[teqc] developing a dgps post processing software

Lalitha myracacc at gmail.com
Mon Nov 8 10:54:23 MST 2010

Thanks to Jim and Param for your detailed answers and sharing the related
links and documents.

I would like to ask your further advice too.
Which is a good option? developing the post processing software newly or
using the existing software's available? I would like to go for some freely
available software for now to try and understand more. Thanks for suggesting
about Gamit/globk. I'm trying to download it.

If developing the software on our own is an option, any idea how big is that
project and how long will it take?

Thanks for your advice and inputs.


On Sun, Nov 7, 2010 at 8:56 PM, Param Gautam <pkrgautam.iitr.03 at gmail.com>wrote:

> dear lalitha, gps data processing do in two steps
> 1. preprocessing using any utility software like teqc, tps2rin etc .
> basically using thsese softwares raw data convert into the usable
> format
> 2. postprocessing, presently people are using gamit/globk and barnese
> software for post processing, gamit is freely available on the website
> and complete manual of processing steps also available there.
> there are different types of receivers are like topcon, trimble ect
> and surveying methods plz go through the attachments you can get some
> idea related to the static , kinematics and rtk gps survey.
> param
---------- Forwarded message ----------
From: James Johnson <jjohnson at ucar.edu>
Date: Wed, Nov 3, 2010 at 3:24 PM
Subject: Re: [gpstech] developing a dgps post processing software
To: GPS/GNSS technical discussions <gpstech at ls.unavco.org>

Dear Lalitha,

In order to write post processing software, you will have to have some
knowledge of the data that is collected by the GPS receiver.  Briefly, there
are two types of measurements available, a range measurement and a phase
measurement.  The range is a measure of the distance between the receivers
antenna and the SV that broadcast the signal.  The phase measurement is a
measure of the phase of the carrier signal.  These two measurements are
available on multiple frequencies as well -- but the typical low cost
receiver only takes measurements on one frequency (refereed to as L1).

In fact, most low end receivers only give you the range (if even that).
 Assuming you have range measurements, you need on more thing -- the
position of the SVs.  The SV positions are broadcast by each SV every 30
seconds in the form of a broadcast ephemeris message that can be used to
calculate the SVs position within several hours of the time the message is
broadcast.  Once you have 4 or more range measurements (4 unknowns, X,Y,Z
and Time) and SV ephemeris, the user position can be estimated with a least
squares adjustment.  This gives you a 'point' position.

For DGPS, one can cancel errors if they already know the position of one
station.  Then instead of solving for absolute position of the rover, you
solve for vector between the known base station and the rover.  This can
also be achieved by estimating errors in the SV orbits and sending a
correction for each observation.  We know where the base station is, we know
the position of the SV and so we know the range we should get.  We
difference our modeled observation with the received observation to come up
with a correction that can be sent to the rover.

RTK uses phase data and requires more expensive receivers.  With RTK, the
one can get cm level positions.

You are really going to have to do some research -- the true answer to your
question is a book.  A good place to start might be this web site:

Best Regards,
Jim Johnson

> On 11/4/10, Lalitha <myracacc at gmail.com> wrote:
> > Hi,
> >
> > How are post processing software are developed. I couldn't find anything
> in
> > the internet about the methodology of the post processing techniques to
> get
> > the differential GPS corrections for the rover station gps data collected
> > against the base station data downloaded from data provider.
> > I know both the files need to be in Rinex format.
> >
> > In the Unavco web site, I could see some useful links to few post
> processing
> > software. I saw different kinds of processing methods like static,
> > kinematic, etc in the Nasa JPL's APPS application. What is RTK. I didn't
> get
> > the exact difference between all these.
> >
> > Where can I find sources for developing a post processing software. Or,
> is
> > there any open source or free tools available that are related to post
> > processing.
> >
> > Appreciate your help.
> >
> > Thanks,
> > Lalitha.
> >
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