The orientation of each transducer with respect to the bow must be determined for processing to proceed. This is done at the time of a UHDAS installation using either previous data or a bottom track calibration run if there is no earlier data from the instrument. A new instrument is calibrated using a period with bottom tracking turned on, or a reciprocal track.
In the determination of the transducer angle, we must use a heading device. By tying the calibration to the accurate heading device, a subsequent change in calibration indicates a change in either the transducer angle or the accurate heading device.
Over time, watertrack and bottomtrack calibration estimates accumulate and we get a better idea of the transducer offset. If an accurate heading device exists and is working, it is not necessary to leave bottom tracking on, and in fact you lose statistical robustness (increase noise) in the ocean velocity data when bottom track is on, because every other ping is looking for the bottom, not measuring the water.
A new calibration value will have to be determined if the accurate heading device is changed or if the transducer alignment is changed (eg. removed and reinstalled).
When to use Bottom track
- if the bottom is in range, and flat, and if there is no
accurate heading device available
if it is known that it won’t hurt other instruments
when leaving port and when returning to port
Limitations of bottom track
doesn’t work well on sloping bottom (works best on a shelf)
- undermines its own water velocity data
every other ping looks for the bottom
- if the bottom is out of range, it lengthens the ping cycle
(looking for the bottom)
can destroy water ping data of other instruments (loud, long ping)
Guidelines for using Bottom Track with UHDAS
- up to a day at the beginning and/or end of a cruise, if the bottom is in range
- longer, if science asks for it (overriding UHDAS recommendations)
Every day an email is generated and sent to the ADCP operator at sea and intersted parties on land. That email includes information about the compter’s health, processing status, and a data snippet that is plotted. On land, the web site with this information is accessible here.
Watertrack and bottom track calibration values are included in the diagnostics. The hope is that values will be:
- close to 1.0 for amplitude (within 1%)
- close to 0.0 for phase (within 0.5deg)
Please note these caveats:
- Some cruises have no bottom track data, some have no watertrack data, simply due to the cruise track.
- Watertrack calibrations are by nature noisy and generally accumulate slowly.
- These data are preliminary; there could be editing that has not yet been done that will clean up these values, for instance some data may not have been flagged as bad below the bottom, and if any underway bias exists (eg. from bubbles) it will hurt the amplitude calibration.
- There could be gaps in the heading correction device that are throwing off the phase calculation
At Sea the most recent watertrack and bottom track calibration assessments are available on the UHDAS home page, as well as a statement about the angles that were determined when UHDAS was installed.
On land recent installations include “cals.txt” in the daily diagnostic files