[time-nuts] OT - GPS and North

[bg at lysator.liu.se]

Hi Florian,

That is basicly the way it is done. The "source" is a magnetic model - a
"formula" with lots of coefficients, you input your position and the
output is an approximation of your magnetic declination at that position.
It can be implemented by precomputing a lockup table (map) with the
required bin size, or store coefficients and do the calculation in the
receiver.

This is actually an exact analogy with the computation of Mean Sea Level
height. The GPS receiver can only measure ellipsoid height, but many
receivers will output a MSL heigt, by computing or search in a precomputed
"geoid separation"-map.

--

   Björn

> Am Saturday 21 November 2009 20:32:11 schrieb J. Forster:
>> OK. Sme GPS receivers have magnetic sensors.  What do they do with/about
>> magnetic deviation.
>>
> Just a wild guess: The GPS receiver also knows its location, and magnetic
> deviation is known to some degree in its variation over earth's surface.
> So,
> why not introduce a map of magnetic deviation that basically tells you: at
> location Lat; Long, your magnetic north points at 357 degrees instead of
> 360.
> Bingo, you're done. That would be especially useful in the pole region,
> where
> deviation becomes large, as the accuracy of magnetic sensors probably is
> in
> the order of a couple degrees.
>
> Sounds reasonable, doesn't it?
>
> HTH,
> Florian
>
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