Patent GB-933986-A (GB)
Title: Electronic surveying systems
IPC: G01S13/36
Filed: 1961-05-29
Abstract: 933,986. Radio distance measuring apparatus. CUBIC CORPORATION. May 29, 1961, No. 19353/61. Class 40 (7). In an arrangement for measuring the distance between two points a master station at one point transmits a carrier frequency f c1  which is frequency modulated by a signal of frequency f m1  and at the second point a slave station, in response to reception of the signal transmitted from the master, transmits a carrier of frequency f c2  frequency modulated by a signal f m2  which is received at the master stations and mixed with its own transmitted signal to produce a signal corresponding to the difference between the frequencies f m1  and f m2  which signal includes information concerning the distance between the two points based on the phase shift produced in the signal of frequency f m1 . To effect a measurement the master station makes four successive transmissions in which the modulation frequency f m1  has respective frequencies of 10 mc/s., 9.000, 9.900 and 9.990 mc/s., and on reception of each of the latter the slave station transmits modulation frequencies f m2  which differ by 1 kc/s. from the respective received frequencies, the carrier frequency f c2  of the slave station being held at a frequency of 48 mc/s. above that (f c1 ) of the master which operates an approximately 9 KMc/s. At both the stations the modulated carriers (f c1  and f c2 ) are mixed and the result of such mixing is shown to result in the production of an I.F. (=48 mc/s.) amplitude modulated by a frequency of 1 kc/s. At the slave station this A.M. signal is utilized to produce its (reply) F.M. signal f m2  at a frequency which is 1 kc/s. lower than each of the received f m1  signals and is also employed to frequency modulate a 70 kc/s. sub-carrier which also modulates the slave carrier f c2 . At the master station the 1 kc/s. A.M. signal derived by mixing its own modulated carrier with the modulated carrier received from the slave station is phase compared with the 1 kc/s. derived from the 70 kc/s. sub-carrier and the phase difference provides a measure of the distance between the two points. In the case of the measurement at 10 mc/s. the accuracy of the result is high (being determined by a highly stable 10 mc/s. reference oscillator) but is necessarily ambiguous for distances separating the two stations of over 50 feet approximately. This ambiguity is resolved by the second measurement which is made with a modulation frequency f m1  of 9 mc/s. giving an effective range frequency of 1 mc/s. and the ambiguity in this measurement is resolved by the third measurement with # m1 =9.900 mc/s. which produces an effective range frequency of 100 kc/s. The final measurement at f m1 = 9.990 mc/s. which is equivalent to a range signal of 10 kc/s. then resolves the ambiguity in the third measurement and yields an unambiguous range of approximately 10 miles. In addition to ambiguity the measurement is inaccurate due to the effect of phase shift in the 70 kc/s. signal and phase shifts inherent in the circuitry and to eliminate these errors a further measurement is made with # m1 =10 mc/s. in which the frequency f m2  of the reply signal is 1 kc/s. above that of the master signal f m1  the difference between this measurement and each of the measurements made with f m2  1 kc/s. below f m1  giving a measurement which is free from errors due to such phase shifts. Identical apparatus (Figs. 1 and 2) is employed at each station, the switches 26 being in the M position when operating as a master station and in the S position when operating as a slave station. For master operation the switch 34, Fig. 1 (which also allows call ringing and speech communication with the slave station) is set to the " measure " position in which the klystron 10 is frequency modulated over lead 36 by the output of mixer 82, Fig. 2, which comprises in accordance with the positions of switches 58 (VF=very fine; F=fine; C=coarse; VC=very coarse) the various f m1  frequencies between 10 and 9.990 mc/s. described above. The f m1  frequencies are derived from an oscillator 62 controlled by crystals 92 . . . 95, the precise frequencies being produced by controlling the latter crystals by means of voltage-sensitive capacitors 85 . . . 87 in accordance with the outputs of phase discriminators 72, 66, 65 and frequency discriminator 64 which supply a voltage to devices 85 . . . 87 when the effective range frequency produced by oscillator 62 differs from that of reference standards 60, 71 or 70 or from 10 kc/s. in the case of discriminator 64. At a slave station the centre frequency of the klystron 10 is maintained at 48 mc/s. above that of the master station klystron (nominal frequency 9 KMc/s.) by means of a loop circuit including the 48 mc/s. and 10.7 mc/s. I.F. circuit and an automatic frequency control circuit 25 (Fig. 1). In response to each measurement signal from a master station the slave station by manual operation of switches 58 and switch 90 transmits a reply signal 1 kc/s. below the received measurement signal (f m1 ) and, additionally, 1 kc/s. above in the ease of the V.F. measurement, the klystron 10 being frequency-modulated by the output of mixer 82 which in addition supplies a 70 kc/s. sub-carrier from oscillator 76 frequency-modulated by a 1 kc/s. signal derived, as described above, from an A.M. detector 22 and supplied to a reactance tube 75. For these transmissions the accuracy of the transmitted frequencies is maintained by the reference standards as described above for master operation. At the master station the 1 kc/s. signal derived from this 70 kc/s. subcarrier in discriminator 46 is supplied via an adjustable calibrated phase shifter 48 to a phase discriminator 43 which has as a second input the 1 kc/s. signal derived from the master station A.M. detector 22 and measurement is effected by adjustment of phase shifter 48 to produce a null indication on meter 52.