Patent GB-1376702-A (GB)
Title: Aircraft guidance system
IPC: F41G7/22
Filed: 1973-02-19
Abstract: 1376702 Pulse Doppler radar WESTING- HOUSE ELECTRIC CORP 19 Feb 1973 7995/73 Heading H4D A guidance system includes a pulsed Doppler radar which tracks a target in range, direction and velocity and which increases the average radiated power when a target approaches nearer than a predetermined range and further includes a guided aircraft with means for receiving signals transmitted from the radar as well as signals reflected from the target and has means for determining the frequency difference between the two received signals. In an air-to-air missile system, the launching aircraft carries a pulse Doppler radar which tracks the target and locks on the missile before launch. When the target range is low enough, the radar p.r.f. and pulse width are increased and the missile launched. The missile carries a C.W. seeking receiver which thereafter tracks the target using reflections generated by the parent radar transmissions. In the search mode, the parent radar measures the target velocity using a bank of contiguous filters covering the expected Doppler frequency range, the velocity is displayed on a C.R.T. against an azimuth time-base derived from the antenna search pattern generator. The radar initially transmits a broad, low-amplitude pulse at a first p.r.f. until the first target echo is detected, this causes the pulse width to reduce, but the amplitude to increase thus retaining the same average radiated power. A train of pulses coincident with the echo pulses is then derived and eventually a second p.r.f. is transmitted. A second pulse train is then derived coincident with the echo pulses due to these transmitted pulses. The true range is then measured by the difference in time between coincidence of the two transmitted trains and coincidence of the two received trains. This range is also displayed on the C.R.T. against azimuth being distinguished from the velocity indication by colour, edge lighting &c. The pilot manually selects a target for tracking by selecting the velocity of the target as displayed and operating a tracking button. The parent radar now tracks the target in velocity using a feedback loop comprising a discriminator connected to the output of a mixer whose inputs are from a variable frequency oscillator and the target signals. The discriminator produces a control signal which adjusts the variable oscillator frequency. The mixer output passes through a velocity gate to the antenna drive which tracks the target using the amplitude modulation present due to the lobing of the antenna feed horn. Range tracking, Fig. 2c.-Time modulator 197 provides a pulse accurately positioned relative to a pulse generated by the coincidence of the first two pulse repetition frequencies supplied over line 356 by means of an error signal developed by detector 352 which receives inputs from the receiver (12, Fig. 2B, not shown) and reference oscillator 366. When the target range falls below a preset value comparator 300 produces an output which de-energizes relay 35. The range gates are initially generated by either of multipliers 360, 362 and generator 372 according to which p.r.f. is being transmitted. At the operation of relay 35, the range gates are generated by multiplier 364 (N 3  > N 1  or N 2 ) and generator 374. Further, the pulses (22, Fig. 2A, not shown) is supplied from oscillator 15 via divider 21, generator 302 and relay contact 304. The average power transmitted by the radar is thus increased, whilst since the range gates are generated from a common synchronizing pulse whatever the transmitter p.r.f., the tracking is not disturbed by changing the p.r.f. Missile receiver, Fig. 2D.-Before launch the missile forward-looking antenna 306 is slaved to the parent antenna (14, Fig. 2B, not shown). Further, signals representing target velocity, range gate pulses and velocity tracking signals are all supplied from the parent radar. After launch, the missile antenna tracks the target in direction using lobing of the antenna feed and tracks in velocity using signals received from the parent radar via rear-facing antenna 316. The relatively high pulse-rate and pulse width enables the missile receiver to operate as a CW seeker. The output of mixer 344 represents the target Doppler frequency which passes velocity gate 318. Discriminator 340 provides a control signal for oscillator 332 which thus tracks the target. Amplitude modulation in the velocity gate 318 output provides tracking signals for the antenna drive.