Patent GB-955281-A (GB)
Title: Improvements in or relating to radio navigation systems
Inventor: WHITE WILFRED ST JOHN
IPC: G01S1/30
Filed: 1961-06-19
Abstract: 955,281. Radio navigation systems. DECCA Ltd. June 8, 1962 [June 19, 1961], No 22019/61. Heading H4D. A radio navigation system for use in a hyperbolic on a two-range position-finding system, comprises a number of spaced transmitting stations each radiating a plurality of signals of slightly different frequencies F 1 , F 2 , F 3  . . . F n , signals of each frequency being radiated from one station at a time in a time sharing sequence in fixed phase relationship, and a receiver comprising, for each pair of stations used to obtain position line information, means producing outputs representative of the phase difference between the signals from the said each pair of stations at any two of the said frequencies, and also first indicating means responsive to one of the said outputs and second indicating means responsive to the difference of the said outputs. The first indicating means will thus give a fine position line reading subject to phase ambiguity whilst the second indicating means will give a coarse position line reading to eliminate the phase ambiguity. The invention is described as applied to a Master and Slaves system transmitting on two different frequencies with a receiver positioned remote from both the Master and Slaves for a hyperbolic system or positioned with the Master for a two-range system. Master, Fig. 1.-This comprises two stable oscillators 10 and 11 generating the two slightly different frequency signals through gates 12 and 13 and amplifiers 14 and 15 to a common aerial 16. The output of oscillator 11 is also heterodyned in mixer 17 with L.F. oscillations from source 18 to produce, at the aerial 16, via gate 19 and amplifier 20, a synchronizing signal only very slightly different in frequency from the signal from oscillator 11, for transmission to the slaves and receiver to control the time sequence of their transmitting and receiving operations. The gates 12, 13 and 19 are controlled by a timer 21 such that the synchronizing signal is first transmitted for 0.1 sec. and the two signals from the oscillators 10 and 11 are then transmitted together for 0.3 sec. Slave, Fig. 2.-In this the synchronizing signal from the master is first received by aerial 30 and tuned receiver 32 and separated out by filter 29 to control a timer 33, controlling in turn, the opening times of the various gates. The signal from the Master oscillator 10 is then received (simultaneously with the signal from oscillator 11) by aerial 30 and is passed via tuned receiver 31 to one side of a phase discriminator 34. An oscillator 35, tuned to the same frequency as oscillator 10, feeds the other side of the discriminator 34 and a signal representative of the phase difference between the two applied signals is passed, via gate 36 opened for 0.3 sec. during the reception period of the signals from the master, to the oscillator, there to shift the phase of the emitted oscillations until the discriminator 34 gives no output. The signals from the oscillator 35 thus become phase locked to those received from the master. Gate 38 is then opened for a period of 0.3 sec. to pass a signal from oscillator 35 to the aerial 40, via amplifier 39. Any phase shifts introduced in this signal by gate 38 and amplifier 39 are detected by receiving the transmitted signal back in aerial 30 and comparing its phase with that of the oscillator signal in a second discriminator 41. Any difference signal produced thereby is passed to a phase shifter 37 to adjust the phase of the oscillator signal accordingly. The phase of the slave signal relative to that of the master signal can be controlled by a goniometer phase-shifter 42- coupled to an indicator 44. A similar circuit is provided for the master signal from master oscillator 11. A second slave station is provided with a different timer such that it emits its two signals immediately after those from the first station have stopped. Receiver, Figs. 3 and 4.-As shown, this is for use with a master station and two slave stations and comprises, as with the slave stations, two separate circuit arrangements, for the two frequencies transmitted, identical except for the difference in the tuning of the receivers and the oscillators. As with the slave stations the synchronizing signal is first received from the master station and separated out in a filter, to control a timer 73, controlling the various gates in the circuit. Taking those signals from the master and slaves having the frequency of master oscillator 10 (those of the other frequency being transmitted, simultaneously with those from the respective station), the signal from the master oscillator 10 is received by aerial 70 and passed via tuned receiver 71 to a circuit comprising phase discriminator 74, oscillator 75 and gate 76, wherein the signals from oscillator 75 are phase locked on to the master signal, as in the slave stations. The signal from the first slave is then received and compared in phase in discriminator 77 with the phase locked signals from oscillator 75, gate 79 being open for the reception period of the signal and passing any phase difference signal from the discriminator 77 to a goniometer phaseshifter 78, to reduce the said phase difference to zero. An indicator 83 connected to the rotor of the goniometer shows the phase difference. The procedure is then repeated for the signal from the second slave using circuit elements 80, 81, 82 and 84. The first slave goniometer phaseshifters 78 and 94 are connected to drive a differential indicator 101 which thus indicates the difference in the phase difference measurements for the two signals from the first slave. Indicator 102 is similarly coupled to the second slave phase-shifters 81 and 97. Indicators 83, 84, 99 and 100 thus give five position indications whilst indicators 101 and 102 give coarse indication. Alternatively, Fig. 4, to this mechanical differential indication, the phaseshifters 78, 81, 94, 97 may be coupled by their rotors to disc digital encoders 110, 111, 112, 113 to give the various phase differences in digital form. The thus adapted phase differences of the first and second slave stations are fed to subtracting means 115 and 125 respectively, to give digital outputs corresponding to the indications of indicators 101 and 102 respectively in Fig. 3. Drum indicators 119 and 129 provided with helical scales 121 and index lines 122 are driven by motors 118 and 128 whose rotational positions are converted to digital form by encoders 117 and 127 for comparison, in comparators 116 and 126, with the digital outputs of the respective subtracting means. Any difference detected in the comparators causes a control signal to be applied to the respective motor to reduce the difference to zero. The indicators 119 and 129 then indicate the required differences in the various phase difference measurements in the form of a coarse position line indication. The fine position line indication in digital form as provided by encoders 110, 111, 112 and 113 may be used in digital computers or digital indicating means.