1,054,454. Doppler pulse radar. LABORATORY FOR ELECTRONICS Inc April 10, 1964 [May 16, 1963]. No. 15011/64. Heading H4D. In a 50% duty cycle airborne pulse doppler radar, leakage between the transmitter and receiver, either due to deliberate overlapping of the transmitter and receiver operating times at low altitudes (see Specification 900,027) or, when the said operating times are nominally abutting, due to the decay of the transmitter energy and the rise of the receiver sensitivity not being instantaneous and so overlapping, is eliminated by heterodyning means, producing a composite signal by mixing local oscillator energy with the leakage energy, and with the energy received after reflection by a target, and means for removing the portion of the composite signal arising from the leakage signal. In the embodiment of Fig. 1, a gate generator 21 produces two complementary squarewave signals which are fed to a delay 23 and an inverter 25 respectively. The delayed square waves (the delay 23 being provided to produce a desired overlap of the transmitter and receiver operating times) gate R.F. oscillator 12 on and off to produce the transmitter pulses at a frequency f which are fed via duplexer 27 to antenna 29. The transmitter energy also passes to the receiver 14 via the duplexer 12 as leakage energy. The reflected transmitter pulses at a frequency of f+fd, where fd is the doppler shift, are received by antenna 29, and fed via duplexer 27 to the receiver 14. The leakage energy and received energy are together passed to a mixer 33 fed at its other input with the signal at a frequency fLO from a local oscillator 30 to produce a corresponding composite signal which is fed to a wide band amplifier 35. Amplifier 35 has an uneven frequency response such that it limits the part of the composite signal due to the leakage signal at a frequency of (f#fLO). This response may be produced by means of an instantaneous gain control circuit sensitive to the increased amplitude of the leakage signal. The output of amplifier 35 is passed through a gate 37 opened by the square waves from inverter 25 such that all signals due to echoes from targets and the signals due to the leakage signals occurring during the period of transmitter and receiver operating time over-lap, pass through the gate. The passed signals are lowered in frequency by mixer 39 and local oscillator 41 and passed through a narrow band amplifier 43 which acts to reject noise signals and which may be provided with an instantaneous gain-control circuit to a carrier elimination filter 45 (possibly as described in Specification 883,398) which removes the part of the signal due to the leakage signal. The remaining part of the signal, containing the Doppler shift information is passed to tracker, computer and speed-indicating means (possibly as described in U.S.A. Specification 2,910,237 and 2,913,179). In the embodiment of Fig. 2 (not shown), one of the complementary square-wave pulses from generator 21 gates a 60 Mc/s. oscillator 61 on and off. The pulsed output of the oscillator is fed via a power divider 63 to one input of a magic T 65. A master oscillator 71 having a frequency controlling feedback loop produces a signal at a frequency of 9600 Mc/s., which is fed via a power divider 69 to the second input of magic T 65, to produce a plurality of pulse signals at the various sum and difference frequencies available with a 60 Mc/s. signal and a 9600 Mc/s. signal. A filter 79 selects one of the difference frequency pulse signals and passes it to antenna 29, via duplexer 27, for radiation. Part of the output of filter 79 passes to the receiver side of duplexer 27 as leakage energy and passes, together with received reflected energy containing Doppler information to a first mixer 33. A signal at 9600 Mc/s. from power divider 69 feeds the second input of mixer 33 to produce a composite signal containing a first 60 Mc/s. part due to the leakage energy and a second 60 Mc/s. + fd part due to the received reflected energy. The first part overlaps the second part slightly due to the non-instantaneous switching-off of the transmitted pulse referred to. The composite signal passes through a wide band amplifier 35 which limits the first part of the signal, to a second mixer 39. The second input of mixer 39 is fed with a signal at 60 Mc/s. from resonant cavity 67 which is excited by a signal from power divider 63. Mixer 39 produces a D.C. signal corresponding to the leakage signal, and a signal at the Doppler frequency fd which is passed to a tracking/computing unit 16<SP>1</SP>. To determine the sense of the Doppler frequency the phase of the output of cavity 67 is compared in discriminator 87 with the phase of the second part of the composite signal as gated through gate 85 by the other of the complementary squarewave pulses from generator 21. The discriminator 87 gives out a positive or negative signal which is fed to the tracking/computing unit 161.