When dealing with installations near metallic magnetic structures, it’s always very important to minimize any potential interference on the instrument’s compass. If this is not handled correctly, the quality of the data may be negatively impacted.
For autonomous applications, the compass calibration procedure is often more than adequate to handle magnetic interference from small local sources, and so it is not addressed here. Here we address a different type of potential interference, which is the one caused by a large structure near the instrument’s installations. Below is a typical example of such installations.
AWAC mounted on monopile and platform
In this case, the AWAC’s compass will be affected by the steel of the platform. This will in turn affect the computed velocity directions in the ENU coordinate system, including the wave direction as well.
In this case, our recommendation is to setup the AWAC to collect data with ENU as normal. Unlike other applications, setting it up in XYZ is not advisable because the correction is much easier later on and we will not be running into any matrix conversion problems. So once the AWAC is mounted and in position, you will need to measure the AWAC's real heading by using some form of external source, preferably something that is not affected by the local magnetic field, such as a GPS compass. The difference between this value and what the AWAC reports will be the bias (offset) introduced by the local magnetic interference and it will need to be added/subtracted from the reported estimates.
If using our post processing software like Storm, SeaState or QuickWave, this is easily done by inputting the offset in the "Compass Offset" field for the applicable software. But, if the customer is using a ProLog and outputting the processed data to any other type of system, such as their own software or datalogger, then they will have to do this computation on their own.
Thankfully, this computation is a straightforward process of a simple Cartesian conversion. It is just a matter of first determining the offset (as explained above) and then adding this value to all of the directional estimates of current and wave direction. For example, if you mount an AWAC on a steel pylon and the compass heading says 140°, yet the externally confirmed value is actually 90°, then they will have to add "-50" degrees to all directional estimates of waves and currents.
Most operational users will likely report data as "speed and direction", so the above conversion applies. However, most "scientific" customers like to report velocity in East and North coordinates. In this case, it's still straightforward, but slightly different. They will have to recalculate with a sin(50) and cos(50) multiplier to the original East/North data.