Patent GB-1181134-A (GB)
Title: Improvements in or relating to Phase Comparison Navigation Systems
Inventor: O'BRIEN WILLIAM JOSEPH
IPC: G01S1/30
Filed: 1967-08-22
Abstract: 1,181,134. Radio navigation. DECCA Ltd. 19 Aug., 1968 [22 Aug., 1967], No. 38674/67. Heading H4D. A receiver, for a phase comparison radio navigation system in which basic frequency signals are radiated from two or more transmitters to provide a basic pattern of position lines and in which a unique frequency is radiated from each transmitter, all of the frequencies being either multiples or sub-multiples of a common fundamental frequency and having a fixed phase relation, comprises a phase discriminator, one input to which is a first locallygenerated signal a phase locked to a basic frequency signal received from one transmitter, a frequency-controlled oscillator b locked to the unique frequency signal from that transmitter, means for rate-aiding signal a by oscillator b, a second phase discriminator, one input to which is a second locally-generated signal c phase locked to a basic frequency signal from a second transmitter, a second frequency-controlled oscillator d locked to the unique frequency signal from the second transmitter, means for rateaiding signal c by oscillator d, and means for providing an integrated phase difference of signals a and c. The invention is particularly applicable to the " Omega " system for example comprising eight stations A to H, Figs. 5, 6, basic frequency signals 10À2 kc./s., 11À67 kc./s. and 13À6 kc./s. being radiated from all stations on a time sharing basis, lane identification signals being radiated on 10À425 kc./s., and each station radiating a unique (" side ") frequency; each station radiates only one frequency at a time. Since the unique frequencies can have much longer transmission times than the basic frequencies, an object of the invention is to make further use of the unique frequencies; each locally-generated signal is rate-aided by an oscillator controlled from the appropriate unique signal, the rate-aiding reducing the problem of ensuring correct counting of cycles of phase change where the receivercarrying craft accelerates or changes course in periods when no basic frequency signals are being received. And the rate-aiding permits narrow receiver bandwidth thereby increasing signal-to-noise ratio. Detailed arrangements, Fig. 4.-In one receiver channels 41, 42, 43 are tuned to respective unique (" side ") frequencies f 1 , f 2 , f 3  and connected to phase discriminators 49, 50, 51; oscillators 52, 53, 54, controlled by the phase discriminators, have their output divided in units 55, 56, 57, the phase discriminators being of the synchronous type to afford a harmonically related phase lock, Figs. 1, 2 (neither shown). The oscillator outputs are mixed with oscillations from 2f oscillators 61, 62, 63. A 45f receiver channel 45 is connected to a phase discriminator 67, the other input to which is cyclically switched to amplifiers 64, 65, 66 in accordance with the cyclic transmissions of the transmissions so that each amplifier provides a signal phase-locked to transmissions from the three different stations. The output of phase discriminator 67 is similarly switched to the 2f oscillators. Position lines are given by integrating phase meters 74, 75, based on a pattern frequency 45f. There are further phase meters 76, 77 integrating the phase changes between oscillators 52, 53 and 52, 54 respectively; under normal conditions discriminators 70, 71 will afford a better reading for applying corrections and (see below) lane identification can be applied to corresponding meters 74, 75, but under high noise the frequency 45f indication will fail first (having the lower duty cycle) but indicators 76, 77 may continue to give position data. Lane identification is afforded from channels 46, 47, 48: signals therefrom are compared with signals at 15f, 5f and f from dividers 82, 83, 84; to give the necessary " standardized " phase shifts through channels 45 . . . 48, a switch 44 allows a short pulse frequency 1f to be applied from a reference source 80; the pulse is derived from divider 84 and should result in zero phase difference at discriminators 85, 86, 87. Any errors are corrected by phase adjustments 91 . . . 94, phase meters, which are also lane meters, 95, 96, 97 being utilized. The lane identification procedure (including divider " notching " i.e. adjustment in steps) is described. Sub-harmonic side frequencies may be utilized, Fig. 3 (not shown), digital counters replacing phase discriminators 70 . . . 73 and indicators 74 . . . 77.