[maemo-developers] N900 GPS - RTCM / DGPS?

[From: Nils Faerber]

Hi!
During the weekend I stumbled across some web-sites again...
For some time I am searching for info on differential GPS (DGPS) to get
higher accuracy.

The first requirement for this would be that the N900 GPS accepts RTCL
messages or similar. Does anyone know if this is the case?
FYI: RTCM is as far as I know the NMEA message type that is used to feed
correction data back into the GPS receiver.

The second question is about the GPS subsystem. The "standard" gpsd for
Linux can read ntrip IP streams and feed the received correction data to
the attached receiver. Can the N900/Maemo5 GPS subsystem do this? And if
yes where would I specify a NTRIP or other DGPS feed?

This is all very interesting!

The most amazing page I stumbled across is this one:
	http://gpspp.sakura.ne.jp/rtklib/rtklib.htm

Usually DGPS correction data must be bought, i.e. it costs quite
significant money to get access to those resources. There are many
national and international institutions that offer this with various
distribution systems - from short-wave radio up to IP and accuracy from
a few meters down to single digit cm! With high precision DGPS solutions
you can achieve accuracy in the few cm range, down to 2cm! But those
devices and access to the correction data is *expensive* - in the range
of 20k€ and upwards.

Using RTKLIB you can use (some) ordinary consumer GPS devices and build
your own reference station! This is incredibly cool since you can
achieve accuracy in the two digit cm range with this - though of course
the absolute position of the moving station can have a shift but the
relative position towards your reference station is really high then. In
my use case where I want to map for OpenStreetMap this a non-issue since
I can later on shift the recorded track easily - the whole track will
have the same offset and all points of the track can be in the cm range
of accuracy.
And this would even work with the N900/Maemo5 device if there would be a
NTRIP RTCM interface.

Cheers
  nils

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