Using a Velocimeter 49
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2.4.2 Distance Check
Bottom check processing determines the distance from the center transducer to the bottom at a
given sampling rate (up to 10 Hz). Bottom check data collection can be either enabled or disabled.
When enabled, velocity profiling is paused for a short period of time (~4ms) while the bottom check
calculation is made. The subsequent velocity sample taken is averaged for a correspondingly shorter
period of time to accommodate for the bottom check while maintaining the same velocity sampling
rate. This shortened period is reflected in the AveragedPingPairs field associated with the velocity
record in the data and displayed in the text output pane.
For the interested reader: The bottom check sample rate used during data collection will be
somewhat different from that selected. Given that the bottom check and velocity sample rates are
interleaved, the actual bottom check sample interval will be an integral multiple of the velocity sample
interval. For example, if the velocity sample rate is 60 Hz (~16.7 ms) and the bottom check sample
rate is 8 Hz (125.0 ms), then the actual bottom check spacing will be 116 ms (~8.6 Hz).
In order for bottom check to be effective over larger distances, the instrument should be carefully
oriented. The central transducer used for bottom check functionality is directional. If the probe is
tilted with respect to the bottom surface, then the signal may be reflected away from the transducer,
preventing the bottom echo from being visible in the profile data.
Tip for Vectrino Profiler users: When configuring bottom check, having the bottom peak centred in
the display window (if the instrument is licensed for profiling bottom checks) with a reasonable
number of low intensity points on either side is optimal. The bottom check process determines a
peak level and curve fits a quadratic to the surrounding points. The vertex of this quadratic is used
when determining the measured distance to bottom.
2.4.3 Velocity Ambiguity and Phase Wrap
An inherent problem with coherent Doppler processing for Velocimeters is the potential of velocity
ambiguity. This arises as a result of the fact that the phase difference can only be determined to
within ±π radians. If the absolute value of is greater than π, phase wrapping occurs and the
measured phase shift has a value . We see this phase wrapping in a
velocity trace as an abrupt, unrealistic change in magnitude and almost always with a change in
sign. While it is possible for phase wrapping to occur without a sign change, this would be the result
of an extremely large wrap where the phase shift exceeds 2π. The ambiguity velocity can be
estimated:
Here, F_source is the frequency of the transmitted wave and C is the speed of sound in water. The
choice of ∆t is crucial for correct operation of the instrument; the size of the separation between
pulses determines the maximum unambiguous velocity that can be measured. Longer lags have
lower maximum velocities, while shorter lags have higher maximum velocities. Conversely, longer
lags will typically have lower noise levels than shorter lags. The lags are controlled by the velocity
range setting.
The simplest way to avoid phase wrapping is to have a little prior knowledge of flow conditions and to
set up the instrument appropriately for the environment. The latter is done in the Deployment
Planning, where you can set the Nominal Velocity Range. This is the maximum velocity you expect
to measure during the deployment or experiment. Note that since the transmitter and receiver are
physically separated, the measured velocity is along the bistatic axis (ref. Velocimeters). By
decomposing the velocity along the bistatic axis onto the vertical and horizontal axes you get the
corresponding maximum vertical and horizontal velocity ranges. These ranges are presented in the
deployment planning dialog (right part).
Measurement uncertainty is roughly proportional to the Nominal Velocity Range – smaller velocity
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