What happens when my instrument is tilted and what actions should I take?Follow
It can be difficult to avoid instrument tilt throughout the entirety of a deployment. In some cases the instrument rigging shifts or is installed at an undesirable angle due to an object or incline on the ocean floor. In cases of massive currents and waves high instrument tilt can periodically occur, displacing the instrument or mooring line. Minor periodic variations in pitch and roll can sometimes be averaged out and the corresponding velocities corrected, however, data with excessive tilt or permanent offsets in pitch or roll should be carefully monitored.
When the instrument tilt exceeds 20-25 degrees the range of the instrument is reduced and the data begins to degrade in ways that are not recoverable. When the tilt is 5-10 degrees it is recommended to carry out thorough quality control measures and consider reprocessing the data. The user should always strive for an installation where the instrument is oriented vertically either towards the surface or downward in reference to mean water level. For information on preventing instrument tilt when mounting or deploying the ADCP, in-depth guidelines can be found in the comprehensive manual. The most typical effects of tilted instruments include depth change of cells, increased sidelobe interference and changes in the direction of measured velocities, all of which are discussed below. In addition, this article discusses options available during data processing.
It is important to note that the limits given should be considered as guidelines and it is the end users responsibility to apply proper quality control measures for all data and consider tilt effects for each individual deployment. In addition, the user should also be aware of the key differences between tilt sensor accuracy (given in the instrument technical specification) and the velocity measurement accuracy.
Remember that one of the key assumptions for velocity measurements using ADCP is that it that the flow is "horizontally homogeneous" - that is, at a given depth, the velocity is constant in the same direction across all the beams. A major consequence of tilted instruments is the change in depth cells, or bins. The pointing angle of the instrument beams are consequently shifted, and the along-beam depth cells are no longer located in the same horizontal layer (see figure below). This may result in a "smearing effect" on the velocity data because the depth cells are incorrectly located at different levels in the water column. Furthermore, if there is strong vertical shear in the water column, and the bins are moving in and out of regions with variable velocities the resulting velocity can be biased. Consequently, it is highly recommended a remapping of the cells from the different beams to its proper vertical location to maintain a consistent profile. Depending on your instrument and deployment setup, there are processing methods available (discussed below) to correct for shift in cell position.
Figure. Position of cells for a leveled and tilted instrument.
Midlife instruments (AWAC, Aquadopp, Aquadopp Profiler)
For the narrow band midlife instruments, remapping of depth cells can be done using the feature "Remove tilt effects" in the post-processing software Storm. This process will remap the depth cells and convert the velocity data to ENU if it is collected in BEAM or XYZ coordinates (see paragraph below Changes of measured velocity direction). There is no option to remap the cells without converting to ENU. For midlife instruments, remapping of depth cells will be carried out on averaged data, because raw data is not available. This means that the remapping is most suitable for cases with static tilt. If the instrument has moved during the averaging period, "smearing" or bias effects in the averaged data must be considered.
For Signature instruments, raw data used to make the average velocity data is available, and so correction can be carried out on each individual ping. The method is referred to as "Bin mapping" and is available in the post-processing software Ocean Contour. After performing bin mapping, the resulting velocities are only provided in ENU coordinates. This is because the repositioned cells are no longer in reference to the instruments orientation as they are in BEAM or XYZ coordinates. If the instrument is set up to output telemetry data, the average ENU data can be bin mapped during onboard processing.
Figure. Signature instrument mounted on buoys have the option of onboard processing which include bin-mapping of each individual ping.
In addition to the horizontal shift of the depth cells, tilted instruments measuring near a boundary can experience an increase in sidelobe interference and a reduction in the effective range. Sidelobe interference is due to energy "leaking" from the direction of the main lobe and interacting with the boundary before the main lobe. A boundary can be the surface, the seabed or a physical object, all of which provide stronger returns than suspended particles. This will contaminate the received signal and make it difficult to measure velocities in this layer. When the instrument is pointing directly at the surface the angle of the slanted beams can result in sidelobe interference that may effect 5-10% of the velocity profile. When tilt occurs this effect will increase and reduce the range further. Unfortunately, it is not possible to reverse the effects of sidelobe interference in post-processing because the receive signal is already contaminated. It is recommended to remove all of the data in the profile where this interference has occurred.
Unlike slanted beams, the centre beam is pointing directly towards the surface or sea floor and will therefore not be affected by sidelobe interference. Sidelobe effects will only occur in the centre beam of the instrument is tilted. In cases where the centre beam is used as an altimeter, valid data can only be achieved within a window of 10 degrees tilt.
Changes of measured velocity direction
If the instrument is configured to collect data in ENU coordinates, the velocity measurements will be corrected for instrument tilt. The UP direction will always be perpendicular to the surface level. As long as the instrument compass is calibrated correctly and not disturbed by nearby magnetic objects, the East and North directions will be relative to the earth's magnetic field. Both BEAM and XYZ coordinates are relative to the instrument. When the instrument orientation changes, so does the orientation of these coordinate systems and the resulting Z-velocities will be relative to the instrument axis and the measured current directions will be changed accordingly. If the data collected cannot be presented in ENU coordinates, corrections for tilt in BEAM or XYZ coordinates will have to be done manually in post-processing.
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