932,294. Radar. REEVES INSTRUMENT CORPORATION. Feb. 21, 1961, No. 6374/61. Class 40 (7). In a pulsed radar system the pulse repetition frequency (PRF) of the transmitted pulses is increased from a first value to a second value when the range of the target approaches the maximum range (or a multiple thereof) determined by the first PRF and, when the target range exceeds such maximum range, the PRF is changed back to the first value, ambiguity due to the presence of other target echo pulses being removed by giving a distinguishable characteristic to some of the transmitted pulses. This arrangement prevents target echo pulses from being obliterated by the transmitted pulses and allows ranging (strobe) marker pulses on the A scope to be aligned with precision on the selected target pulse. An additional advantage is that a high PRF is employed even when operating at extremely long ranges, resulting in a high signal-to-noise ratio. Fig. 3 shows a simplified embodiment in which the PRF generator 21 is arranged to operate at one or other of two PRF's having periods T 1 and T 2 under the control of relay arm 33 in accordance with the actuation of cam switch 29 by hand crank 27 which also operates variable time delay devices 22, 23 producing strobe pulses of periods T 1 and T 2 respectively, the arrangement being such that when the range of the selected target approaches the maximum range determined by T 1 the cam switch is operated and generator 21 changes over to period T 2 and pulse generator 43, which produces intensifying ranging (strobe) pulses for A-scope 42, is switched over from control by the output of device 22 to control by the output of device 23. This mode of operation continues until the range of the target (not tracked by strobe pulses of period T 2 ) exceeds that determined by period T 1 at which point cam switch 29, 30 opens and operation in the first mode with pulses of period T 1 is restored. In either mode the output of PRF generator 21 is supplied to the radar transmiter 36 via a circuit 34 which operates to code the transmitted signal by adding to e.g. every fifth or sixth pulse, a second pulse as shown by pulse pairs 11 (Fig. 2). These added pulses which have a low repetition frequency determined in practice by the maximum frequency of which the system is to operate, are supplied to a variable time delay circuit 37 controlled by crank 27 and to a sweep generator 38 producing the horizontal sweep on the long range A-scope 44. The variably delayed L.F. pulses from circuit 37 are combined with target echo pulses (from radar receiver 40) in a circuit 41 and supplied to the vertical deflection circuits of scope 44 as a negative pulse to distinguish them from target echo pulses. The latter pulses are supplied directly to the vertical deflection circuits of A-scope 42 (the horizontal sweep of which is produced by generator 35 at the period T 1 or T 2 of operation of PRF generator 21) and, in operation of the system, when the L.F. strobe pulse from device 37 has been set to coincide with the selected coded target pulse on scope 44 (11<SP>1</SP>, Fig. 2), the range indicator 26 provides a coarse indication of target range within the accuracy of L.F. ranging systems. Further operation of crank 27 to align the intensifying pulses from generator 43 with the selected target pulse on A-scope 42 then produces a range measurement on indicator 26 within the accuracy of the fine ranging system the range (time intervals St 1 , St 2 and St 3 ) being measured without ambiguity due to the presence of intervening echo pulses such as 12<SP>1</SP>, 15<SP>1</SP> (Fig. 2). In a second embodiment (Figs. 4, 5 and 6) the coding of the transmitted pulses is effected by the periodic omission of a single pulse such omission being effected at a (low) frequency equal to that of coarse ranging strobe pulses. As shown, two pulse trains A and E of periods T 1 and T 2 respectively are derived by frequency division in ¸5 divider 52 and ¸4 divider 53 of the output of a 2,000 c.p,.s. pulse repetition frequency (PRF) generator 51 and supplied alternately, in accordance with the position of relay-operated switch 81 to a gate circuit 95 which, under the control of a low-frequency pulse generator 91 (waveform I), coincidence circuit 92 (waveform J) and monostable multi-vibrator 93 (waveform K) periodically gates out one of the input pulses (A or E) to produce pulse train L which, after delay in device 96, are supplied to the radar transmitter 97 transmitting the waveform M. (When the switch arm is on contact 55 the referenced outputs of the various " blocks " of Fig. 4 are as shown in Fig. 5, whilst when the switch arm is on contact 57 the outputs are as shown in Fig. 6 in which a prime indicates that a difference exists between similarly referenced waveforms.). For range measurement during operation in each mode (i.e. transmission of the M or M<SP>1</SP> pulse trains) strobe pulses are produced by means of amplitude comparators (multiars) 58, 62 in which the amplitude of saw-tooth waves from generators 54, 56 controlled respectively by pulse trains A and E are compared with D.C. voltages derived from respective potentiometers 60, 64 the sliders of which are operated by hand crank 135 (or by the Auto range tracking device 122). The outputs of the comparators then consist of variable strobe pulses D and H which are supplied, selectively in accordance with the position of switch arm 82, to a mono-stable multi-vibrator 130 the output V of which is supplied to a combining circuit 117 together with coarse range pulses R (to be described) to produce beam intensifying pulses W for the fine range A-scope 125 the horizontal sweep of which is effected by generator 128 controlled by pulses trains A or E. The coarse range pulses R are produced by comparing in a further amplitude comparator (multiar) 104 the amplitude of a saw-tooth wave O (from generator 100 operating at the repetition frequency of low frequency generator 91) with a D.C. voltage from a potentiometer 105 controlled by hand crank 135 via a 4 to 1 bevel gears 108, 109 respectively the drive shaft 107 including a cam member 110 co-operating with a stop 111 to avoid damage by over driving. The output of comparator 104 (pulses 2) is then supplied to a mono-stable multi-vibrator 112 producing the pulses R which in addition to being supplied to the combining circuit 117 (as described above) are fed to a further combining circuit 114 together with the output of a gate 124 comprising the received radar echo (plus noise) signals of waveforms S but from which the transmitter pulses M are gated out by means of gating pulses T thus producing the waveform U controlling the vertical deflections of the long range scope 103 the horizontal sweep of which is produced by the sawtooth wave from generator 100 via isolating amplifier 101. In operation of the system coarse range measurement is effected by operating crank 135 to position the strobe pulse R on the long range scope 103 in co-incidence with the " missing " target pulse (since the transmitted pulses are coded by the omission of pulses at the frequency of L.F. generator 91) and by further adjustment of crank 135 to cause the intensifying pulses W to intensify the selected target pulses on A-scope 125 an accurate indication of range is provided by range indicator 73. For automatic range tracking the pulse train X is derived from the D or H pulses and supplied via gate 116 and delay device 132 to a conventional automatic range tracking arrangement 122 mechanically coupled via shaft 133 to differential 134 a friction collar 140 holding the hand crank stationary.