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@marullo.it wrote:

--

Chris Albertson
Redondo Beach, California

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@marullo.it wrote:

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@gmail.com
To: "Discussion of precise time and frequency measurement"
time-nuts@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@marullo.it
wrote:

the

just

the

--

Chris Albertson
Redondo Beach, California

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