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 <giuse...@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 > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > -- ===== Chris Albertson Redondo Beach, California _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.