Patent GB-623378-A (GB)
Title: Improvements in or relating to direction indicating devices for radio signals
IPC: G01S1/02
Filed: 1946-01-31
Abstract: 623,378. Radio navigation. DECCA RECORD CO., Ltd., and O'BRIEN, W. J. Jan. 31, 1946, No. 3188. [Class 40 (vii)] Relates to systems for determining the bearing of a mobile receiver with respect to a rotating narrow-beam beacon in which the beacon transmits an omni-directional reference pulse signal when the beam is in a datum direction, e.g. north, means being provided at the receiver for indicating the time interval between the reception of the reference signal and the pulse signal produced by the rotating beam. According to the present invention, the omni-directional transmission includes an additional identifying signal to distinguish the reference signal from the beam signal. As described, the identifying signal comprises two additional pulses, one preceding and the other succeeding the reference pulse, and all the signals are displayed on the circular trace of a cathode-ray tube at the receiver, the separation of the reference and beam signals indicating the bearing. In Fig. 2 (not shown), the circular trace is produced by applying phase quadrature voltages from a sine-cosine potentiometer to the deflection plates of the cathode-ray tube, the potentiometer being driven by an electric motor through a variable-speed gear-box and the speed of the drive being adjusted to synchronism with the beacon rotation so that the signals displayed on the trace are stationary. The signals may be displayed by brilliance modulation so as to appear as bright spots on a blacked-out trace, Fig. 3, or as gaps in a brilliant trace, Fig. 4 (not shown), or the trace may be normally visible as a fine line and the signals applied to one of the anodes to cause defocusing with a consequent widening of the trace, Figs. 5 and 6 (not shown). When the trace is synchronized, its phase is adjusted by means of an epicyclic gear in the gear-box or by means of phase-shifters in the outputs from the potentiometer so that the reference signal 18a, Fig. 3, appears at the top of the trace and the position of the beam signal 1a then indicates the bearing on a transparent scale 52. In order to provide-optimum coverage, the beacons may be arranged in groups of six, the stations in each group being arranged at the apices of a hexagon and the hexagonal groups being symmetrically arranged to provide hexagonal spaces between the groups, Fig. 9 (not shown) ; alternatively, the stations of each group may be arranged along the sides and at the apices of parallelograms, Fig. 10 (not shown). The spacing of the identifying pulses 17a and 19a, Fig. 3, with respect to the reference pulse 18a is made a characteristic of a group and the beacons of any one group are distinguished by their differing rotation rates. In order to observe signals from two beacons in quick succession for the purpose of obtaining a fix, a cathode-ray tube is employed, Fig. 7, in which the circular trace is produced by passing a D.C. current through a deflector coil 67 which is rotated by either of the clutch plates 62 or 63 which are driven through epicyclic gears 71 and 72 and a three-position gear-box 65 from a synchronous motor 61 energized from a 150 c/s. oscillator 60. The epicyclic gears are adjusted for correct phasing and synchronism of a selected pair of beacons, the signals from which may then be observed by engaging the appropriate clutch plate. In this embodiment the received signals may be displayed as described above or they may be superimposed on the D.C. deflection current to produce an inward or outward radial deflection of the trace, Fig. 8 (not shown), which may also be intensity modulated and the bearing scale may be produced by feeding pulses to the deflector coil which are obtained photo-electrically by means of rotating apertured light-masking discs coupled to the gear trains. At the beacon, Fig. 1 (not shown), the rotating beam is produced by a wave-guide aperture radiating into a cylindrical parabolic reflector and the reference and identification signals are produced by a commutator ganged to the rotating aerial system.