Patent GB-1345595-A (GB)
Title: Methods for determining the distance travelled by a vehicle over a reference surface and systems for performing siad methods
IPC: G01S15/60
Filed: 1971-06-17
Abstract: 1345595 Distance measurement; pulse radar INSTITUT FRANCAIS DU PETROLE DES CARBURANTS ET LUBRIFIANTS 17 June 1971 [24 June 1970] 28521/71 Heading H4D [Also in Division G4] The distance travelled by a vehicle (e.g. a ship) over a reference surface is determined by transmitting an acoustic or electromagnetic wave-energy signal obliquely towards the surface, measuring the Doppler frequency shift of waves received by reflection from the surface, and computing the distance travelled from the measured shift, the duration of the transmitted signal being made substantially equal to the two-way travel time of a reference signal previously transmitted towards and reflected from the surface. In the Fig. 4 embodiment (and Fig. 3 timing diagram) the reference signal In, comprising a train of oscillations of frequency F 0  from source 1, is transmitted by transducers 4a, 4b oppositely inclined to the vertical. The echo of this signal initiates transmission of the main signal E n  which persists for a time T ne  equal to a known fraction k 1  (#1) of the two-way travel time T n  of the reference signal, both times being measured by counter-timers. The echo signals of this main signal are amplified at 5a, 5b and passed via switches 10a, 10b to phase-locked loop frequency-trackers 6a, 6b during a time gate R n  (a known fraction k 3  of T n  in width), which gate excludes spurious transmission breakthrough. Tracker 6a locks on to the frequency F 0  + #f n  (where #f n  is the Doppler shift), and tracker 6b locks on to F 0  - #f n . The trackers memorize these frequencies until a subsequent time gate R n  occurs, and their outputs are demodulated at 11 to give a Doppler frequency signal 2#f n  which is extracted by low pass filter 12 and passed to bi-directional counter 13 which counts the Doppler frequency cycles during an interval equal to the recurrence period of successive main transmission signals E n . The counting direction of counter 13 is determined by a control signal from comparator 18 which, together with counters 16 and 17, compares the transmission frequency F 0  of oscillator 1 with the received echo frequency F 0  + #f n  from tracker 6a. Scale changer 14 converts the counted number to an indication of distance, expressed in appropriate units, and device 15 displays the result. Details of a digital unit 2, which controls the transmission/reception/measurement sequence, are given (Fig. 5, not shown). In a modification, several main transmission signals E n  may follow each reference signal-In. Additional information, as to the dip of the reference surface or the rolling angle of the vehicle, can be obtained by comparing wave propagation times over the two inclined paths associated with transducers 4a, 4b. In an alternative embodiment (Fig. 2, not shown) the main transmitted signal immediately follows the reference signal (which is a brief wave train of different carrier frequency from the main signal) and persists until the echo of the reference signal is received.