Patent GB-1446794-A (GB)
Title: Speed measurement system
IPC: H03L7/06
Filed: 1974-10-09
Abstract: 1446794 Doppler speed measurement RAYTHEON CO 9 Oct 1974 [18 Oct 1973] 43760/74 Heading H4D A speed measurement system, e.g. for a ship, comprises a pulse Doppler sonar system including a transducer system for irradiating the transmission medium and receiving echoes therefrom, a circuit for storing a digital signal representing the duration of a predetermined plurality of cycles of the received echoes, a measuring circuit arranged to make successive measurements of the duration such that the digital signal is periodically updated in accordance with one such measurement, the value of the digital signal being used to control the period of a periodic signal in a further circuit, and a circuit arranged to count the cycles of the periodic signal occurring within a predetermined interval of time to obtain a further digital signal representing the speed of the vehicle. In Fig. 1 the transducer system comprises transducers 26, 28 which transmit 200 kHz pulses produced by transmitter 34 at a variable p.r.f. in beams arranged longitudinally and angled fore and aft at, e.g. 30 degrees to the vertical. The transducers are mounted in an oil filled housing 30 along with temperature sensitive thermistor 42 which provides a voltage proportional to the oil and hence the ocean 22 temperature. The respective echoes received from, e.g. ocean bed 64 are fed from each transducer via T-R switches 36, 38, along lines 58 to receiver 32. The echoes are Doppler frequency shifted respectively positively and negatively in accordance with the ship's speed, such that when subtracted from each other, they provide a measure of the ship's speed invariant to a pitching motion of the ship. In the receiver 32, respective transducer signals are fed in separate channels 1 and 2 through a minimum threshold gate comprising comparator 70 controlled by variable reference voltage 72, are mixed down to 8 kHz in mixer 74 and digitalized in comparator 78. Each channel is fed to a Doppler extractor 84 wherein the duration of a predetermined number of cycles, e.g. four, is measured, during each of the received echoes and a comparator compares the values of the successive measurements, an equality of which indicates that the measurements are adequately noise free. These values are stored and updated as a digital signal M and are used to provide data for the Doppler combiner where the respective Doppler signals are subtracted. The digital signals M from each channel are fed into the Doppler combiner inputs G1 and G2 and preset modulo counters 152, 154 to count modulo, e.g. 1 MHz clock pulses from clock 90 which are set with a phase difference # 1  - # 2  between the channels, as fed from outputs J1 and J2 so that the outputs of these counters are proportional to the respective Doppler frequency, and may be subtracted in an updown counter 156, the phase difference preventing overlapping 1 MHz pulses. The measurement of Doppler frequency is averaged over, e.g. 6 secs. as per key signal from clock 90. The polarity of the count is obtained in a register 164 and a signal MSB indicating the ship's direction of motion is obtained therefrom to feed to the display 40. A rate multiplier 172 clocked by temperature corrected clock pulses from thermistor 42 via A/D converter 100 feeds Doppler information to a fixed frequency oscillator so that the frequency thereof is multiplied by the count of the up-down counter (156). The resultant oscillation is then applied to a counter which counts the cycles of this oscillation for the 6 sec. interval to provide the number of cycles per unit time which are proporttional to the ship's speed and up dates a register (166) which feeds a speed display 40. The distance travelled is also displayed, and the depth is measured in depth processor (Fig. 6, not shown) which measures the pulse echo time.