Patent GB-804605-A (GB)
Title: Radio location system
IPC: G01S13/82
Filed: 1955-04-15
Abstract: 804,605. Radio navigation. SEISMOGRAPH SERVICE CORPORATION. April 15, 1955 [April 23, 1954], No. 10940/55. Class 40 (7). Relates to a C.W. hyperbolic radio navigation system in which a pair of position-indicating signals is transmitted from each of three spaced transmitters, the signals being heterodyned at a mobile receiver to produce beat frequency signals which are compared in phase with corresponding reference signals derived at some of the transmitters and transmitted therefrom as modulation signals to give a first pair of coarse and fine position indications relative to a first pair of transmitters and a second pair of coarse and fine position indications relative to a second pair of transmitters. According to the present invention at each transmitter the position indicating signals are applied to a single aerial through a linear signal channel including a push-pull class A or class B power amplifier. The invention is described as applied to a system in which frequencies (F3 + 420), F1 and F2 are transmitted continuously from spaced transmitters 10, 11 and 12, Fig. 1, respectively and frequencies (F1 + 240) and (F2 + 600) are transmitted alternately from transmitter 10, a frequency F3 being transmitted alternately from transmitters 11 and 12 in synchronism with the alternate transmission of frequencies (Fl + 240) and (F2 + 600). As described F1 = 2195.76 kc/s., F2 = 2413.4 kc/s. and F3 = 2299.58 kc/s. When the frequency (Fl + 240) is being transmitted from transmitter 10, the frequencies (F3 + 420) and (F1 + 240) received at the transmitter unit 12 from the transmitter 10 and heterodyned with the frequencies F3 and F1 received from transmitter 11 and the frequency F2 transmitted from transmitter 12 is modulated by the resultant beat frequencies 420 and 240. At the mobile receiver 13, Fig. 2, the frequencies (F3 + 420) and (F1 + 240) from transmitter 10 are heterodyned with the frequencies F3 and F1 from transmitter 11 to give 420 c/s. and 240 c/s. position signals; reference signals of corresponding frequency are obtained by demodulating the F2 signal from transmitter 12. The 240 c/s. position and reference signals are compared directly to give a fine position indication relative to transmitters 10 and 11 and a coarse indication is obtained by heterodyning the 420 and 240 c/s. position signals and comparing the resultant 180 c/s. beat signal with the corresponding 180 c/s. beat signal obtained by heterodyning the 420 and 240 c/s. reference signals. When the frequency (F2 + 600) is being transmitted from transmitter 10, corresponding fine and coarse position indications relative to transmitters 11 and 12 are produced by utilizing the 420 and 600 c/s. signals obtained by heterodyning the frequencies (F3 + 420) and (F2 + 600) from transmitter 10 with the frequencies F3 and F2 from transmitter 12, the fine indication being obtained by direct comparison of the 600 c/s. position and reference signals and the coarse indication being obtained by heterodyning the 600 and 420 c/s. to give 180 c/s. signals. In a modification, Fig. 3 (not shown), the frequency (F3 + 420) is transmitted intermittently so that during the periods of (F3 + 420) transmission the system functions as above but during the periods when (F3 + 420) is not transmitted only 600 or 240 c/s. position or reference signals are produced giving fine indications only. Detailed description.-Transmitters, Fig. 1. At the central transmitter unit 10 frequencies of (F1 + 420) and (F2 + 600) from oscillators 18, 19 are applied alternately through switch 20 (periodic time quarter of a second) to a linear mixer 15 together with the continuous output of frequency (F3 + 420) from oscillator 14, the output from mixer 15 being applied to aerial 17 through a linear amplifier 16. The end transmitter unit 11 comprises oscillator 29 (frequency F3) coupled directly to a linear mixer 22 and an oscillator 21 (frequency F1) coupled to mixer 22 via modulator 21a, the output from mixer 22 being applied to aerial 30 via linear amplifier 23. The unit 11 also comprises receiver 24 (tuned to F2) and receiver 25 (tuned to frequency F3) coupled respectively through 600 c/s. and 420 c/s. band-pass filters 2b and 25a and gate circuit 28 to modulator 21a. An output from the 600 c/s. filter 26 is also applied to unit 27 which controls receiver 25, gate cirucit 28 and oscillator 29 such that receiver 25 and gate circuit 28 are operative and oscillator 29 is inoperative only when a 600 c/s. signal is applied to control unit 27. The other end transmitter unit 12 comprises corresponding oscillators 38 (frequency F3) and 31 (frequency F2) and receivers 34 (tuned to F1) and 35 (tuned to F3), the control unit 37 being such that receiver 35 and gate circuit 39 are operative and oscillator 38 is inoperative only when a 240 c/s. signal is applied from band-pass filter 3b to control unit 37. When switch 20 at