Patent GB-906563-A (GB)
Title: Improvements in or relating to radio navigation systems
Inventor: O'BRIEN WILLIAM JOSEPH
IPC: G01S1/30
Filed: 1960-02-15
Abstract: 906,563. Radio navigation. DECCA Ltd. Jan. 19, 1961 [Feb. 15, 1960], No. 5375/60. Class 40 (7). A phase comparison radio navigation system comprises a master station transmitting alternately two frequencies mf and (m - 1)f, a spaced slave station transmitting alternately two frequencies nf and (n  1)f during corresponding periods, all the transmitted frequencies being mutually locked in phase, and a mobile receiver comprising an oscillator of frequency f which is phase-locked to both the received master frequencies mf and (m - 1)f, means for storing a representation of the phase of the received slave signal nf and phase indicating means utilizing said stored phase representation and the received slave signal (n  1 )f to determine isophase position lines corresponding to those which would be provided if the master and slave stations each transmitted a signal of frequency 1f, i.e. the " virtual " or " characteristic " frequency of the system which determines the spacing between isophase lines of equal value is 1f. Alternatively the slave station may transmit the frequencies (m- 1)f and mf during the alternate periods in which the master station transmits the frequencies mf and (m - 1)f. If the master and slave stations are fixed the system will define hyperbolic position lines but if the master station is located on the craft carrying the mobile receiver then the system will indicate the distances of the craft from the slave stations, i.e. it defines circular position lines. As described, the system comprises a master station and two or three spaced slave stations designated red, green and purple, which normally transmit frequencies 6f, 8f, 9f and 5f, respectively, the normal transmissions from all the stations being interrupted simultaneously for short " alternate " periods, e.g. of duration 1 sec., to transmit " alternate " frequencies 5f, 9f, 8f, 6f, respectively, so that each station transmits substantially continuously. The end of a normal transmission period is signalled to the slave stations and to the mobile receiver (1) by reversing the phase of the normal 6f signal from the master station for a period of <SP>1</SP>/ 10  sec. or (2), if there is no purple slave, by the frequency change from 6f to 5f. Phase indications of higher characteristic frequencies (e.g. 8f and 24f for the master-red slave pair) are also produced and the phase indication of characteristic frequency 1f is used to resolve ambiguities in these intermediate and fine phase indications. For use in a navigation system the alternate frequencies may be transmitted repeatedly at predetermined intervals but in a survey system they may be transmitted only on request. As described, the means in the receiver for storing a representation of the phase of a received slave signal of frequency Nf during a normal transmission period may comprise (1) a separate oscillator of frequency Nf or a frequency Nf derived from the 1f oscillator which is locked in phase to the received signal Nf, Fig. 3, or (2) a phase meter indicating the phase of the received slave signal Nf relative to a signal Nf derived from the 1f oscillator, said meter having an index marker which is aligned with the pointer of the meter during a normal transmission period, Fig. 5. Alternatively, phase can be preserved by means of a continuous signal of a submultiple frequency of the signal whose phase is to be preserved or as the combination of two continuous signals of different frequencies, one of which can be adjusted in phase. Mobile receiver, Figs. 3, 5 and Fig. 4 (not shown).-Fig. 3 shows a receiver for use with a system comprising a master station and red and green slave stations. An output from the 1f oscillator 161 is applied through a six-step phase-shifter 162 to a pulse shaper 163 and the sixth harmonic of the output from the pulse shaper 163 is selected by an amplifier 164 and applied to a phase discriminator 165 together with the received normal master signal 6f, an output from the discriminator 165 being applied through a switch 171 to lock the phase of the 1f oscillator 161 with the received 6f signal. The phase reversal of the 6f master signal at the end of each normal transmission period is also detected by the discriminator 165 which applies a corresponding control signal to a switch unit 178 which opens the switch 171 during the phase reversal periods. During the alternate transmission periods the received master signal 5f is heterodyned in a mixer 174 with the 6f output from amplifier 164 to give a 1f output which is applied together with a 1f output from the pulse shaper 163 to a phase discriminator 176 whose output controls the phase shifter 162 to lock the phase of the 1f oscillator 161 with the 5f master signal. In a modification (described in connection with the red slave station, Fig. 2, not shown) the phase discriminator 176 controlling the phase-shifter 