[time-nuts] OT- RMC sentence and magnetic variation/compensation in GPS module

[David Martindale]

Garmin receivers provide that data in their RMC sentence.  They
apparently calculate it automatically using a mathematical model of
the earth's magnetic field, so its accuracy would depend on the
accuracy of the model.  The GPS system itself does not provide a way
to measure variation.

With the handheld models, this estimated variation is used to display
magnetic headings and bearings, if that's what you select instead of
true.  It's certainly convenient to use all magnetic values when your
compass reads only magnetic bearings (e.g. inexpensive compasses, most
sighting compasses).  The handheld receivers also allow you to set the
variation manually, if you know its value more accurately than the
estimate the receiver calculates.

Even the Garmin board-level receivers (e.g. GPS 25) output variation,
probably because Garmin already has the model for their other
receivers.

But omitting the magnetic variation data is apparently pretty common,
and it hardly seems like an essential function of a GPS receiver since
this can't be obtained as part of navigation.

     Dave

On Sat, Feb 5, 2011 at 6:32 AM, Giuseppe Marullo <giuseppe at marullo.it> wrote:
> Hi all,
> I am unable to understand how to get this field from a RMC sentence, on the
> GPS I am using (LS20031) the field is empy so I am wondering if this is just
> a problem of implementation or what.
> AFAIK, the data is highly  variable (location and time) so how is the GPS
> module  able to provide it? Is it something provided by satellites? Will the
> module compute it from the path comparing the bearing with an onboard
> compass?
>
> Could some good soul shed some light on the matter?
>
> Thanks in advance.
>
> Giuseppe Marullo
>
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