[GRASSLIST:384] Re: DGPS

[Hamish]

> does someone know something about 12 channel gps receiver (like garmin
> in my case) data and DGPS using postprocessing???

Generally, you either have to do DGPS in real-time, either with a
connected DGPS receiver or with a EGNOS (or WAAS,MSAS(MTSAT)) enabled
receiver. I imagine new handheld units probably have WAAS/EGNOS support
built in but no support for a DPGS input signal.

If you don't have either of those, you will need to have a survey-grade
GPS which will log the pseudoranges from each individual satellite.
Corrections are applied to each sat signal and then new calculations are
made for t,x,y,z based on that corrected data.

With a concurrent well surveyed base-station nearby recording positional
error (but not pseudoranges) you can have a try at doing some post-
processing to clean it up, but you have to make the assumption that
you are seeing the same error at both spots (ie both GPSs are using the
exact same satellites for their calculations). Some people use this as
"better than nothing", but I think it is too big an assumption for
general use.

If you do have recorded pseudorange data, you can buy correction data
after the fact from a number of vendors who operate recording base
stations. (some of us are lucky, for me there is one just on the roof of
our building; check to see if your local geology dept or survey school
has one (eg for tectonic monitoring)).


> I have seen that there is the possibility to convert garmin data to
> rinex 
> (
> http://artico.lma.fi.upm.es/numerico/miembros/antonio/async/node3.html
> )
> but I haven't still found a free (GPL) software able to perform DGPS 

If someone has found a way to get the pseudoranges out of a commercial
grade GPS's firmware, that would be great news. In the past it cost
hundreds of dollars extra for the firmware version (I refuse to call it
an upgrade) that lets you at that info.

"gpsd" has some DGPS basic support built in, maybe have a look at that.
http://gpsd.berlios.de/


good luck,
Hamish