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

[Alan Melia]

Try this site as an alternative for local magnetic data
http://www.ngdc.noaa.gov/geomagmodels/IGRFWMM.jsp?defaultModel=WMM

Alan G3NYK
----- Original Message ----- 
From: "Chris Albertson" <albertson.chris at gmail.com>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Sent: Saturday, February 05, 2011 5:47 PM
Subject: Re: [time-nuts] OT- RMC sentence and magnetic
variation/compensation in GPS module


The GPS receiver can only compute the direction to true north if the
antenna is on a moving vehicle.  The GPS looks backward to where you
were in the last few seconds and computers the direction of travel.
On a fixed antenna this can't work.

But even at best the GPS' idea of "north" is poor.  I had a sailboat
and what you'd do there is install a NMEA fluxgate compass. Many of
these contained gyros too.  Then you program your other instruments to
ignore the GPS NMEA heading and use the better compass data

But "magnetic variation" can be can be gotten from a map.  It is a
very slowly changing function of your location.  The GPS could have
the data in it's firmware.  Paper charts will always have the
variation in degrees and degrees per year printed on them.   I've also
seem some older GPS where the use must enter the variation in  a setup
menu.  Compensation would always need to be entered by hand.  You get
it from measuring your specific compass after turning onn all the
normal electronics.

My bet is that this unit does not have a variation map in its firmware
and you have to set it my hand or maybe the unit will pass it through
it it has a NMEA input  Some GPS's can listen on an input port and
will pass this data through if they either can't generate it or if the
user has programmed the GPS to pass the external data.


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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-- 
=====
Chris Albertson
Redondo Beach, California

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