Patent GB-1237419-A (GB)
Title: Pulse doppler radio navigation system
IPC: G01S1/40
Filed: 1968-12-13
Abstract: 1,237,419. Radio navigation. INTERNATIONAL STANDARD ELECTRIC CORP. 13 Dec., 1968 [15 Dec., 1967], No. 59343/68. Heading H4D. In a radio navigation system of the Doppler type two aerials at a radio beacon are oppositely located on a circle and are rotated about the centre thereof, or their rotation may be simulated, and one aerial is energized with each first pulse of a sequence of pairs of R.F. pulses, whilst the other aerial is energized with each second pulse of the sequence. Typically, the diameter of the circle is three wavelengths, the aerials are rotated at 15 r.p.s., each pulse has a duration of 3.5Á sec., the two pulses of a pair are separated by 12 Ásec., and the pairs of pulses are repeated at 60 to 800 Ásec. intervals. At a receiving station the two pulses of a pair have a time separation of 12 Ásec.Œ#t, the time #t being a phase shift to be measured for determining the bearing of the station with respect to the beacon. As described, the two received pulses at an intermediate frequency trigger respective oscillators 01, 02, Fig. 1, by means of an arrangement comprising series-connected flip-flop stages FF1, FF2 and a monostable stage MF. The oscillators 01, 02 are of different frequencies, for example, 10 MHz and 9À95 MHz, and they store, respectively, the phases of the first and second pulses of a received pair of pulses. The two oscillations are mixed in a unit M and a 50 kHz signal is extracted via a low-pass filter TP, said signal beginning at the moment of occurrence of the triggering pulse for the oscillator 02. The time interval between said triggering pulse and a zero cross-over of said 50 kHz signal is an indication of the phase difference between the oscillators O1, O2. In order to prevent ambiguities in measurement, circuit means shown within a dot-dash rectangle SA provide a "sliding temporal window" on each side of the triggering pulses, to set predetermined limits to the phase excursions of said pulses during measuring periods, that is, the time intervals between successive pairs of pulses. The positive-going cross-overs of the 50 kHz signals from the filter TP produce pulses at the output of a shaper PS which are fed to one input each of AND gates 1, 2. The trigger signal from the flip-flop stage FF2 is led to a "start" terminal of a digital counter, and also to an input of a flip-flop stage FF3. A saw-tooth generator ST is caused to produce a saw-tooth oscillation due to initiation by said trigger signal from the flip-flop stage FF2, and it is stopped by a pulse appearing at the output of the AND gate 1. The output of the saw-tooth generator ST is passed to a first input of a differential amplifier DV and also to a sample and hold circuit SH. A capacitor C stores the value delivered by the circuit SH, and is actually integral with said circuit. A voltage divider D reduces the voltage of the capacitor C to such an extent that the voltage produced by the saw-tooth generator ST reaches the said reduced voltage at approximately 100 electrical degrees before the instant when the said generator is switched off. The output of the divider D is passed to a second input of the differential amplifier DV, output from which is fed to second inputs of the AND gates 1, 2. Output from the AND gate 1 causes the flipflop stage FF3 to switch over to its other stable position, and causes the peak value reached by the sample and hold circuit SH to be transferred to the capacitor C. Output from the AND gate 2 is led to the "stop" terminal of a digital counter. Waveforms at various points in the circuit are also described (Fig. 2, not shown).