[Openstreetmap] How do I talk to the GPS to get the info we need?

Matt Amos matt at matt-amos.uklinux.net
Mon Nov 15 00:14:50 GMT 2004

On Saturday 13 November 2004 12:58, SteveC wrote:
> * Petter Reinholdtsen (pere at hungry.com) wrote:
> > [Jo Walsh]
> >
> > > slightly arcane format, but there you are. gpsbabel definitely
> > > supports this. as for post-facto DGPS correction it's a good
> > > idea; the nearest DPGS server to london that i know of it in
> > > italy though. we'd toyed with the idea of having a whip-round
> > > to set one up in limehouse...
> >
> > Do you know how to query the GPS using NMEA-0183 v2.20 in a way
> > that get it to report accuracy info and enough info to be able to
> > post-process the collected data with DGPS corrections?
> Matt was saying something along the lines of 'you can get that data
> in binary mode' IIRC... Matt?

you _need_ to have pseudo-range (PR) and availability/ephemeris data.

iirc, you can't get this with NMEA, you need to find out what chipset 
is in your GPS unit and use appropriate drivers. Garmin drivers exist 
somewhere on the net and i'm writing SiRF drivers at the moment. any 
other chipset, YMMV.

its worth noting that "simple" DGPS with subtraction of the relative 
points is a very bad idea, see:
note: when he says "there is no pseudo range data output from a normal 
consumer type GPS receiver" he is _wrong_. you can get it from the 
above mentioned chipsets and probably others as well.


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