangle represents the area of variation of the two shift frequencies in each pair of corresponding filters. Thus, in one filter of a pair the bandwidth is m to n, such that f may vary from m to n and still pass through, and f<SP>1</SP> in the other filter of the pair may vary from m to q. The line D joining the abutting corners of F<SP>1</SP> the rectangles has a slope of -. If the target F introduces a frequency shift of h, then the resultant shift frequencies will lie along line E parallel to line D, and shifted therefore along the f<SP>1</SP> axis by an amount h(F<SP>1</SP>/F - 1). Thus if   h (F<SP>1</SP>/F - 1)<qm the referred to erroneous passage of the shift frequencies will occur. To overcome this, frequency difference means 441, Fig. 5, are substituted for the AND gates 340A &c. each feeding a filter 455 having a centre frequency and bandwidth respectively equal to the differences of these two characteristics in the two filters concerned in banks 430 and 430<SP>1</SP>. For one of the filters 455 to give an output, the shift frequencies must lie within an area such as sprm. Fig. 7 illustrates a modification of the arrangement of Fig. 8 illustrates a modification of the arrangement of Fig. 5, using three transmitted frequencies and a second stage of difference frequency means. In the arragement of Fig. 9, the shift frequency f is first heterodyned by a frequency F1 so as to have a possible range of F1-#o to F1-#n and is then passed to a bank of filters having the ascending bandwidths shown. The output of each filter is then heterodyned with such a frequency that the heterodyned frequency ranges of each filter is identical, such that identical filters can be used to feed the AND gates or difference frequency means. If the target responder multiplies the transmitted frequency by a small factor, rather than add to it, the described arrangements will not detect this interference. If, however, as shown in phantom in Fig. 3, the two transmitted frequencies are randomly varied, keeping their ratio, with corresponding variation of the scale of frequency selecting means 250, then such interference is detected.