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…or, how well is the Earth magnetic field mapped?

Three hallotrons (simple, cheap, robust, tiny, low-power devices) aligned in three axis can be used to determine the direction of local vector of Earth magnetic field.

GPS can be used to determine the location of the satellite. (This is trickier due to mechanisms that disable all commercial GPS chips at these speeds and altitudes, to prevent military use, but still possible.)

Knowing the location, direction of Earth magnetic field at the location, and the measurement from the hall sensors, one should be able to determine attitude of the satellite (excluding spin around precisely the axis of the magnetic field vector) quite accurately.

Now the problem lies in that "direction of Earth magnetic field at the location", as the solar wind bends the field into quite a fancy shape:

How precisely is this shape mapped, and how constant/variable is it over time?

…or did I miss something else; is this a viable method of attitude determination at all?

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Yes, actually, and it is commonly used for LEO satellites. I know of a number of satellites that use magnetic field information to determine their location. It won't provide absolute attitude, but it is sufficient that another point of reference, such as the Earth's curvature or sun's position, is sufficient to determine where the spacecraft is pointed.

It tends not to be used by further out spacecraft, which tend to be larger and benefit more from having a star tracker or similar system. Plus, as you mentioned, the magnetic field varies the further you go. It tends to follow the same lines, however, which allows one to determine the direction of the spacecraft relative to the magnetic field.

See Spacecraft Magnetometer.

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  • $\begingroup$ The linked article concentrates on scientific use of magnetometers. It has one tiny mention of use "as a compass". $\endgroup$
    – SF.
    Jul 1, 2016 at 6:27
  • $\begingroup$ Do you have more references on using magnetism for attitude corrections? $\endgroup$
    – Magic Octopus Urn
    Oct 9, 2018 at 18:48
  • $\begingroup$ It is called magnetorquers, there are lots of references. $\endgroup$
    – PearsonArtPhoto ♦
    Oct 9, 2018 at 19:22

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