I should have excerpted all of K8RI's post, particularly this:
> My Garmin Vista H has electronic compass separate
> from GPS. (not all Garmin do) so it has some kind of
> magnetic sensors.
> I know I can turn it on and off separately, to save battery power.
Because the option to turn it on or off is programmed, it pretty much has to
be a power hungry flux gate type.
There are MEMS devices with 20 nW power consumption which the manufacturer
wouldn't bother to power down. With <= 0.3 mT field sensitivity, one can
make a handy compass out of one of them.
-Doug Ronald (AE6SY)
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