Pre-deployment operations and calibrations
4. Pre-deployment operations and calibrations#
4.1. Battery degaussing#
Degaussing battery packs before installation is recommended. After each degaussing of the battery pack, a careful compass calibration needs to be performed in order to take into account the change in the magnetic environment where the compass is located.
4.2. Ballasting requirements#
Good ballast to ensure stable flight
Record accurate mass
4.3. Compass calibration#
We used to rely on pre-deployment compass calibrations but now perform it in-situ running two back-to-back dives at the start of the deployment using 2 different pitch and roll combinations.
We return the collected magnetometer data, then process it offline and upload a hard-iron and soft-iron correction file.
Eliminates the problem of subsequent hard-iron distortion caused by large magnetic objects like ships and rocks.
We also measure any residual hard-iron distortions due to battery pack locations and correct for that (typically none on Seaglider, somewhat present for Deepglider).
Local magnetic anomalies are measured before every deployment by moving a glider through a wide range of pitches, headings, and rolls in an area free of stray magnetic fields and estimating the local anomaly field through least squares. This first-order correction is stored on board the glider. A residual heading- and pitch- dependent error with RMS size of 1.5° remains [Rudnick et al., 2018]; this residual error curve is recorded and applied in post-processing. A constant magnetic declination for the deployment region is stored onboard the glider; spatially varying magnetic declination can be applied in post-processing.
Here basic recommendations about compass calibration for the seaexplorer:
The compass have to be calibrated in the following cases:
After payload hardware modification
Upon change of mission location
After a ballasting procedure (because of added lead weights distribution)
At least once a year.
It is also recommended to:
Do the procedure in an area of undisturbed magnetic field (e.g. forest, on a beach, at sea).
To level the vehicle
To know the true north
To set the actuators in neutral position
4.4. Compass error#
There is a way to quantify compass error after calibration. That may be worthwhile to be addressed. People often do that on land. Our team is checking the compass error all the time before deployment. After calibration, the compass error is around 1 deg or less. After a month-long mission, the error does not change much in our case. If not doing the compass calibration, the error goes up to 5deg usually and up to more than 10 deg (an exceptional case).
If it helps, I added figures of compass error measured before and after compass calibration.
I have looked up the old compass error estimates according to pitch, since Pierre asked about it. Note that those results are before compass calibration. Once being done with calibration, such pitch-dependency of compass error is not much significant.