Patent GB-889843-A (GB)
Title: Improvements in or relating to radio navigation aids
Inventor: JOLLIFFE SIDNEY ARTHUR WALTER
IPC: G01S1/30
Filed: 1959-07-17
Abstract: 889,843. Radio-navigation. MARCONI'S WIRELESS TELEGRAPH CO. Ltd. Dec. 15, 1959 [July 17, 1959], No. 24613/59. Class 40(7). A radio-navigation system of the hyperbolic position line type comprises at least two spaced transmitters radiating similar frequency-modulated signals having a fixed predetermined phase relationship so that the position of a mobile (e.g. aircraft) receiver may be determined (using prepared charts) from the frequency difference between the received signals. The transmitted signals comprise two main carriers of different frequency which are amplitude modulated by subcarriers of the same frequency which are themselves frequency modulated. The frequencies are so chosen that errors due to the Doppler effect are negligible even when the receiver is carried by the fastest of aircraft. The modulation signal may be of sawtooth wave form or, preferably, sinusoidal to reduce bandwidth requirements and the fixed predetermined phase relationship is preferably an out of phase condition to avoid the production of an "zero beat" position line in the useful area served by the system. By employing at least three spaced transmitters on a line at right angles to a required course direction three laterally spaced route lines characterized by particular beat frequencies between each of the three possible pairs of transmitters may be established and the position along any such route line determined by the crossing thereof by (hyperbolic) position lines due to a different pair of transmitters. In the case of the two transmitter system, the receiver, which combines the two received frequency modulated signals in a twoterminal mixer to produce a beat frequency for application to a frequency measuring device, is provided with means consisting of an automatically operated change-over switch at the mixer input to ensure that one of the two terminals always receives either the higher or lower of the two frequencies producing the beat. This avoids ambiguity due to phase reversals. Fig. 1 shows a simplified arrangement of the two-transmitter system which 'comprises two geographically spaced aerials A, B, radiating, respectively, carrier waves of different -frequency FC1, FC2 amplitude modulated in 3, 4 by frequency modulated signals produced in 5; 6 by frequency modulation of a sub-carrier wave from source 7 by means of a modulation signal from source 8, a delay device 9 being included in the circuit between source 8 and one of the modulators (e.g. 6) to avoid the production, as mentioned above, of an "zerobeat" position line in the service area of the system. The position of a mobile receiver within the latter area may then be determined by (using prepared charts) measurement of the beat frequency produced by combining the two received frequency modulated signals since each value of beat signal is indicative of a particular position line. (e.g. the hyperbolae X<SP>1</SP> or the straight line X). Fig. 2 shows a receiver suitable for use in the system and which comprises two channels A, B (appropriate to the transmitters A and B each of which includes R.F. circuits 10 (A, B), frequency changing and I.F. amplifiers 11 (A, B) and detectors 12, (A, B). The outputs of the latter, which may be frequency multiplied by the same factor in multipliers 13 (A, B), are then combined in a mixer 14 the beat signal from which, after shaping as required in 15, is fed to a counting circuit 16 which provides a measure of the beat frequency. To prevent ambiguity due to phase reversals, as mentioned above, the input circuit to mixer 14 includes a change-over switch 17 (shown as a mechanical switch although, in practice, an electronic switch is employed). This is operated, whenever the instantaneous frequency of the modulation signals in the two channels "cross-over", by separately detecting such signals at 18A, 18B, comparing the phases in a phase comparator 19, squaring the output of 19 and then differentiating the squared waveforms in a circuit 20 to produce from the leading end trailing edges, sharp pulses of opposite polarity which operate the switch control 21. These pulses may also be employed to reset the counter 16. A receiver for use in the three transmitter system described above is provided with a further set of the elements 10 ... 12 of Fig. 2 for the third transmitter and the three detectors 12 may be selectively coupled in pairs, by means of a manually operated switch, to the inputs to the frequency multipliers (Fig. 5, not shown). An arrangement of the three transmitters is 'described with reference to Fig. 4 (not shown).