Patent GB-610091-A (GB)
Title: Improvements in or relating to radio direction indicating systems and apparatus for measuring short intervals of time
IPC: G01S1/24
Filed: 1946-03-07
Abstract: 610,091. Measurement of short-time intervals. DECCA RECORD CO., Ltd., and O'BRIEN, W. J. March 7, 1946, No. 7152. [Class 37] [Also in Group XL (c)] In a radio navigation system of the kind in which positional information is obtained by measuring on a craft the intervals of time between the reception of pulse signals transmitted from two or more spaced transmitting stations, the interval between transmission of corresponding pulses from the transmitters being constant, each individual carrier-wave cycle in the pulses from one station corresponds to a similarly placed cycle in the pulses from another station and the radiations from the transmitters comprise a series of pulses having the same pulse repetition rate, pulse duration and carrier frequency. The transmitters may comprise one master station and the required number of slave stations. At each slave station a crystal oscillator 19, Fig. 3, whose frequency corresponds as closely as possible to the master station signal has its output passed through a frequency corrector 20 which comprises a goniometer arranged to advance or retard the phase of the output in steps of 3 degrees at a constant rate, adjustable from zero to 30 steps a minute. The timing of the steps is controlled by a monitor receiver 28 at some distance from the transmitter, this receiver also controls a random phase variation corrector 21 so that the output passed to pulse modulated amplifier 22 and thence to transmitter 23 is exactly synchronised with the master transmitter. The modulation signal is obtained from a fre. quency divider 25 delivering 200 microsecond pulses at the rate of 50 per second. The divider is synchronised with the carrier wave by coupling its input to the output of phase corrector 21 through phase adjuster 26 to which manual adjustments are made in accordance with the indication given by monitor 27 which includes a cathode-ray tube displaying a superimposed image of the carrier signal output of modulator 22 and the pulse output of divider 25. The carrier waves are thereby brought into a predetermined relation with the modulation pulses, the relation being the same for all the stations. The receiver, Fig. 1, comprises a crystal oscillator 1 providing a stable 10 Kc. output which is applied to divider 4. The 1000 cycle output is fed to a phase-shift goniometer 5 and thence through a quadrature network 6 to deflection coils 7 and 8 of a cathode-ray oscilloscope giving a circular trace line on the screen. The output of divider 4 is passed through a further divider 9 providing a 200 cycle output for the field coils of goniometer 10. The rotor coil of this goniometer provides a manual control for the continuous variation of the output signal phase and provision is also made to alter the phase of the output by four steps of 360 degrees. The output of the gonio. meter 10 feeds a saw-tooth generator 11 whose output provides radial deflection for the oscilloscope through switch 12. When switch 12 is closed the trace becomes a five-turn spiral. A further divider 13 delivers a rectangular pulse during one cycle of the output 10 and zero output for the succeeding three cycles. The four-step control allows the rectangular pulse to be placed in any desired one of the four successive cycles. The output of divider 13 is applied to switch on the beam of the oscilloscope during every fourth cycle of the output of goniometer 10 and also to a gain control in the output of the receiver 15 so that incoming signals are only applied to the radial deflection circuit for one cycle in four. The result of these arrangements is effectively to expand the length of the trace in order to provide greater accuracy. A circular scale is mounted in front of the tube in order to measure the distance between points on the trace thereon. In operation the goniometer 10 and the four-step control are set so that incoming signals from the master station come on the centre ring of the spiral and with switch 12 then opened the intersection of the zero signal trace and the finish of the third half-cycle of signal is brought into line with the scale zero. The length of trace and thus the time interval to any other received signal can then be found, altering the four-step control if necessary. The master station signals may be identified by superimposing coded keying thereon. Any lack of synchronism between the time-base crystal oscillator and the pulse recurrence frequency of the transmitter, which causes rotation of the display, must be corrected. This may more easily be done by changing switch 18 to feed coils 8 with 10 Kc. signals when any movement of the display is amplified on the more complex trace produced, Fig. 3 (not shown).