GB607371A - Improvements in or relating to radio navigational aids or systems - Google Patents

Abstract

607,371. Radio navigation. FLETCHER, H. Jan. 31, 1946, No. 3181. [Class 40 (vii)] A short-base hyperbolic radio beacon comprises two spaced transmitters which transmit carrier waves of different frequencies, one of the carriers being modulated by a signal, the frequency of which is sub-harmonically related to the frequency difference between the carriers by an integral factor greater than two. The transmitters, Fig. 3, comprises two master oscillators transmitting on frequencies separated by a frequency difference f. The outputs from the oscillators are frequency multiplied an integral number of times (in this case 3) and then fed to the transmitting aerials through power amplifiers. The master oscillator outputs are also applied to a mixer, the output of which (of frequency f ) is used to modulate the output of transmitter B. The spectral distribution of frequencies is as shown in Fig. 4, AC representing the carrier transmitted from A, BC the carrier wave transmitted from B and BS2 the side band (assuming single side band modulation is being used). At the receiver, Fig. 5, the beat between AC and either BC or BS2 is selected by the filter F2. The beat between BS2 and BC is selected by the filter F1 and frequency multiplied by M1 to make it equal in frequency to the output of F2 with which it is compared by the phase meter PM1. The phase of the output of F1-M1 is independent of azimuth since it comprises the modulation of the carrier BC, but the phase of the output of the filter F2 is dependent on the difference between the paths from each transmitter to the receiver and hence is dependent on azimuth as described in Specification 578,479. The reading of the phase meter is consequently a measure of the direction from the transmitters to the receiver. The phase difference between the two lowfrequency signals is proportional to the carrier frequency used and so the discrimination is high, and ambiguities caused by different directions having the same phase differences (apart for a difference of 2#) occur. To overcome this a low-frequency generator, Fig. 3, provides a signal which modulates the carrier wave BC and thus produces the side-band BS3. The L.F. signal is frequency multiplied an integral number of times (in this case 5) and then used to modulate the carrier AC, thus giving rise to the side band AS. At the receiver these modulations are separated out by filters F3, F4, frequency multiplied to bring them to a common frequency and compared by the phase meter PM2. Thus a " coarse " hyperbolic grid is laid down which is used to give an approximate direction and thus remove the above-mentioned ambiguities.