Noisy Velocimeter Data; Grounding Problems; Phase Wrapping - Nortek Vector Comprehensive Manual

5.3.5

Noisy Velocimeter Data

Noisy data may be caused by:
Not enough particles in the water. Velocimeters cannot measure velocity properly if the water has
too few scatterers. Data will be questionable when signal levels are down around the noise level
(less than 60 counts).
The Velocity Range is set too high.
The sampling volume is located beyond the bottom (typically if the test bucket is too small).
Reflection and reverberation conditions at the bottom creating pulse-to-pulse interference between
acoustic pulses. Try moving the probe up or down relative to the bottom.
Probe malfunction. Run the
probe really is malfunctioning, report the probe check result to Nortek.
Grounding
5.3.6

Grounding Problems

Tests in laboratory tanks can sometimes lead to grounding problems that show up as elevated noise
levels, but only after the instrument is submerged in water. You will not automatically see the
increased noise level in your data if your signal from the water is above the noise, but the increased
noise level could look like signal. If grounding problems cause elevated noise levels, you may be able
to reduce your problems by coiling your cable into a tight bundle and raising the cable above the floor
(i.e. placing it on a chair). Check out the
us for further guidance. Keep in mind that grounding problems occur around man-made structures,
and are not normally a problem in the field.
Vector: One way to tell the noise level in the tank is to collect with the transmitter out of the water
but the receiver arms in the water. The signal you now see is noise only. Grounding problems will
disappear when you disconnect the Vector from the computer and from the external power supply.
Collecting data to the internal recorder using batteries can be an option to remove possible grounding
problems in indoor environments.
Vectrino Profiler: If you encounter noise problems when synchronizing the sensor using a TTL
source: try terminating the Sync- line at the TTL input by connecting a 120 ohm resistor in series
with a 1nF capacitor between the Sync- line and the ground.
5.3.7

Phase Wrapping

Please check out the
Instruments" (available on the web) for more details regarding phase wrapping. The essentials are
that the Doppler phase shift is computed using the covariance method, and this restricts solving the
phase difference within the range of [-π, π] or as reported by the planning software. This introduces
the problem of ambiguous determination of the phase shift; if the phase difference goes outside the
range of ±π, the measurement uniqueness is lost, something known as velocity ambiguity.
We see this phase wrapping in a velocity trace as an abrupt, unrealistic change in magnitude,
usually 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, in
which case decorrelation would likely occur first.
It is generally best to avoid phase wrapping issues as the time spent correcting them is often
significant. The best tip is to have some prior knowledge about the area of interest, so that you can
set the Velocity Range properly.
To identify wrapping: If you are lucky, velocities were fairly strong so the wraps go from a positive
velocity to a negative velocity and are quite obvious. A simple sign filter (i.e. positive velocity
expected, wrapped points have a negative sign) will work fairly efficiently to identify wrapped points.
For smaller velocities where it is a bit harder to identify wrapped points by sign alone. Playing with
various forms of local median filters to identify wrapped points has varying degrees of success,
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Probe Check
problems.
Principles of Operation
(Online or Data Collection > Start Probe check). If the
Probe Check section for a figure. Also, feel free to contact
and "A Practical Primer for Pulse Coherent
Troubleshooting Guide 113