Patent GB-776737-A (GB)
Title: Radio location system
IPC: G01S19/48
Filed: 1955-12-16
Abstract: 776,737. Radio - navigation. STANDARD TELEPHONES & CABLES, Ltd. Dec. 16, 1955 [Dec. 21, 1954], No. 36151/55. Class 40(7) A radio navigation system comprises a beacon station and a receiving station. The beacon includes means for radiating energy according to a moving multilobed radiation pattern and also a spaced receiver which receives the signals from the beacon aerial, detects the amplitude modulation caused by the movement of the radiation pattern and thus obtains a reference signal which is transmitted. The receiver includes means for receiving, detecting, and comparing the direct signals from the beacon and the reference signal. Various methods of transmitting the reference signal may be used. The beacon transmitter may be pulse modulated and the reference signal used to time modulate the pulses (Fig. 1, not shown) or the output of the spaced receiver 34 may be used to frequency modulate the beacon signals in transmitter 31. In systems where the multilobed beacon pattern is continuously rotated and bearing information can be obtained in any direction from the beacon the receiving station is provided with an aerial 35. receiver 36 and separate detectors 37 and 38 for the reference signal and beacon signal envelope. The amount of delay necessary to provide maximum auto-correlation of the reference signal, which delay represents the beacon rotation period, is determined by means of an adjustable delay device and an auto-correlation detector 42 having an indicator 42a. One form of delay device consists of an endless band of magnetic tape 40 driven by motor 43. The reference signal is recorded by head 39 and picked-off by head 41. The spacing between the two heads is adjusted for maximum correlation and then represents the rotation period. The motor drive 43 may be made responsive to the output of correlator 42 to ensure that the magnetic tape speed is adjusted to compensate for any slight variations in rotation rate and maintain the head spacing truly representative of 360 degrees of pattern rotation: The beacon signal envelope is cross-correlated with the responsive signal and before being fed to the correlation detector is passed through an adjustable delay device. This device is adjusted for maximum correlation and as shown consists of a head 44 recording on a second track on tape 40 and a movable pick-off head 45 feeding the delayed signal to the cross-correlation detector 46. The ratio of the spacing between heads 44 and 45 to that between heads 39 and 41 is a measure of the bearing of the receiver from the beacon. Defining a track in azimuth: To define a course 49 the beacon may comprise a transmitter 55 feeding spaced aerials 50, 51, 52 and 53 through individual goniometers. A motor 60 synchronously moves each end of the goniometers to cause a multilobed radiation pattern to sweep back and forth. across the course. line. The reference signal is derived from receiving aerial 65 on the course line and modulated on to a separate radio frequency signal radiated by aerial 69. Arrangements at the receiver (Fig. 4b, not shown) are similar to those described with reference to Fig. 3. The reference signal pick off head is adjusted along the reference signal track on the tape to give maximum auto-correlation. Two cross-correlation detectors are, however, provided fed respectively from pickoff heads spaced from the beacon signal recording head by a little less and a little more than the spacing between the reference signal recording heads. When the two detectors give equal indications the receiver is on the course line and deviation to one side or the other is indicated by an increase in the appropriate indication and a decrease in the other. In an alternative form of receiver a plurality of non-movable beacon signal pick-off heads are arranged along the tape, each provided with its own cross-correlation detector and indicator.