Comprehensive Manual12
© 2018 Nortek AS
The decision on which coordinate system to use should be based on what the aim with the data
collection is. When handling data, XYZ or ENU coordinates are the most practical for a user. Beam
coordinates are used when dealing with ambiguity (phase wrapping) and higher order turbulence
calculations, as these are the "rawest" format.
Along with determining a phase shift (and then the velocity vector), the instrument will also typically
compute the speed of sound, measure the return signal strength, and measure the attitude sensors
such as pitch, roll, and compass heading (if equipped). For more about this; check the auxiliary
sensors section.
To assure high quality velocity data from a Velocimeter, it is important to configure it properly. The
specifics for each instrument version is described in Using a Velocimeter.
1.4 Auxiliary Sensors
Along with determining a phase shift, the instrument will also typically compute the speed of sound
(needed to change the phase shift to an actual velocity) and measure the return signal strength
(reported in arbitrary units as counts and output as the variable Amplitude). Vectors also measure
the attitude sensors such as pitch, roll, and compass heading (if equipped) to aid in the last step in
processing; coordinate system transformation.
Pressure (Vector)
The pressure sensor measures the hydrostatic pressure and reports in units of dBar. Vectors
sample pressure at the same rate as velocity.
The instrument do not set the pressure offset automatically, so this must be done using the software
right before deployment to corrects for local atmospheric pressure. For more about this, check out
the Service Manual. Note that a Nortek pressure sensor cannot measure negative values.
Temperature
The instruments use the speed of sound to convert time to distance and Doppler shift to velocity, and
since speed of sound depends on temperature, the data from the temperature sensor is of vital
importance. The temperature is measured by a thermistor embedded in the probe and reported in
degrees Celsius. The instruments sample temperature every second. The time response is about 10
minutes for the Vector, 5 minutes for the Vectrino and the Vectrino Profiler.
Magnetometer and Tilt (Vector)
The Vector compass is a system including a 3-axis magnetoresistive magnetometer, a liquid tilt
sensor, algorithms to convert data from these sensors into heading and instrument tilt, and a
calibration algorithm. The magnetometer measures the magnitude of three components of earth's
magnetic field, and the tilt sensor measures two components of tilt and detects up/down orientation.
The Vector (with compass) combines this information to compute the instrument's tilt and heading,
then uses the tilt and heading to correct measured velocities to ENU coordinates.
Tilt sensor:
The tilt sensor measures the orientation of the instrument and comes in two versions; vertical
orientation and horizontal orientation. The tilt sensor is calibrated within +/- 30º, but the accuracy is
significantly better for smaller tilt angles because of the technology used to measure tilts.
If an instrument with vertical orientated tilt sensor is to be mounted horizontally, the tilt sensor data
will be un-useable and the data would need to be collected in XYZ or BEAM coordinates. It is
possible to change the orientation of the sensor, but the instrument would need to be returned to
Nortek. In addition to the physical replacement of the instrument would need to go through
calibration.
The tilt sensors measure tilt relative to earth gravity. If the instrument is moving (e.g. when mounted
on a buoy), the tilt sensor cannot differentiate the acceleration from the buoy from the acceleration of
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