UASD Glossary: Doppler Shift
UASD Glossary: Doppler Shift

The change of phase with time of an echo is measured by comparing the phase of echoes in consecutive pulses within a pulse set.
The time separation is determined by the pulse repetition rate (typically 100 pulses per second giving dt=10ms).

A more precise estimate of Doppler phase shift is available in the B-mode sounding modes where the same frequency is repeated in a completely separate pulse set transmitted several pulses after the first pulse at that frequency. Assuming that the same echoing region is responsible for all echoes at that frequency, this second measurement of phase difference over, effectively, a longer pulse separation, will be larger and therefore more precisely measured (given the fixed minimum phase detection accuracy of about 4).

The advantage of a more precisely defined phase change and its corresponding Doppler phase shift must be weighed against the disadvantage of aliasing effects where phase rotations of 2 are "lost" in the measurements.


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