Patent GB-763195-A (GB)
Title: Pulse radar systems
IPC: G01S13/52
Filed: 1954-01-11
Abstract: 763,195. Pulse radar. SPERRY RAND CORPORATION. Jan. 11, 1954 [Jan. 12, 1953], No. 807/54. Class 40(7) In a long-range, e.g. 200 miles, pulse radar a mecury delay line is utilized during an initial predetermined portion of the pulse recurrence period, e.g. corresponding to a range of 0-75 miles, to effect moving target discrimination (M.T.I) by comparing with pulses from the same target received during adjacent reccurrence periods and during the remainder of the recurrence period, e.g. corresponding to a range of 75-200 miles, the same delay line is utilized to add, i.e. integrate, corresponding echoes received during any two successive recurrence periods to improve the signal/noise ratio of the echoes; the same delay line is also utilized to determine the recurrence frequency of the system. If co-herent detection is employed during the M.T.I. period of operation, the reference oscillation is switched off during the integration period. The change over in operation from M.T.I. to integration is effected by gating circuits controlled by gating pulses produced by a generator synchronized with the transmitter trigger. Fig. 1 (not shown) illustrates an application of the invention to a conventional M.T.I. system in which corresponding video echo pulses produced during any two successive recurrence periods are subtracted to give a resultant output which depends on the radial velocity of the target and Figs. 2 and 3 show applications of the invention to an M.T.I. system as claimed in Specification 763,196 in which an echo pulse received during one recurrence period is subtracted from the mean of the corresponding echo pulses received during the preceding and succeeding recurrence periods. In Fig. 2, the transmitter (not shown) is triggered by a pilot pulse generator 40 which also synchronizes the control gate pulse generator 52 controlling gates 43, 48 and 53, and the reference oscillator in the receiver, the " Feed Forward " gates 43 and 48 being open only during the M.T.I. period and the " Feed Backward " gates 53 being open only during the integration period. The video signal is applied through adding circuit 29 and modulated at 32 on to an X sub-carrier from generator 33, the modulated signal being combined at 34 with an output from the pilot pulse generator 40 and modulated at 35 on to a carrier from generator 36. The output from modulator 35 is applied through. the delay line 30 (delay = recurrence period) to detector 37 and. the pilot pulse component of the output therefrom is applied through the automatic frequency control circuit 41 to the pilot pulse generator 40, the X subcarrier component being applied through filter 38 to give a " once delayed " B echo pulse which is applied to cancellation circuit 51. During the M.T.I. mode of operation the B pulse is also applied through gate 43 and modulated at 44 on to a Y sub-carrier from generator 45, the resultant signal being applied through circuits 34, 35, 30, 37 and Y filter 46 to detector 47 to give a " twice-delayed " C pulse which is combined in the averaging circuit 50 with an undelayed A pulse obtained by applying the video input through gate 48, the output from circuit 50 is then subtracted from the B pulse in circuit 51. During the integration mode of operation there are no signals at points A and C and the " once-delayed " B pulse from the output of detector 39 is fed back through gate 53 and combined in adding circuit 29 with the undelayed echo pulse to provide the desired integration, the output being again taken from circuit 51. In Fig. 3 the band width requirements of the delay circuit are reduced by employing single side band modulation and two different carrier frequencies, e.g. 12.25 Mc/s and 10 Mc/s, the video signal being applied through a low-pass filter, e.g. cutoff frequency .5 Mc/s; to reduce cross talk. The gate circuits are shown schematically as ganged switches 71a, b, c and d. As shown an output from the low-pass filter 60 is combined in adding circuit 61 with an output from the transmitter trigger generator 80 and modulated at 62 on to a 12.25 Mc/s carrier from generator 63, the modulated signal being applied through amplifier 64 and drive stage 65 to the delay line 66 (delay = recurrence period). One output from the delay line is applied to detector 75 to give the trigger pulse component which is applied through cathode follower 77 and amplifier 79 to the trigger generator 80 and another output from the delay line is applied through a 12.25 Mc/s filter 68 to detector 69 to'give a " once-delayed " B pulse of negative polarity Which is applied to combining circuit 70. In the M.T.I. mode of operation an output from filter 68 is applied through switch 71a and amplifier 72 and is heterodyned in frequency changer 73 with the 22.25 Mc/s output from oscillator 74 giving a modulated 10 Mc/s signal which is applied through switch 71b and circuits 65, 66 and 67 and 10 Mc/s filter 68<SP>1</SP> to detector 69<SP>1</SP> to give a " twice-delayed" C pulse of positive polarity which is applied to combining circuit 70 together with an undelayed A pulse of positive polarity obtained by applying an output from the low-pass filter 60 through switch 71c. In the integration mode of operation a "once-delayed " pulse from cathode-follower 77 is applied through a compensating delay line 78 (delay = 2Ás) and switch 71d and combined in circuit 61 with the undelayed echo pulse. Suitable circuits for the adder 61 and combining circuit 70 are illustrated in Figs. 4 and 5 (not shown).