The heading indicates an instrument's orientation relative to the magnetic north pole. In practice, it works like a traditional mechanical compass with the X-axis as the magnetic needle. Output data is given in degrees and corresponds to the readings on a compass. Together with pitch and roll it tells the overall orientation of an instrument.
There are no "right" values of the heading unless defined by the user. It simply states which direction the X-axis is pointing. The heading can indicate whether there has been much movement or not during deployment. Buoy-mounted instruments will naturally have some heading variations, but for a bottom-mounted deployment, the reading should be steady. A distinct change of heading could be a result of something displacing the mount or magnetic objects nearby interfering. A shift in heading also means that the BEAM coordinate system and the XYZ coordinate system are shifted. Hence, the direction of velocity vectors in these systems will vary within one measurement series. The given velocity directions in ENU coordinates will not be altered by varying heading, as it is not relative to the orientation of the instrument. (Read more about the different coordinate systems here) But if a change of heading is due to a relocation of the instrument, the measurement area is also changed and this cannot be corrected. On the other hand, currents vary slowly over horizontal displacements, and small changes of position might not affect the result very much and can be used further - but this must be assessed on a case-by-case basis.
Heading data are used when converting to ENU coordinates. If desired to represent current data in this coordinate system, it is necessary to have accurate heading values. Knowing the heading can also be beneficial if one wants to orient the XYZ velocities relative to axes or reference points in the immediate area. If you intend to use the compass, make sure to perform a compass calibration just before submergence. This should be done with all magnetic materials attached and fixed relative to each other, just like they will be when the measurements are taken. The possible offsets can come from the two sources hard iron and soft iron. Also, be aware that a battery change can shift the magnetic field around an instrument. For this reason, you should perform the compass calibration each time you change batteries. When components disturb the magnetic field near an instrument, the consequence is unrealistic values of heading and further wrong directions of speed in ENU coordinates. For Signature instruments, it is possible to recalibrate the compass in Ocean Contour after deployment if the instrument has been rotated 360° while measuring (typically on its way down or up).
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