Patent GB-973083-A (GB)
Title: Improvements in or relating to radio navigation systems
Inventor: WHITE WILFRED ST JOHN
IPC: G01S5/14
Filed: 1960-05-20
Abstract: 973,083. Radio navigation. DECCA Ltd. May 17, 1961 [May 20, 1960], No. 17919/60. Heading H4D. Relates to a path-difference or distancemeasuring radio navigation system in which signals from spaced Master and Slave stations comprising a first set of signals are compared in phase at a mobile receiver to determine the equi-phase position line passing through the receiver, the effective comparison frequency being such that there are a plurality of complete cycles of phase change in traversing the required operational area of the system; when the Master and Slave stations are at fixed positions the system defines hyperbolic position lines, Fig. 1 (not shown), and when the Master station is on the same craft as the mobile receiver the system defines circular position lines, Fig. 2 (not shown). According to the present invention lane identification, i.e. resolution of phase ambiguities, in such a system is effected by transmitting a second set of signals differing in frequency from said first set to provide an additional phase comparison having a different effective comparison frequency, the two phase comparators respectively controlling two indicators each having a relatively movable index and scale, the two scales being calibrated in accordance with their respective effective comparison frequencies in the same units of distance or distance-difference (hereinafter referred to only as distance) and each scale being long enough to cover the complete range of phase change in the required operational area of the system so that the two indicators give identical distance readings only when there are no phase ambiguities. If one or both of the phase measuring means sould be in error by a complete cycle, the scale indications will not agree and the correct reading can be obtained by slipping one or each phase measuring means an integral number of phase cycles. The frequencies of corresponding signals in the two sets of transmitted signals may be quite unrelated and may differ by about 5-10%. A phase scale may also be provided on each indicator. When the phase ambiguities have been resolved the two indicators may be used to display information relating to two pairs of Master and Slave stations to provide position fixing. As described, each indicator may comprise (1) a strip of film or tape moved longitudinally past a fixed transverse index, Figs. 6 and 7; (2) a rotatable drum with a helical scale, Fig. 8 ; or (3) a disc having a spiral scale co-operating with a multi-arm rotatable pointer, each revolution of the pointer corresponding to several complete cycles of phase change. The invention may be used in a system in which the component signals in each set of transmitted signals are radiated continuously or intermittently on different but related frequencies but as described the same two frequencies are radiated sequentially from the two stations. Indicators. In the system described, each phase difference output is in the form of a shaft rotation, one complete rotation corresponding to 360 degree phase change, i.e. one phase lane. As shown in Fig. 7, the two indicators comprise two perforated strips of tape or film 100, 130, arranged side by side for parallel movement past a common fixed transverse index mark 131, each strip, e.g. strip 100, Fig. 6, being driven in accordance with its respective phase shift 112 by means of a worm 107 and sprocket 104 and a servo comprising a drive motor 108 energized by the differential output from a comparator 109 responsive to digital outputs from digitisers 110 and 111. The tape 100 is stretched tight between two spools 101, 102, each provided with a stall torque motor 103. Each strip carries a phase scale 123 which is read against the fixed index 131, and the desired condition of identical readings on the two distance scales 123 at the index 131 may be checked automatically by photo-cells 134 sensing markings 132 on the strips 100, 130. In Fig. 8 the indicators comprise two parallel drums 140, 141, each driven as in Fig. 6, and having a helical line 142 with the phase and distance marks on opposite sides thereof which are read off against an index mark 143 parallel to the drum axis. System operation. Fig. 5 and Figs. 3 and 4 (not shown), show the equipment for dealing with one set of transmitted signals in a system comprising a Master station and two Slave stations sequentially transmitting a frequency F +f, e.g. F = 2 mc./s., f = 132 k./cs., each station transmitting for a 0.3 sec. period and the cycle being initiated by a trigger signal F + f + 50 c./s. of duration 0.1 sec. from the Master station. In such a system the effective comparison frequency is F + f but the actual comparison frequency is f. At the mobile receiver, Fig. 5, the received signals are reduced to f (or f + 50 in the case of the trigger signal) by a local oscillator 73 of frequency F and a mixer 74 and the trigger signal is selected by a timing unit 76 which opens the gates 80, 84, 88 during the transmission periods of the Master and the first and second Slaves respectively. During the Master transmission an oscillator 78 of frequency f is phaselocked to the signal f derived from the received Master signal by means of a first discriminator 77 and during the first Slave transmission period an output from the oscillator 78 is brought into a predetermined phase relation with the signal f derived from the received first Slave signal by a phase-shifter 83 automatically controlled by a second phase discriminator 81, the output shaft of the phaseshifter 83 controlling the indicator 85 as described above. An indicator 87 is controlled during the second slave transmission period in a similar manner by a phase shifter 86 and a third phase discriminator 82. A local signal F +f is produced by combining outputs from the oscillators 73 and 78 in a mixer 90 and the receiver is " referenced " by operating switches 71 and 79 thereby applying the local signal F+f to the RF input 72, a phase-shifter 91 between the oscillator 78 and mixer 90 being automatically controlled by the first discriminator 77 to equalize the phases of the reference signals applied to the two sides of the two phase discriminator and indicator circuits 81, 83, 84 and 82, 88, 86 respectively. At the Master station, Fig. 3 (not shown), the transmitted signals F +f and F + f + 50 are produced by applying signals F, f and f + 50 from stable oscillators to a mixer through gates controlled by a timing unit; a simulated Slave signal of adjustable phase is also generated for checking purposes.  Each Slave station, Fig. 4 (not shown), is similar to the mobile receiver, the locally generated signal F+f produced in a mixer (51) corresponding to mixer 90, Fig. 5, being applied to the transmitter through a gate (52) which is opened during the appropriate transmission period by the timing unit, but in this case the phase-shifter (50) corresponding to the phaseshifter 91, Fig. 5, is controlled by the second phase discriminators (55) and the phase shifter (57) corresponding to the phase shifter 83, Fig. 5, is controlled manually to adjust the pattern produced by the Master and Slave transmissions. The third phase discriminator (60) operates as in Fig. 5 and is used to monitor the other Slave. The operating frequency of the system is readily changed by changing the frequency of the F oscillators at the transmitters and receiver. Lane identification is obtained by duplication of the Master and Slave transmitters, the two transmitters at each station operating simultaneously at different frequencies F +f and F<SP>1</SP> + f coupled to a common aerial and controlled by a common timing unit.