Effect of permanent magnet and solid core magnetorquer on magnetometer reading

Question

Permanent magnet as passive stabilization and magnetorquer with solid core as active stabilization has been used over the year in satellites.But the would have magnetic field lines, wouldn't they affect the reading of magnetometer. The magnetometer is used for taking the reading of earth magnetic field.

There is possibility of switching off the magnetorquer with air core while taking reading of magnetometer, but what about the other two case of actuator one with permanent magnets and second with the solid core (Ferrite) magnetorquer ?

Is placement of magnetometer important? If yes, where should one put the magnetometer ideally? Can one take reading of Permanent magnet and solid core, and subtract it from overall reading of magnetometer? but how reliable would be that, as the earth magnetic field magnitude would be quite less than that of permanent magnet.

Answer 1

This is an excellent question, and in fact one that plagues many small satellite designs! There are a number of means that are typically done to adjust for the effect of a magnetorquer/ magnetometer reading issues.

1. Don't use the magnetometer while the magnetorquer is being used. If you don't require a 100% duty cycle of the magnetorquers, this can be the simplest solution.
2. Bias out the magnetorquer by measuring it's effect on the ground, and using math to remove the effect. This isn't as perfect, but is useful in particular with permanent magnets.
3. Put the magnetometer as far away from the main body of the spacecraft as possible. This is often done by sticking it on the end of a boom, sometimes shared by the antenna, other times as a stand alone boon. This is usually used in conjunction with the other items.
4. Use more than one, and figure out the correct reading.
5. Place the magnetometer outside of the Faraday cage of the satellite. This will prevent the inner electronics from affecting things. This won't protect against permanent magnets, as blocking magnetism is really hard, but it can protect against some spurious readings created by electric fields, to a degree.

In practice, a combination of these is typically done. The best bet is to place it outside of the spacecraft as far away from any magnetic materials as you can do, and test it in a number of orientations on the ground to try and determine the truth from whatever it is reading.