Patent GB-978112-A (GB)
Title: Improvements in or relating to radio navigational systems
Inventor: WHITE WILFRED ST JOHN
IPC: G01S1/30
Filed: 1961-06-19
Abstract: 978,112. Radio location systems. DECCA Ltd. June 7, 1962 [June 9, 1961], No. 22090/61. Heading H4D. A phase comparison radio navigation system having a master station transmitting signals of a given frequency and two or more slave stations transmitting signals of the same frequency in fixed phase relationship with the master signals in a cyclic time-sharing sequence, is characterized in the provision of at least one further station transmitting signals of substantially the same frequency as the master station signals and the arrangement of the slave stations to additionally transmit signals locked in phase to the transmissions from the further station or stations, all the transmissions taking place one at a time in a cyclic time-sharing sequence controlled by synchronizing signals from the master station or alternatively from any of the other stations. Such a sytem comprising a master station, two slave stations and a further or mistress station, may be used in a variety of ways; for example, Fig. 1 shows a two-range system wherein the position of a craft carrying the master station MR and a phase-measuring receiver may be found from the two intersecting sets of circular isophase lines centred around the two slaves 61 and 52 and the position of a second craft carrying the mistress station, MS, and a receiver, may also be similarly found. In another system, Fig. 2, the master station MR and the two slaves S1 and S2 may combine to form a normal hyperbolic navigation system for any receivercarrying craft R, whilst the position of a craft carrying the mistress station MS and a receiver may be more exactly found by the two-range system using the two slave stations S1 and S2. The time-sharing transmission sequence of the various elements of the system is shown in Fig. 7. A synchronizing signal is first sent out from the master station for 0.1 sec., followed immediately by a phase comparison signal of slightly different frequency for 0.3 sec. The two slave stations receive the synchronizing and comparison signals and lock their oscillators into phase with the comparison signal. The received synchronizing signal then causes the locked slaves to each emit 0.3 sec. duration signals, one after the other at the same frequency as the master comparison signal. The master comparison signal is locked into a receiver and the phase thereof compared with those of the two received slave signals. The master synchronizing signal is also received by the mistress station and causes it to emit its own phase comparison signal of substantially the same frequency as that of the master comparison signal, causing the two slaves to emit a second set of phase-locked 0.3 sec. duration signals for detection in a receiver as before. Master station, Fig. 3.-This comprises a high frequency oscillator 20 feeding, through a gate 29 opened by a timer 28 during both the synchronizing and the phase comparison signal times, into a mixer 25, there to beat for the first 0.1 sec. with the output from oscillator 21 via gate 23, to produce the synchronizing signal; and for the next 0.3 sec. with the output from oscillator 22 via gate 24, to produce the phase comparison signal-the outputs from the oscillators 21 and 22, differing by the required small amount, e.g. 60 c/s. To simulate a slave return signal for test purposes the output from oscillator 22 may be fed to the mixer 25 via a phase shifter 31 and a gate 30, opened for 0.3 sec. during the first or second slave signal period as required. Mistress station, Fig. 4 (not shown).-This is substantially the same as the master station except for an additional receiver stage to receive and filter out the master synchronizing signal for application via a 0.9 sec. delay means to the timer 28. Oscillator 21 and gate 23, although provided in the mistress station, are largely redundant and are only used when the mistress station is used as a master station. Slave stations, Fig. 5.-Each of these comprises a master oscillator and control unit 64 and a mistress oscillator and control unit 65 connected to a common I.F. receiver and transmitter unit. The synchronizing and comparison signals are first received from the master, heterodyned in a mixer 42 and passed through an I.F. amplifier 44. The synchronizing signal is filtered out and passed to a timer 45 to control the various gates in the station. The I.F. comparison signal is fed to one side of a phase discriminator 46 and has its phase there compared with that of an equifrequency signal from a stable oscillator 47. Any out-of-phase discrepancy between the two signals causes a signal to pass via a gate 48, opened during the comparison signal period, to control the oscillator and remove the discrepancy. The oscillator 47 is thus locked in phase with the received master signal. The timer 45 now opens gates 52, 56 and 68 for the appropriate slave transmission period and a signal from the oscillator 47 passes, via phase shifter 50 and gate 68 to be heterodyned back to the normal frequency for transmission via gate 52 and amplifier 53 from aerial 54. The transmitted slave signals are received back by the aerial 40 and appear as the I.F. at one input of a frequency discriminator 55. The other input of the discriminator 55 is fed from the oscillator 47, and, during the slave signal transmission period, the output of the discriminator should be zero. Any phase shift introduced by the transmitting unit produces an output from the discriminator 55 which is fed to control the phase shifter 50. In order to adjust the position of the equiphase lines an additional phase shift can be introduced through the phase shifter 50 by means of a phase shifter 57 between the oscillator 47 and the discriminator 55, the amount of introduced phase shift being shown on an indicator 58. The relative phase of the signal from the other slave may be found by a third discriminator 60 involving the use of a gate 62 opened during the transmission of the other slave signal. The whole procedure is repeated on receiving the mistress phase comparison signal using the unit 65 and gate 57. Receiver, Fig. 6.-In this the phase of an oscillator 78 is locked on to that of the master phase comparison signal as in the slave stations and provides a reference phase signal for application via phase shifters 83 and 86 to one side of phase discriminators 81 and 82, servo gates 84 and 88 being opened for the two slave transmission periods respectively, whereby the phase shifters 83 and 86 are caused to react and show their degree of reaction on indicators 85 and 87 as phase differences between the master (or mistress) signal and the respective slave signals. The two indicators are checked by means of switches 71 and 79 beating the signals from oscillator 78 with those from oscillator 73 and applying the resulting signal to the aerial input. Any phase difference across the discriminator 77 causes the phase shifter 91 to react, reducing the difference to zero, whence the indicators 85 and 87 should also indicate zero. Specification 973,083 is referred to.