Patent GB-581568-A (GB)
Title: Improvements relating to arrangements for determining distance and bearing by electromagnetic waves
IPC: G01S13/84
Filed: 1941-11-14
Abstract: 581,568. Wireless systems for determining distances and bearings. STANDARD TELEPHONES & CABLES, Ltd., and JACOBSEN, B. B. Nov. 14, ]941, No. 14708. [Class 40 (v)] A wireless system for measuring the distance between two points, of the kind in which an original signal transmitted from one station is received and retransmitted by a second station, either as a modulation of the same or a different carrier wave, and on reception at the first station is compared with the original signal, is characterised by the provision of means at the second station in the path of the signal to alter by a known amount the apparent distance travelled by it, which may be varied according to a predetermined time schedule, thereby rendering the indications useless to unauthorized persons. In the system shown in Fig. 1, signals emitted from a transmitter TA at a first station A (which may be carried by an aeroplane) are demodulated by a receiver RB at the second station B, passed through a phaseshifting device N and modulate the carrier from a transmitter TB, this carrier being preferably of a different frequency from that of transmitter TA. The retransmitted signals are demodulated in a receiver RA at the first station and are compared with the original signals in a comparator C in which the phasechange or delay is measured, the corresponding distance between the stations being shown on an indicator I. A similar phase-changing network to N may be used at station A, so that by adjusting both networks according to a pre-arranged time schedule the effect of N on the distance measurement may be compensated so that no allowance has to be made for it. Instead of demodulating the signals at station B, the received carrier may be retransmitted after passage through a highfrequency amplifier with a network corresponding to N. Preferably continuous wave modulation is used, but pulse or frequency modulation may be employed. In a modification, allowing null methods of observation at station A, the network N at station B is replaced by a phase changer which cyclically varies the phase through 360 degrees and sends out a reference signal (for example, by applying a blocking voltage to a valve in the transmitter) at some point in each cycle. The comparator C measures the time interval between the reference signal and the occurrence of phase opposition of the original and recovered signals. The timing of the reference signal with respect to phase variation is varied to render the indications valueless to unauthorized persons. In another modification, station B is as shown in Fig. 1, whilst at station A a cyclically variable phase changer is interposed between the receiver RA and the comparator C, and a timing device is employed which measures the interval between a reference signal emitted by the phase changer and the occurrence of phase opposition in the comparator. In this form, the phase changing network may be manually adjusted to obtain phase opposition. A phase-shifting network is introduced between the modulator and transmitter to compensate for that produced by the network N at station B. In a further system the transmission from station A is modulated at a frequency of 200 per second by a high-stability oscillator in conjunction with a frequency divider. The output of a similar oscillator and frequency divider at station B is first adjusted by the operator by means of a comparator to agree in frequency and phase with the received signals, and is then supplied to modulate a transmitter. The signals received at station A are passed through a cyclic phase-changer and compared with those from the frequency divider by a null method as above described. Adjustable phaseshifting networks are provided at both stations. Two fixed stations may be used in order to enable the bearings of a movable station to be determined by distance measurements on both.