Page 52 - Electronic Navigation Cyber Book

(a) can operate in either W/T or G/T mode. G/T speed is also measured
with respect to a water mass;
(b) measures a time delay between transmitted and received pulses;

(c) produces a speed indication, the accuracy of which is subject to all the
environmental problems affecting the propagation of an acoustic wave
into salt water. See Chapter 2.

• Doppler frequency shift is a natural phenomenon that has been used
for many years to measure velocity. If a transmitter (TX) and receiver
(RX) are both stationary, the received signal will be the same frequency
as that transmitted. However, if either the TX or the RX move during
transmission, then the received frequency will be shifted. If the TX and/or
RX move to reduce the distance between them, the wavelength is
compressed and the received frequency is increased. The opposite effect
occurs if the TX and/or RX move apart.
• A Doppler speed logging system:

(a) transmits a frequency (typically 100 kHz) towards the ocean floor and
calculates the vessel's speed from the frequency shift detected;

(b) measures both W/T and G/T speed;

(c) produces a speed indication, the accuracy of which is subject to all the
environmental problems affecting the propagation of an acoustic wave in
salt water;
(d) uses a Janus transducer arrangement to virtually eliminate the effects
of the vessel pitching in heavy weather;

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