Patent GB-1347124-A (GB)
Title: Bearing measuring system
IPC: G01S19/48
Filed: 1971-08-13
Abstract: 1347124 Radio navigation; automatic phase control INTERNATIONAL STANDARD ELECTRIC CORP 13 Aug 1971 [14 Aug 1970] 38109/71 Headings H3A and H4D A bearing measuring apparatus co-operates with a beacon emitting a complex signal comprising main and auxiliary reference pulse trains having repetition rates equal to the frequencies of a fundamental and a harmonic wave also forming part of the complex signal and said apparatus comprises a receiver for said complex wave, first means to produce first and second signals equal in frequency to said fundamental and harmonic waves and phase-locked to the main reference pulse train and second means responsive to said first and second signals and said received complex signal to provide the bearing of the apparatus from the beacon. In an airborne TACAN receiver, Fig. 1, the 15 Hz 135 Hz amplitude modulation components are compared in correlators 17, 19 with phase shifted signals from oscillators 14, 15 phase locked to the reference pulse trains. The error signal generated by correlator 17 drives motor 20 which varies electromechanical phase shifter 16 to produce zero error. When the phase error falls below a predetermined threshold, amplifier 21 switches to using the output of the 135 Hz correlator 19. Motor 20 also drives resolver 23 and synchro differential 24, which also has a magnetic compass input, to indicate course deviation and true bearing respectively. Phase-locked oscillators 14, 15, Fig. 3.-Amplitude limited signals comprising pulse-pairs are passed to decoder 11 which produces at the output of amplifier 33 single pulses coincident with the main reference pulse bursts. The pulses trigger bi-stable 34 to produce a 7À5 p.p.s. square wave which passes to comparator 39 wherein it is compared to a 7À5 p.p.s. signal derived from oscillator 35. The error signal output from the comparator 39 controls the phase and frequency of oscillator 35. When this error signal falls below a predetermined threshold switch 38 changes to a fine mode operation. The oscillator 35 output is passed via phase shifter 45 and squarer 46 to two monostables 47, 48 which generate "early" and "late" gates which enable AND gates 49, 50 whose second inputs are the decoded video input. Any error in the position of the gates is detected by comparator 39. The 135 Hz reference signal is produced in a similar fashion. In the coarse mode however the oscillator 40 output is divided by eighteen to produce the 7À5 p.p.s. signal used in the phase comparison. In the fine mode, AND gates 56, 57 are inhibited during the main reference pulses by the inverted outputs of the early and late gate monostables 47, 48. Correlators 17, 19, Fig. 4.-In the coarse mode, the 15 Hz component in the video input is neutralized by a second input to summing amplifier 61 such that its output contains no 15 Hz component. The amplifier output is passed to hold circuit 64 which retains the pulse amplitude until the video input has been validated. Boxcar 65 output is filtered to remove the 15 Hz component which is passed to detectors 68-71. The second inputs to the detectors are quadrature signals and their complements derived from the 15 Hz reference signal via phase shift element 16. The detector outputs represent the amplitude and phase errors in the neutralizing 15 Hz signal applied to summer 61. The amplitude error is used to vary the gain of amplifier 62 and the phase error controls motor 20 to correct the phase shift element 16. When the amplitude of the phase and amplitude errors fall below predetermined thresholds, switch 93 changes to fine mode operation wherein the 135 Hz component is utilized.