Patent GB-635001-A (GB)
Title: Improvements in or relating to radio navigational systems
IPC: G01S1/02
Filed: 1947-03-20
Abstract: 635,001. Radio navigation. DECCA RECORD CO., Ltd., and O'BRIEN, W. J. March 20, 1947, No. 7712. [Class 40 (vii)] A radio navigation system of the hyperbolic type comprises means for radiating signals of different but harmonically-related frequencies from spaced transmitters and a receiver comprising means for comparing the phase of a pair of the received signals wherein means are provided for bringing the two signals to a common comparison frequency which includes a frequency dividing circuit dividing the frequency of one of the received signals, the transmitter which radiates this signal also radiating a signal to synchronize the divider output to prevent phase ambiguities. The transmitters. The transmitting system may comprise a master and two (or three) spaced slave stations. Using numerical examples of frequencies, the master transmitter (Fig. 3) may comprise a 60 kc. oscillator and amplifier 40 and 41 feeding antenna 43 through a reject filter 42. This 60 kc. signal is received at the spaced slave stations and used as described in Specification 620,479 to control the radiation of signals of 80 and 90 kc. from the respective stations in fixed multiple phase relationship (Figs. 4 and 5, not shown). The 80 and 90 kc. signals from the slave stations are received by antenna 65 at the master station and combined in mixer 68 to give a 10 kc. beat note which is fed to second mixer 69. Mixer 69 is also fed with the 60 kc. radiated signal which is picked up by loop 70 and fed through amplifier 71 and adjustable phase-shifter 72. The 70 kc. output of mixer 69 is selected and passes through electronic phase adjuster 73, amplifier 74 and 60 kc. reject filter 75 before being applied to antenna 43. The phase relationship between the 70 kc. and the other radiated signals of the system is maintained constant by phase discriminator 76 which controls phase adjuster 73. This discriminator is fed on the one hand from mixer 68 and on the other with the 10 kc. beat note between the 70 and 60 kc. signals radiated as picked up by loop 70, the signals being combined in mixer 78, and applies its output to the adjuster 73 to maintain the multiple phase relationship of the radiated signals. In commencing operation of the system the 80 and 90 kc. transmitters may synchronize under a number of different conditions, in the case of the 80 kc. transmitter causing differences of 120 degrees phase shift in the 10 kc. output of mixer 68. Such changes are shown to produce no difference in the phase relationship shown by the receiver indicator. Any adjustment of phase shifter 72 does, however, alter the phase relationship between the beat notes, which relationship can be adjusted to a desired value by this means. If a third slave station is used this would radiate 50 kc. signals and the 70 kc. signals received from the master station would serve for synchronizing in a similar way to that in which the receiver is synchronized so that the 50 kc. radiated signal would not be locked incorrectly with the 60 kc. received signal. Alternatively the synchronizing may be done by manual monitoring. The receiver (Fig. 1). Signals from all the stations are received on antenna 10 and separated by tuned amplifiers 11-15 tuned to 70, 80, 60, 90, and 50 kc. respectively. Considering only the phase relationship determination between the master and the 80 kc. slave transmission the 60 and 80 kc. received signals are mixed in mixer 16 and the 20 kc. beat note selected and applied to phase discriminator 18. The 60 kc. signal is also fed to a mixer 19 where it is combined with a 40 kc. signal to produce the other 20 kc. beat signal which is applied to discriminator 18. The 40 kc. signal is obtained by taking the 20 kc. beat signal from amplifier 20, mixing it with the 60 kc. incoming signal, and selecting the 40 kc. beat by amplifier 22. As the circuit 19-22 serves to divide the frequency of the 60 kc. input by three there is a three to one ambiguity in the 20 kc. output synchronization and to remove this the 70 kc. signal is applied to the mixer 19. As positive peaks of the 60 and 70 kc. signals coincide only once every 6 cycles of the lower frequency the mixer may synchronize in every alternate correct relationship. The other two phase relationships are similarly determined, the 70 kc. signal being applied to the appropriate mixer in each case to prevent incorrect synchronization. Although the phase relation between the 60 and 80 kc. signals is determined at 20 kc. it is shown that the comparison actually obtained is on the basis of 80 kc., while for the other signals the actual reference frequencies of comparison are 90 kc. and 50 kc. respectively. Specification 635,009 also is referred to.