Patent GB-1423156-A (GB)
Title: Radar target acquisition circuit
Inventor: GELLEKINK B
IPC: G01S13/70
Filed: 1972-12-11
Abstract: 1423156 Radar HOLLANDSE SIGNAALAPPARATEN BV 11 Dec 1972 [15 Dec 1971] 57155/72 Heading H4D In an automatic range tracking pulse radar the tuning of the tracking gate pulse P3 is initially set by an acquisition circuit 42 which produces a manually adjustable acquisition gate pulse P4 of greater duration than the tracking gate pulse P3 and comprising a sequence of narrow gate pulses P<SP>1</SP>a . . . P<SP>1</SP>z, the timing of the tracking gate pulse P3 being initially set by that one of the narrow gate pulses P<SP>1</SP> which coincides with the echo pulse; as described the timing of the acquisition gate pulse P4 is variable in discrete steps #t which may be less than the duration #T of the narrow-gate pulses p<SP>1</SP>, Fig. 1, or equal to #T, Fig. 2 (not shown). The invention is described as applied to a known form of monopulse Doppler MTI system for tracking also in azimuth (B) and elevation (E) in which the RF # output from the monopulse comparator 5 is converted to IF and applied to a coherent (phase sensitive) detector 22 whose output is applied in parallel to two box car detectors 24, 25 which are enabled respectively by early and late tracking gate pulses P1 and P2 from a first gate pulse generator 3 which also produces the tracking gate pulse P3 of duration equal to the combined durations of P1 and P2, said generator 3 being triggered by a range strobe pulse RP from a time modulator 4c (in the range servo 7) which is triggered by the transmitter trigger pulses S. If a video echo pulse EP from the detector 22 overlaps both P1 and P2 then the resultant stretched outputs from the box car detectors 24, 25 are converted by filters 26, 27 into corresponding CW Doppler frequency signals and applied to a sum and difference circuit 28 whose outputs are applied to a phase sensitive detector 33 to give a DC range error signal e(r) which, when mode selector switch 37 is in position b (normal tracking mode), is applied via first and second integrators 36, 38 to control the time modulator 4a. In the monopulse azimuth channel the corresponding box car detector 14 is enabled by the tracking gate pulse P3 and the resultant Doppler frequency signal is applied together with the sum output from circuit 28 to a phase sensitive detector 32 to give a corresponding DC azimuth error signal for the azimuth servo 6; the elevation channel (not shown) operates in a corresponding manner. Separate AGC circuits 23 and 31 are provided for the IF and LF stages. During the initial acquisition mode the switch 37 is first set to position a and the time modulator 40 is adjusted manually to make the output range strobe pulses (now denoted ReP) correspond approximately to the estimated target range Re as determined from the PPI display 45 of an auxiliary search radar. Each of said strobe pulses ReP is applied to stop a clock pulse counter (clock pulse repetition period #t) which was started by the previous trigger pulse #S and the resultant range count N(Re) is reduced in a subtractor 49 by the amount N(#R) corresponding to about half the duration of the acquisition gate pulse P4. The resultant reduced range count N(Re - #R) is stored in a register 53 and during the next interpulse period when the instantaneous count output from the counter 46 reaches the reduced range count value N(Re - #R), a digital comparator 47 enables a second gate pulse generator 51 to generate the sequence of narrow gate pulses P<SP>1</SP>a . . . P<SP>1</SP>z which are applied sequentially to respective narrow range gate channels 43a ... 43z each comprising a box car detector, Doppler filter and quantizer. Each said gate channel 43 is response to the video echo pulse output EP from the coherent detector 22 in the monopulse E channel and is coupled to a respective logic circuit 55 set by the trigger pulses #S such that when one of the gate pulses P<SP>1</SP>, e.g. pulse P<SP>1</SP>j, coincides with the echo pulse EP then the corresponding logic circuit 55j produces an output signal OP which is applied together with the strobe pulse ReP to a time discriminator 59 to give an analogue error signal corresponding to the time interval between the signals OP and ReP, said error signal being applied via the switch 37 (now in position c) to adjust the time modulator 40 until the tracking gate pulse P3 overlaps the echo pulse; when this condition obtains the switch 37 is moved to position b and normal tracking ensues. The output signal OP also actuates a blocking circuit 58 to disable the higher order logic circuits 55 to the right of the active logic circuit 55j. If when shifting the acquisition gate pulse P4 said pulse P4 becomes coincident with a fixed target echo then the resultant transient output from the corresponding narrow gate channel may be identified as the response from a moving target and such false MTI signals are suppressed by (i) a threshold circuit 52 such that a new reduced range count value N(Re - #R) is only transferred to the register 53 if it differs from the existing value in the register 53 by more than a selected amount and (ii) a disabling circuit 54 which temporarily disables the narrow range gate channels when a new reduced range count value is entered in register 53. In the Fig. 2 modification (not shown) the number of narrow gate pulses P<SP>1</SP> (produced by the second gate pulse generator 51) is greater than z and the timing of said pulses P<SP>1</SP> is fixed relative to the transmitter trigger pulses #S, a sequence of z contiguous pulses P<SP>1</SP> being selected (for application to the z narrow gate channels 43a . . . 43z) by a wide acquisition gate pulse P4 initiated as above by the comparator 47. In this ease false MTI signals are suppressed by temporarily blocking a number of narrow range gate channels at each end of the wide acquisition gate pulse P4.