Patent GB-656108-A (GB)
Title: Improvements in or relating to radio navigational systems
IPC: G01S1/30
Filed: 1948-10-08
Abstract: 656,108. Automatic phase control systems. DECCA RECORD CO., Ltd., O'BRIEN, W. J., BRIDGES, D. E., HENDLEY, D. A., EASY, M. H., and VICKERS, J. Oct. 8, 1948, No. 26339. [Class 38 (iv)] [Also in Group XL (c)] The invention relates to continuous wave radio-frequency navigation systems of the type in which synchronized signals of different but harmonically-related frequencies are radiated from spaced transmitters as described in Specification 620,479. In such a system wherein one (master), station radiates a stable master signal and a further (slave) station a signal maintained in fixed multiple phase relation with the master signal the slave station signals are derived from a stable oscillator whose frequency is normally controlled by the received master frequency to maintain synchronism but whose stability is sufficient to maintain the master-slave phase relationship where reception of the master signal is interrupted. The invention is described in connection with a coarse-pattern lane identification system which is a development of that described in Specifica - tion 630,698. In this system where more than one frequency is radiated by the master and slave stations spurious beat frequencies of the original master signal frequency which affect phase syn - chronization may be received at the slave station. Making use of the present invention, reception of master frequency signals for synchronizing the slave station transmissions is stopped during slave identification periods. The embodiment described comprises a chain of one master and three slave stations normally transmitting signals of the sixth, eighth, ninth and fifth harmonics of a fundamental frequency respectively. The three slaves are known as the red, green, and purple stations. A complete cycle of lane identification with respect to the red, green, and purple contour patterns is made every minute. Commencing with the red identification, the master signal frequency is first changed in frequency to 60 cycles below normal for <SP>1</SP>/ 25  of a second to "warn" the slave stations and receiver, next the master radiates 5th and 6th harmonic signals and the red station 8th and 9th harmonic signals for half a second; normal transmission is then resumed. Fifteen seconds later a <SP>1</SP>/ 25  second warning signal of normal frequency plus 60 cycles is radiated by the master followed by radiation of 5th and 6th harmonic signals from the master and 8th and 9th harmonic signals from the green slave station for half a second for green lane identification. Fifteen seconds later again the master radiates a warning signal of normal - 60 cycles for <SP>1</SP>/ 25  of a second immediately followed by normal + 60 cycles for <SP>1</SP>/ 25  second, followed by the half-second purple lane identification period when 5th and 6th harmonic signals are radiated by the master and 8th and 9th harmonic signals by the purple slave. The use of warning signals differing slightly in frequency from the master signals to control switching means at the slave transmitters and the receivers is claimed in Specification 656,127, [Group XL (c)]. The Master Transmitter. The master station is provided with a stable crystal oscillator 1 from which is derived by divider 2 and pipper 3 a series of short pulses containing at least the first ten harmonics of the fundamental in correct (zero), multiple phase relation. From the output of pipper 3 the sixth harmonic signal is derived in amplifier 4 and fed through phasing amplifier 5, goniometer 7, gate 8, and drive units 9, 10, 11 and 12, to feed antenna 6. A loop 14 associated with the aerial feeds back the radiated signal through amplifier and adjuster 16 to be compared in phase with the output of amplifier 4 by discriminator 17 which feeds a phase-controlling voltage to control the phase shift in phasing amplifier 5 to maintain the radiated signal in phase with pipper 3. A suitable phase control amplifier is described in Specification 656,124. The amplitude of the control voltage is shown on meter 19, and goniometer 7 may be manually adjusted to keep the voltage low. During lane identification periods the master station also radiates fifth harmonic signals which are derived from the pipper signals in amplifier 4<SP>1</SP> and feed through apparatus 5<SP>1</SP>, 7<SP>1</SP>, 9<SP>1</SP>, 10<SP>1</SP>, 11<SP>1</SP> and 12<SP>1</SP>, corresponding to similarly-numbered apparatus for the sixth harmonic. A gated amplifier 28 is also included and the phase of the radiated signal maintained constant with respect to the pipper output by units 161, 171 and 181. The switching sequence is controlled by switches 21, 22 and 23 mounted on shaft'24 rotated by motor 20. The movable arm of switch 21 makes contact with one of the fixed contacts at the commencement of each lane identification period and triggers ¢ second timer 25 and <SP>1</SP>/ 25  second timer 26. The end of the