CIMU Magnetometer Calibration

The magnetometers are subject to variations in the Earth's magnetic field strength and to magnetic disturbances due to electrical currents and and nearby soft iron or other ferrous objects. The closer a disturbance source, the smaller it can be and still affect the north-finding. Generally, you should calibrate the magnetometers when you move the system a geographically significant distance (e.g., 50 km or more).

You should choose a location to run this calibration that is not affected by ferrous objects such as dock structures, rebar, steel boat hulls, electrical conduits, etc. It should be either the location you will be using (e.g., the boat) or an area as much like the one you will be using as possible.

Moving a Boy Scout compass around in the area you plan to use for the calibration can turn up unexpected magnetic anomalies. If the needle deflects in unexpected ways you should determine the cause and/or choose a new location.

Calibration is straightforward. Click the "Calc Mag Cal" button, then pickup the device and wave it around in the air. Once you start moving the device, you will see dots appearing on the display, as shown in the next figure.

When you do the magnetometer calibration, be sure there are no soft-iron or ferrous objects in the vicinity of the Transceiver. One oft-overlooked such object is a metal match band.

Conceptually, the dots are appearing on the surface of a sphere. The goal is to get dots evenly distributed over the surface of the sphere. The sphere moves as the device moves, and dots on the close side of the sphere are darker gray, and the dots on the far side of the sphere are lighter gray.

A good strategy is to point the device at each of the six cardinal axes (up, down, east, west, north, south) in no particular order. Rotate the device slowly around its own long axes as it is pointed in the six directions. Then, randomly move the device in space to try to fill in the blanks.

When the system collects 1000 points, it will stop taking new points and give you the option of committing to the calibration, as shown in the next figure:

If you like what you have collected, you can click the "Commit Mag" button and the parameters will be sent to the device to overwrite the previous parameters. If you prefer, you can also try the data collection again by clicking "Calc Mag Cal." If you're finished, click the "Exit" button.

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