Patent GB-1323829-A (GB)
Title: Methods and apparatus using the doppler effect for determining distances travelled by a vehicle
IPC: G01S15/60
Filed: 1971-04-19
Abstract: 1323829 Electric measurement INSTITUT FRANCAIS DU PETROLE DES CARBURANTS ET LUBRIFIANTS 19 April 1971 [2 Feb 1971] 23518/71 Heading G4H [Also in Division H4] A system for determining the distance travelled by a vehicle over a reference surface from the Doppler shift of waves transmitted obliquely towards and reflected from the surface is characterized in that the transmitted signal is a sequence of n wave trains each of length # and with a predetermined wave train recurrence period, the sequence is terminated at the time of reception of the reflected waves, the transmission sequence is followed by a reception period of duration dependent on n, then by a further transmission sequence, and the ratio between the recurrence period T REP  of the sequences and the actual transmission time n# of a sequence is maintained constant. This ensures a measurement accuracy substantially independent of the distance between the vehicle and the surface. The waves may be acoustic or electromagnetic. In the Fig. 4 embodiment (with waveforms as in Fig. 2), a transmission sequence is preceded by transmission of a recognition signal S generated in oscillators 1a-1m and fed to transmit/receive transducer 3 via switch K1 closed for time T by sequence controller IV. Wave trains E 1 , E 2  ... E n  of the sequence are then transmitted at intervals T when controller IV repetitively closes switch K2 to feed the output of oscillator 2 to transducer 3. The resulting echoes are applied via band-pass amplifier 4 and amplifier 5 (with a stepped, time-varied gain to compensate for wave attenuation during the propagation time) to synchronous demodulator 6 and limiter 7 which extract the Doppler frequencies. The recognition signal is detected by means of a band-pass filter 8a and an energy detector 9a which are switched into circuit via switch K3 during the gaps between wave trains of the sequence (thus avoiding spurious detection of, e.g. transmitter break-through signals). Detector 9a produces an output pulse r 1  in response to the recognition signal, which terminates the transmission sequence and initiates a reception period T F  during which switch K4 is closed so that zero-crossings of the Doppler frequency signals can be detected at 9b and counted at 10. T F  is automatically adjusted by counters in controller IV (details given, Fig. 7, not shown) to equal (2n-1)T, so that T F  plus the overall transmission time (n+1)T, i.e. a sum of 3nT, is proportional to the total wave train transmission time n#. The count at 10 is thus proportional to the distance travelled in the time T REP  and is multiplied at 11 by a scale factor before application to display unit 12. The distance of the vehicle, e.g. a ship, from the surface, e.g. the ocean bed, is determined by counting at 15 the number n of wave trains E in the sequence, counting at 13 clock pulses from source 14 for the period between the end of the sequence and the first pulse E of the next sequence, and combining the counts in a scaledetermining unit 30 feeding display 16. In a modification T REP  may be made equal to pnT where p is an integer greater than three. The selection of p = 3 in Fig. 4 provides a reception period T F  sufficiently long for the detection of echoes resulting from side-lobes of the transmission.