Patent GB-1065818-A (GB)
Title: Radio location ranging system
IPC: G01S13/84
Filed: 1964-08-04
Abstract: 1,065,818. Radio navigation. SEISMOGRAPH SERVICE CORPORATION. Aug. 4, 1964 [Oct. 2, 1963], No. 31131/64. Heading H4D. In a radio position-finding system comprising a mobile unit 10, Fig. 1 (not shown), and two fixed transponder stations 11 and 12, the mobile unit transmits first and second CW interrogating signals of different frequency, during the first half of a transmission cycle (see Fig. 4, not shown) and each transponder transmits first and second reply signals in reply to and phase locked to the mobile unit interrogating transmissions and with the same frequencies as the interrogating signals, during consecutive halves of the second half of the transmission cycle. The first reply signals are compared in phase at the mobile station with a locally generated reference signal phase locked to the signals transmitted by the mobile unit; to produce, for each transponder station, a fine distance indication, and the first reply signal from each transponder station is compared in phase with  the corresponding second reply signal, to produce for each transponder station, a coarse,  but unambiguous, distance indication.  In the mobile unit, Fig. 2, a crystal oscillator 15 produces a signal at 2200 kc/s. which is  gated through gate 16 during the first half of the transmission cycle, by a signal from master sync. generator 18, to pass to a converter 19, where it is mixed with the 200 kc/s. output of oscillator 20, to produce first and second signals at frequencies of 2000 kc/s. and 2400 kc/s. respectively. These are transmitted to the transponder stations. At transponder station (11), Fig. 3 (not shown), the two signals are received and passed via tuned amplifiers (55) and (56) to converters (57) and (58) where they are mixed with the outputs of 2200 kc/s. oscillator 61 to produce signals at 200 kc/s. These signals are further heterodyned down to 315 c/s. by means of converters (62) and (63) and oscillator (64) operating at 200À315 kc/s. Reference signals at 315 c/s. are produced by converters (71) and (85), by mixing the outputs of oscillators (64) and (68), the latter operating at 200 kc/s. and applying its output to the converters via adjustable phase shifters (69) and (70). The phase of the 315 c/s. signal from converter (62) is compared with that of the reference signal from converter (71) in phase discriminator (73), and any detected difference energizes a motor (74) which in turn adjusts phase shifter (69) until the two signals are in phase. Phase shifter (70) is similarly adjusted until the reference signal from converter (85) is in phase with the signal from converter (63). Gate circuits (59), (67) and (89) are opened during the first half of the transmission cycle, under the control of gate sync. generator (60), which is synchronized with the mobile station by the termination of the signal from detector (62). The gate sync. generator (60) produces a pulse which is applied via switch (78) to open gate (77) for the third quarter of the transmission cycle. The 2200 kc/s. signal from oscillator (61) is applied through gate (77) to converters (76) and (81), where it is heterodyned with the phase locked outputs of oscillator (68) to produce the two reply signals at 2000 kc/s. and 2400 kc/s. The transponder station (12) is identical with station (11) except that switch (78) is in its other position such that the pulse from generator (60) is delayed by one quarter of the transmission cycle and the reply signals accordingly occupy the last quarter of the transmission cycle. The reply signals from station (11) are received at the mobile unit, and pass, via tuned amplifiers (25) and (26) to converters (28), (29), (44) and (45) where they are heterodyned down to 315 c/s., using the outputs of oscillators (15) and (30), gate (17) being open during the second half of the transmission cycle. A reference signal at 315 c/s. is produced by converter (36) by heterodyning the outputs of oscillators (20) and (30) and is compared in phase with the 315 c/s. signal from converter (29) in discriminator (35). The resultant phase difference provides a fine distance to transponder indication on indicator (38), a counter (40) being advanced one step for every 360 degrees phase change to provide a lane count. The phases of the two 315 c/s. signals arising from the reply signals are compared in discriminator (41) to provide a coarse distance to transponder indication on indicator (42) and counter (43). Discriminators (47) and (50), controlling indicators (48) and (51), and counters (49) and (52), are likewise provided for indicating the distance to the transponder (12).