ADCPs are used to measure water current velocities by utilizing the principle of the Doppler effect. The output velocities can be given in the three coordinate systems BEAM, XYZ, and ENU. BEAM coordinates provide velocities along the beams, while velocities in ENU and XYZ coordinates are typically split into horizontal (ENU-plane or XY-plane) and vertical (along the U or Z axis). The horizontal velocities are further often given with one direction and one magnitude. Note that in XYZ coordinates the velocities are truly horizontal and vertical only when the instrument is leveled.
When deploying instruments, the user often has some idea of what velocities to expect. The magnitude and direction of velocities vary a lot from site to site. The more familiar one is with the measurement area, the better the basis one has for using velocity readings as part of data quality control because unexpected values can indicate that something has happened.
The averaged vertical velocities in oceans are usually close to zero. Whenever this is not the case, it may be because something is disturbing the measurements. Among other things, elevated vertical velocities can be caused by sidelobe interference, flow disturbances from blockages, turbulence, and when the instrument itself is moving.
The horizontal velocities can be both low and high, depending on the site. Determining whether one velocity reading is realistic requires knowledge of the area. Looking at velocity profiles will be able to provide important information anyway. For instance, if the whole profile has a quite high speed except for a few adjacent cells where the velocity is close to zero, it might be something interfering in these cells. A stationary blockage would provide a reading of zero at its location even though there are strong currents around it. Sidelobe interference and drag down are two common things that can provide abnormally high velocities.
Trustworthy heading readings are important to gain proper directions in ENU coordinates. If you believe something is wrong with your velocity directions, it may be due to wrong heading data. One example of this is of the currents go towards the west, but you expect them to go towards the south based on the topography in the area. All magnetic materials around the instrument can disturb the compass. Compass calibration is hence important in order to obtain reliable heading data and furthermore right directions in ENU coordinates.
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