Patent GB-1468018-A (GB)
Title: Phase comparison radio navigation systems
IPC: G01S5/10
Filed: 1973-11-16
Abstract: 1468018 Radio navigation DECCA Ltd 18 Nov 1974 [16 Nov 1973] 53229/73 Heading H4D A phase comparison radio navigation system features a prime transmitting station and, spaced therefrom, one or more secondary transmitting stations, all said stations radiating phase-locked signals in a time-shared sequence. At the prime station timing signals are derived by a digital counting means from the signals of a source having a frequency related to that of the radiated signals. In a receiver for determining the phase differences between the signals from the stations a timing cycle corresponding to that at the prime station is derived by a counter from the output signals of a means which is responsive to the received signals of said radiated frequency to produce signals of a related frequency. Synchronization is effected by means of a timing identification signal from the prime station. As described, the prime station commences each sequence by transmitting in successive 20 msec. intervals first a synchronizing signal, then an unmodulated signal of frequency f1, and then an unmodulated signal of frequency f2. The secondary stations follow in turn, but do not transmit synchronizing signals. The maximum number of transmitting stations is six. The 20 msec. synchronziing signal from the prime station is of frequency f1, but a phase displacement of 72 degrees is effected during two 5 msec. intervals (Fig. 1, not shown). At the prime station a 10 MHz master oscillator 14, Fig. 2, feeds two synthesizers 10, 11, which develop respective outputs of frequencies F1 (=f1+100 kHz), F2(=f2+ 100 kHz), and a 100/1 frequency divider 15, which develops an output of frequency 100 kHz. Electronic switches 31, 32 feed balanced modulators 17, 18 from respective synthesizers 10, 11, and from the divider 15 to produce an output of frequency f1 or f2, in accordance with the prime station transmitting sequence. A digital phase shifter 16 is brought into circuit during those parts of the synchronizing signal where a 72 degree phase displacement is required. A 10 MHz feed from the oscillator allows adjustment in steps of 1/100 cycle to be effected by a push button 25. Operation of the switches 31, 32 and of the phase shifter 16 is effected by a digital timing control 30, which receives a feed of frequency 4 kHz from the synthesizer 10. At a receiver a voltage controlled 10 MHz oscillator with an associated 100/1 frequency divider is provided for each transmission of different frequency (i.e. f1 and f2) for each station. As shown in Fig. 4, the 10 MHz oscillator 111 controlled by signals of frequency f1 received from the prime station feeds two synthesizers 107, 108 which develop outputs of respective frequencies F1, F2. These are heterodyned with incoming signals in a mixer 103 which provides an output of frequency 100 kHz. After processing in a squarer 105, it is applied as one input of a phase lock discriminator 106, the other input thereto being obtained from the frequency divider of the 10 MHz oscillator corresponding to the station and frequency being received, by operation of the appropriate logic gate of a gating circuit 150. The error output of the discriminator 106 is fed through a switching circuit 153 to the said 10 MHz oscillator, so that it becomes phase-locked to the received signal. The gating and switching circuits 150, 153 are operated, by a timing control unit 151, which receives an input of frequency 4 kHz from the synthesizer 107. Synchronizing is effected by means of a discriminator unit 172, having one input from the frequency divider 112 associated with the oscillator 111 and another input from the squarer 105. The discriminator unit 172 senses the phase modulation in the synchronizing signal from the prime station, and provides a reset signal for initiating the cycle of the timing control unit 151. A manually operable switch 173 allows the feed from the discriminator 172 to the timing control unit 151 to be disconnected, but is effective only after the reception of the next resetting pulse. The receiver at a secondary transmitting may be similar to that described above (Figs. 3A, 3B, not shown).