output pulse from timer 26 triggers <SP>1</SP>/ 25  second timer 27. The pulses from timer 25 open gate 28 in the fifth harmonic channel so that fifth harmonic signals are radiated for lane identification and the timing pulses from timers 26 and 27 are fed to gate 8 in the normal sixth harmonic channel to stop the radiation of that signal whilst warning signals are being radiated and also to control gates 30 and 30<SP>1</SP> to feed warning signals of sixth harmonic +60 cycles and/or sixth harmonic - 60 cycles from generators 29 and 291 to aerial 6 in the correct sequence. The Slave transmitters. The control part of each slave station comprises an oscillator and phase locking unit (Fig. 2). This unit consists of a pipper 40, similar to unit 3 of the master station, fed with signals of fundamental frequency by frequency-divider 41. This divider is fed with sixth harmonic signals from crystal oscillator 43 through signal peaker 45 and control switch 42. Divider 41 introduces a six-to-one phase ambiguity after division and correct phase lock of the fundamental frequency output is maintained, as described in detail in Specification 656,126, [Group XL (c)], by a once-per-minute synchronization with the beat signal derived by heterodyning the received fifth and sixth harmonic signals transmitted by the master station during a lane identification period. Signals from the master station are received on aerial 46 and fed through fifth harmonic channel 47, 48, and 49 and sixth harmonic channel 51, 52, and 53, to mixer 50 which feeds the fundamental beat frequency to the amplifier 54, phase adjuster 55 and peaker 45 feeding the synchronizing signal through control switch 42 to the divider. Discriminator 56 fed with the output of divider 41 and mixer 50 is coupled through switch 58 to indicator 59 which shows if the correct phase lock has occurred. Switch 58 is controlled to energize the motor only during the reception of appropriate signals and the indication is held for sufficient time to be read. To synchronize the oscillator 43 with the normal sixth harmonic master signals fast and fine control signals are applied to the oscillator. To obtain the fast control signal the received signal from amplifier 53 is derived from the output of pipper 40 through amplifier and phase-adjuster 63, goniometer 62 and amplifier 61. The error signal derived by the discriminator is fed through switch 65, which is open during lane identification periods but closed during normal transmission, to error voltage switching circuit 64 providing a control to the oscillator circuit to maintain phase synchronism. The fine phase control signal is obtained by multiplying the frequency of the signals from amplifiers 53 and 61 before applying them to a second discriminator 69 followed by a D.C. amplifier 70 and switch 72 operating in the same way as switch 75. The fine control voltage is indicated on meter 71. The fine control system used, in which a D.C. potential is applied across the oscillator crystal to vary its frequency slightly is described in greater detail in Specification 656,125, [Group XL (c)]. Control of the various switch units is by timers 75, 76, 77, and 78, triggered by sixth harmonic plus or minus 60 cycle signals received from the master and passed respectively by crystal filters 73 and 74. The output of pipper 40 is used to drive the slave transmitter unit (Fig. 3) which is similar in construction and arrangement to the parts of the main transmitter bearing the same references. Amplifier 4 generates a signal of the frequency normally radiated by the slave transmitter and a separate set of apparatus (or in the case of the purple slave two separate sets of apparatus), indicated by the rectangle 85 generates the lane identification signals. The <SP>1</SP>/ 25  second warning signals passed by filters 73 and 74 (Fig. 2), serve to operate gate circuits (such as 28), to prevent radiation by the slave station during lane identification on patterns other than its own and to provide the correct pair of frequencies for radiation during its own pattern lane identification period. The slave phase control arrangement maintaining the radiated signal in phase with the pipper output differs from that used at the master station in that two discriminators are used. The second (fine control), discriminator 84 operating at a multiple of the slave frequency has its output clipped to about a tenth of its maximum and then combined with the output of discriminator 83. Normally the output of discriminator 84 never reaches the clipping level and provides the main control. For large errors, however, the control voltage is mostly the output of discriminator 83. Thus an erroneous phase lock due to the frequency multiplication proceeding discriminator 84 is prevented. The control voltage is indicated by meter 86. The Specification also describes the receiver (see Group XL (c)), with reference to Fig. 5 (not shown), and the part of the description contains references to Specifications 656,123, 656,128, 656,129 an