Patent GB-1209025-A (GB)
Title: Improvements relating to target location systems
Inventor: WHITE ERIC LAWRENCE CASLING
IPC: G01S15/10
Filed: 1966-10-21
Abstract: 1,209,025. Target detection and location system. ELECTRIC & MUSICAL INDUSTRIES Ltd. 23 Oct., 1967 [21 Oct., 1966], No. 47413/66. Heading H4D. General.-The Specification describes a withinpulse frequency modulated radar or sonar target detection system, wherein each pulse, e.g. 0À1 sec. long, is divided into intervals during which it has a constant frequency, no two intervals having the same frequency. All described embodiments are sonar systems transmitting pulses having the frequency variation shown in Fig. 1A. The order of frequencies is selected by quasi-random and the frequencies are separated by equal amounts (90 c/s.). Generation of the pulse is such that the phase from one interval to the next is continuous (Fig. 1B). The echo of the pulse is detected by matched filter techniques or by correlation techniques, Doppler shift also being detected. Production of pulse.-Four methods of producing the transmitted frequencies are described:- (A) Fig. 2. The frequencies present in the pulse centre around 9900 c/s. and are: 9540 c/s.; 9630 c/s.; 9720 c/s.; 9810 c/s.; 9900 c/s.; 9990 c/s.; 10,080 c/s.; 10,170 c/s.; 10,260 c/s. A generator 1, thus produces a frequency of 9000 c/s., and this is fed to an upper side band mixer 30 for selective mixing with the frequencies 540 c/s. &c. produced by mixers 9/12. " By 10 " divider 2 produces a 900 c/s. signal which is fed to one input of each of the mixers 9/12. Frequencies of 90 c/s., 180 c/s., 270 c/s. and 360 c/s. are produced by divider 3, and multipliers 6, 7 and 8 respectively and are fed to the second inputs of respective mixers 9/12. Upper and lower side band products together with the output of divider 2 provide the required range of frequencies 540 c/s. to 1260 c/s. A resetting pulse train of 10 p.p.s. is produced by divider 4 and this is fed to reset a 9-stage shift register 5 shifted by the 90 c/s. output of divider 3 such that a signal appears at each stage of the register in succession for periods defining the interval of the pulse, Fig. 1A. The 540 c/s. to 1260 c/s. range of frequencies are fed via respective gates 21/29 to a combined second input of mixer 30. The gates are opened in the correct order to produce the frequency variations of Fig. 1A, by the outputs of the register, by means of a " random " selection matrix, whereby a selected register stage output is connected to a selected gate. The pulse output of mixer 30 is fed to a transmitting transducer, via gate 32 enabled for 0À1 sec. every 30 sees. (B) Fig. 3. Each frequency 9540 c/s. &c. is generated directly by an oscillating circuit 35 including turning capacitors 51/59. The " random " selection matrix connects each capacitor in turn to the oscillating circuit, according to the predetermined programme. (C) Fig. 4. Each frequency 9540 c/s. &c. is produced by dividing the 1À09 Mc/s. output of oscillator 1 by corresponding factors of 114, 113, 112 . . . 106, by means of binary divider chain 60. Use of all the stage of divider 60 gives a division of 128 and to produce the lesser factors in the required order, selected ones of the stages are preliminarily reset by the output of the chain under the control of signals from the selection matrix enabling the selected ones of gates 62/66 feeding the output of the chain to the resetting inputs of the stages. (D) Fig. 5. The selection matrix is omitted, and the pulse is continuously generated by mixing a 9000 c/s. signal with the output of a frequency synthesizing network, comprising a 300-stage shift register 70 shifted along by a 300 c/s. signal in 0À1 see., each stage output being connected via a weighting resistor to a combining amplifier 75. Thus, the 540 c/s.- 900 c/s. - 1260 c/s. - 630 c/s. &c. signal required to produce the frequency variation of Fig. 1A is synthesized as comprising 300 portions each 1/3000 sec. in duration and having amplitude determined by the weighting resistors. Detection of echo.-Several methods of detecting the echo and extracting Doppler information are also described:- (E) Fig. 6. The received echo is heterodyned down to a centre frequency of 900 c/s. by means of oscillator 104 and mixer 103 and is then fed to a transversal filter comprising a tapped delay line 105. If no Doppler shift were present a single channel of suitable weighting resistors connected to respective tappings would produce an output from combining amplifier 107 and threshold device 108 on reception of an echo of the pulse of Fig. 1A. To allow for Doppler frequency shift, 40 channels of weighting resistors are provided each detecting echos shifted by increments of 10 c/s. Output, from a specific threshold device indicates a target with a corresponding velocity. (F) Fig. 7. The various frequencies are separated out by filters 109/117, having bandwidths wide enough to accommodate any Doppler shifts and the filter outputs connected to a tapped delay line 105 via a " random " matrix corresponding to that used in transmission. (G) Fig. 8. To separate out Doppler frequency shifts with a resolution of 10 c/s., the nine contiguous filters of Fig. 7 are sub-divided into 112 overlapping filters 118, each 90 c/s. wide and having centre frequencies spaced by 10 c/s. Filters having pass bands 295/385 c/s., 385/475 c/s., 475/565 c/s. . . 1015/1105 c/s., i.e. the filters occupying the corresponding lowest parts of the nine pass bands of Fig. 7 (suitably heterodyned down), are connected to a first random matrix and delay line assembly. Successive sets of nine filters are similarly connected to matrixes and delay lines, whereby forty outputs are available corresponding to forty target speeds. (H) Fig. 9. The transmitted pulse, heterodyned down in mixer 127 is sampled in gates 128 by 1 Ás pulses at intervals of 333 Ás from generator 129. Each sample passes through a 332 Ás delay 120 to arrive back at the delay input via gates 123, 124 immediately before the next sample. The samples are successively combined at the input of the delay, whereby when the transmitted 100 ms pulse has finished, there are 300 samples circulating in the loop 120-123-124 at the rate of once. per 332 Ás and representing a compressed version of the transmitted pulse. The thus repeated compressed pulse is fed to a correlating bank described below. The received echo pulse is heterodyned down at 103 and is passed to 40 sampling/ delay/loop channels 130/131/132/133 similar to that described for the transmitted pulse, whereby circulating repeated compressed versions of the echo pulse are produced. The sampling rate varies about a mean of 3000 kc./s. from the 40 channels to allow for 40 different Doppler shifts. The compressed echos are multiplied with the compressed transmitted pulse in multipliers 134 and the product integrated at 135, to analogue a correlation function, whereby the output of one of integrators rises to a maximum at the time that one of the repeated compressed transmitted pulses coincides with the compressed echo pulse, the channel of said one integrator indicating the Doppler shift of the echo pulse.