162 is responsive to the 5f master signal from amplifier 172 and a 5f component of the output from the 1f pulse shaper 163. Reference signals of frequency 8f, 1f, 1f and 9f derived from the 1f pulse shaper 163 (and hence all phase-locked to the master signals) are selected by amplifiers 191, 196, 189 and 180, respectively, and the received slave signals 8f and 9f are selected by amplifiers 185 and 183, respectively. During normal transmissions the 8f output from an oscillator 193 is phase-locked to the red slave signal 8f by means of a phase discriminator 194 and applied to a phase discriminator 190 together with the 8f reference signal to give an intermediate red phase indication of characteristic frequency 8f, similarly an intermediate green phase indication (characteristic frequency 9f) is provided by applying the 9f reference signal from amplifier 180 to a 9f amplifier 182 through a calibrated phase-shifter 181 which is controlled by a phase discriminator 184 to bring the output from the amplifier 182 into a predetermined phase relationship with the 9f green slave signal from the amplifier 183. In order to prevent the phase control of the 8f oscillator 193 and the 9f phase-shifter 181 being affected by the short duration alternate slave signals, long-time-constant circuits may be employed or the outputs from the discriminators 194 and 184 may be disconnected during the alternate periods by the switching unit 178. In practice identical circuit arrangements would be used for the intermediate red and green phase indications. Fine red and green phase indications of characteristic frequency 24f and 18f, respectively, may also be produced by known frequency multiplying techniques. During the alternate transmission periods the red slave 9f signal from amplifier 183 is combined in a mixer 198 with the 8f output from the oscillator 193 to give a 1f signal which is applied through an amplifier 197 to a phase discriminator 195 together with the 1f reference signal from the amplifier 196 to give a coarse red phase indication of characteristic frequency 1f and a corresponding coarse green phase indication is produced by the 8f green slave signal from amplifier 185 and the corresponding components 186, 187, 188, 189. The discriminators 195 and 188 are rendered effective only during the alternate transmission periods by means of the switching unit 178 and the phase indication produced during each alternate transmission period is retained during the following normal transmission period by using a phase meter with no return spring on the pointer. The fixed phase shifts in the receiver are " referenced," i.e. standardized, by operating a switch 169 to apply an output from the 1f pulse shaper 163 to the amplifiers 172, 166, 185, 183 and adjusting phase controls thereon to produce zero outputs from the associated phase discriminators 175, 165, 190 and 184, respectively. When there is a purple slave the 5f and 6f signals therefrom may be handled, Fig. 4 (not shown), in the same way as the red slave signals. In a modification, Fig. 5, the 1f reference pulses are derived as in Fig. 3 and during normal transmission periods the medium red phase indication is given by the angular position of the pointer 224 of a meter 223 controlled by a phase discriminator 221 responsive to the received normal 8f red slave signal from amplifier 220 and the 8f reference signal from amplifier 222, the components 220, 221, 222 corresponding to components 192, 190, 191 in Fig. 3. A calibrated rotatable ring 228 is then adjusted around the fixed circular scale 225 of the meter 223 to align an index marker 229 on the ring 228 with the pointer 224. The medium green phase indication is given in a corresponding manner by the pointer 234 of a meter 233 controlled by the 9f amplifiers 230, 232 and phase discriminator 231, an index mark 239 being aligned with the pointer 234. During the alternate transmission periods the pointers 224 and 234 are set, respectively, in accordance with the phases of the green 6f and red 9f signals and the coarse red phase indication is given by the difference between the positions of the pointer 234 on meter 233 and the index 229 on the meter 223, the position of the pointer 234 on the meter 233 being transferred mentally to the scale 228 on the meter 223; similarly the coarse green phase indication is given by the difference between the positions of the pointer 224 on the meter 223 and the index 239 on the meter 233. The mental transfer may be avoided by interchanging the connections to the two meters during the alternate transmission periods so that the coarse phase indications are given directly by the positions of the pointers 224 and 234 on their respective rotatable scales 228 and 234 but in this case phase standardisation must be effected by adjustable phase-shifters in one input to each of the discriminators 221 and 231. Transmitting stations, Figs. 1 and 2 (not shown).-At the master station, Fig. 1 (not shown), the 6th and 5th harmonics of a 1f master oscillator and pulse