transmitter unit 10 is such that frequency (F1 + 240) is transmitted, receiver 34 at unit 12 receives the frequency together with frequency F1 from transmitter unit 11 and the resultant 240 c/s. beat signal actuates control unit 37 to open gate 39, render receiver 35 operative and render oscillator 38 inoperative; a 240 c/s. output from filter 3b is also applied through gate 39 to modulator 31a. The receiver 24 at transmitter unit 11 receives only the frequency F2 from transmitter unit 12; accordingly there is no 600 c/s. output from receiver 24 and hence receiver 25 and gate circuit 28 are inoperative but oscillator 29 (frequency F3) is rendered operative. Receiver 35 at unit 12 receives this frequency and also the frequency (F3 + 420) from transmitter unit 10 to give a 420 c/s. beat signal which is applied through filter 35a and gate 39 to modulator 31a. Thus during the (F1 + 240) transmission, unit 10 transmits frequencies (F1 + 240) and (F3 + 420), unit 11 transmits frequencies (F1 and F3) and unit 12 transmits frequency F2 modulated by 240 c/s. and 420 c/s. reference signals. Similarly during the F2 transmission, unit 10 transmits frequencies (F2 + 600) and (F3 + 420), unit 12 transmits frequencies F2 and F3 and unit 12 transmits frequency F1 modulated by 420 c/s. and 600 c/s. reference signals. The mobile receiver unit 13, Fig. 2, comprises three receivers 42, 43 and 44 tuned respectively to frequencies in the region of F1, F3 and F2. In all conditions of switch 20 at unit 10, receiver 43 receives frequencies F3 and (F3 + 420) and produces a 420 c/s. position signal which is applied to mixers 55 and 57. During the (F1 / 240) transmission, receiver 42 receives frequencies F1 and (F1 + 240) from transmitter units 11 and 10 respectively and produces a 240 c/s. position signal and receiver 44 receives the modulated F2 from unit 12 and produces the 420 c/s. and 240 c/s. reference signals. The 240 c/s. position and reference signals are selected by filters 48 and 50 respectively and compared in phasemeter 49 to give a fine indication relative to transmitter units 10 and 11. A 240 c/s. output from filter 48 is also heterodyned in mixer 57 with the 420 c/s. position signal from receiver 43 to give a 180 c/s. position signal which is selected by filter 58 and compared in phasemeter 59 with a 180 c/s. reference signal obtained by heterodyning in mixer 63 the 420 c/s. and 240 c/s. reference signals from receiver 44; phasemeter 59 thus gives a coarse indication relative to transmitter units 10 and 11. Similarly during the (F2 + 600) transmission, receiver 44 receives frequencies F2 and (F2 + 600) from transmitter units 12 and 10 respectively and to give a 600 c/s. position signal and receiver 42 receives the modulated F1 signal from transmitter unit 11 to give 420 c/s. and 600 c/s. reference signals. The 600 c/s position and reference signals are selected by filters 47 and 45 respectively and compared in phasemeter 46 to give a fine indication relative to transmitter units 10 and 12. A 600 c/s. output from filter 47 is also heterodyned in mixer 55 with the 420 c/s. position signal from receiver 43 to give a 180 c/s. position signal which is selected by filter 54 and compared in phasemeter 53 with a 180 c/s. reference signal obtained by heterodyning in mixer 56 the 420 c/s. and 600 c/s. reference signals from receiver 42 ; phasemeter 53 thus gives a coarse indication relative to transmitter units 10 and 12. Modifications.-In Fig. 3 (not shown) the (F3 + 420) oscillator 14 is intermittently operative, each 1 second operative period being followed by a 3-second inoperative period. During the operative periods the system functions as described above with reference to Figs. 1 and 2 but during the inoperative periods a fine indication only is produced by the 240 c/s. signals during the (F1 + 240) transmission and by the 600 c/s. signals during the (F2 + 600) transmission. The end transmitter units 11 and 12 differ also in that the F3 oscillators 29 and 38 at units 11 and 12 respectively are continuously operative, the outputs from these oscillators being applied to mixers 22 and 32 through gate circuits controlled by units 27 and 37 in the same way as the oscillators themselves are controlled in Fig. 1. In Fig. 4 (not shown) the frequencies (F3 + 420), (F1 + 240) = (F3 + 420 - 104 kc/s. and (F2 + 600) = (F3 + 420) +114 kc/s are produced at transmitter unit 10 by separately amplitude modulating frequency (F3 + 420) with signals of frequency 104 kc./s. and 114 kc/s., the unwanted frequency components being rejected by suitable filters. At transmitter unit 11 the frequency F3 is produced by a 103.82 kc/s. oscillator coupled to a modulator and filter which replaces mixer 22 and a similar arrangement is employed at transmitter unit 12, the frequency of the oscillator being 113.82 kc/s. ; these oscillators are controlled by units 27 and 37 in the same way as the oscillators 29 and 38 are controlled in Fig. 1. Specifications 452,235, [Gro