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CASIUS (Calibration of Attitude Sensors in USBL Systems) primarily computes the misalignment between the transceiver and the ships motion reference units. It also provides an indication of system performance, errors in dimension control, deployment pole integrity, GNSS quality, MRU errors and noise interference sectors. From these result corrections can be made to improve the systems performance an accuracy.
The scatter distribution is summarised as a statistic compared to water depth so 100m depths should result in a scatter of approximately 10cm 1DRMS. If the calibration lines exceed ½ the water depth horizontal offset, refraction errors can bias the data.
Calibrating in shallow water <50m tends not to produce consistent results because the GNSS and MRU error is invariably larger than the acoustic jitter and misalignments are lost in the noise.
- Check the computed offsets don’t differ significantly from measured values.
- Check computed sound speed agrees with expected. This is often scaled in error to attempt to resolve data outside ½ waterdepth.
- Check processing results agree with the tidal model result confirming reliable GNSS height.
- Check the error result is within expected error budget.
- Check all clusters are similar levels of noise as some observations, using astern propulsion, can suffer aeration.
- Check which data collection points have outliers and does this correlate to certain vessel headings?
- Is the data more spread in a certain axis or geometry as this indicates whether Pitch and Roll out perform Heading error.
- Poor repeatability can indicate pole movement.
Larger spread on lines exceeding ½ water depth.
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