Patent GB-808678-A (GB)
Title: Automatic echo pulse recapture circuit for tracking radar systems
IPC: G01S13/70
Filed: 1956-08-27
Abstract: 808,678. Pulse radar. STANDARD TELEPHONES & CABLES Ltd. (International Telephone & Telegraph Corporation). Aug. 27, 1956, No. 26101/56. Class 40 (7). Relates to an automatic range-tracking pulse radar having velocity " memory " comprising a variably-delayed range gate such that when an echo is within a tracking zone interval centred on the centre of the range gate, the timing of the range gate is automatically adjusted to maintain its centre coincident with the echo and if the echo fades the gate will continue to move at the rate that obtained prior to the echo failure so that it will lock and the echo when it re-appears. In such a system, if the radial component of target velocity changes during the period of echo failure, then when the echo re-appears it may be outside the tracking zone and according to the present invention automatic lock-on under such circumstances is effected by a pair of recapture gating pulses symmetrically time displaced about the centre of the range gate, the time interval between the recapture pulses, i.e. the " recapture zone," being greater than the duration of the tracking zone. In a first embodiment, Figs. 1 and 2, the range gate comprises a wide gate pulse, Fig. 2C, produced by a generator 2, Fig. 1, and a narrow sampling pulse, Figs. 2D and 2G, produced by a generator 3 and occurring at the centre of the wide gate, the timing of the pulses being controlled by a time modulator 1 triggered by the " main bang," i.e. the transmitter trigger pulse. The wide gate pulse C gates on the I.F. stage of the receiver and any echo pulse passed by the receiver is shaped into an S-shaped waveform, Fig. 2E, by a generator 5 and applied together with the sampling pulse S to a main time discriminator 4 to give a D.C. error voltage which is integrated twice at 7 and 8, giving a range voltage which controls the modulator 1 to effect normal range tracking; in this system the width of the tracking zone, Fig. 2G, is substantially equal to that of the S waveform, Fig. 2E. The recapture pulses RP1 and RP2, Fig. 2F, are coincident respectively with the leading and trailing edges of the wide pulse C and are derived therefrom by a generator 9 and applied to an auxiliary time discriminator 10 coupled to an integrator 11, an output from the S-generator 5 being also applied to the auxiliary discriminator 10 through a gate 6 which is so controlled by a sum output from the main discriminator 4 over a lead 4b that the gate 6 is closed only when an echo is within the tracking zone. Consequently if an echo reappears outside the tracking zone but within the recapture zone (e.g. at position E1 or E2, Fig. 2G) a positive or negative output will be produced by the integrator 11 depending on whether the echo drifts into coincidence with recapture pulse RP1 or RP2 and this output triggers a generator 12 to produce corresponding voltage of fixed amplitude which is applied to the modulator 1, thereby giving the gates a step displacement such that the echo is within the tracking zone. Gate 6 then closes and normal tracking is resumed. In a second embodiment the normal tracking range gate comprises two narrow contiguous early and late gate pulses RP1, RP2, Fig. 4B, produced by separate time modulators 13 and 14, Fig. 3, which are controlled in like manner by the integrated error output from a discriminator 15 to which the gate pulses and echo are applied; in this system the tracking zone constitutes the interval bounded by the leading edge of the early gate pulse RP1 and the trailing edge of the late gate pulse RP2. A further sum output is produced by the discriminator 15 if an echo pulse is coincident with the gate pulses RP1, RP2 and this output is applied via a lead 15b to block the action of a bias source 23. If the echo being tracked fades then the blocking voltage vanishes and progressively increasing and decreasing displacement voltages are applied from the source 23 to the modulators 13 and 14 respectively causing the gate pulses RP1, RP2 to move slowly to positions RP1<1>, RP2<1> respectively, Fig. 4C, so that if the echo re-appears in the recapture zone constituted by the interval bounded by the pulse positions RP1<1>, RP2<1> it will eventually drift into coincidence with one of the gate pulses resulting in an error signal from the discriminator 15 which adjusts the timing of the gate pulses so that the echo is within the tracking zone. The resultant sum output from the discriminator 15 again blocks the source 23 and normal tracking becomes operative. Alternatively, the displacement voltages from the source 23 may be such that the gate pulses RP1, RP2 are shifted quickly to positions RP1<1>, RP2<1> and then move slowly inwards.