Patent GB-670338-A (GB)
Title: Improvements in or relating to radio navigation systems
Inventor: EARP CHARLES WILLIAM
IPC: G01S1/40
Filed: 1949-01-12
Abstract: 670,338. Radio navigation. STANDARD TELEPHONES & CABLES, Ltd. Jan. 6, 1950 [Jan. 12, 1949], No. 856/49. Class 40 (vii). The invention relates to a radio beacon having a plurality of aerials equally spaced round a circle which are consecutively energized one at a time in cyclical succession round the circle to radiate a normally unmodulated train of pulses which, as received at a distant station is time-phase modulated, the phase serving to indicate the azimuth bearing of the station relative to the beacon. According to the invention a receiver is located at the centre of the circle for receiving the train of pulses and deriving therefrom a correction waveform corresponding to the time-phase modulation due to inaccuracy of timing of the radiations from the beacon aerials and the correction waveform utilized to correct the error in azimuth indication due to the timing inaccuracy. The correction waveform may either be used at the beacon to control the time of energization of the aerials in such a sense as to balance out the inaccuracy or it may be radiated to the distant station so that correction may be applied there. The invention may be applied to a multi-channel pulse beacon of the type described in Specification 635,487, the relevant portions of one such beacon being shown. A 10 kc/s. generator K and pulse generator PG feed pulses to delay network DN1 and thence to channel modulators C1, C2, C3, C4 and C5. These are normal elements in a time-sharing multichannel transmitter and the output therefrom (except C1) together with double synchronizing pulse DP is applied to modulate transmitter O. In the present example the normally unmodulated output of C1 is not fed to transmitter O but is applied to a series of gate circuits G1, G2, G3, G4, G5. These gate circuits are controlled by pulses of 2 kc/s. frequency derived from unit PG through a divider and tapped off at points along a second delay line DN2 so that the pulses passed by the gate circuits consecutively energize in cyclic succession transmitters ABCDE spaced round a circle. The output of the divider is filtered at F and the 2 kc/s. wave used to modulate a reference channel C2. Channels C3, C4 and C5 modulated from sources 3, 4 and 5 may be used for purposes unconnected with the azimuth beacon. According to the invention a receiver R is placed in the centre of the circle and the output therefrom gated at G by lengthened pulses derived from DN1 so that beacon pulses only are passed to trigger demodulator T. This is triggered in one direction by the timing pulses from DN1 and in the other by the received beacon pulses so that pulses which are width-modulated according to any timing inaccuracy are obtained. Lowfrequency filtering at LFF gives the error wave which in this embodiment is fed back to C1 to modulate the pulses fed to the beacon aerials in such a way as to cancel out the timing inaccuracies and cause true bearings to be indicated at distant receivers. In a second method of correction the received pulses from the gate G time-phase modulated in accordance with any timing error are time-shifted as a whole by a delay line and then applied to transmitter O in place of one of the further channels (Fig. 2, not shown). At the receiver for this system pulses received over the error signal channel and the beacon pulses are used to trigger a trigger demodulator to produce a width modulated pulse train in which the width modulation is dependent on the bearing and is freed from inaccuracies due to mis-timing of the beacon transmitters.