Wednesday, December 16, 2015

Testing gnss-sdr with prerecorded data

Under the documentation section of gnss-sdr.org there is a post entitled "First positioning fix using Galileo" where they document getting a position fix using (at the time) all four Galileo satellites (there are now ten in orbit). They provide not only the position fix but the raw I/Q data which can be fed into gnss-sdr to replicate the fix. This makes a good first test of a gnss-sdr installation.

The page is not incredibly verbose so I will add some verbosity here. First off download and decompress the data from the link provided on the page. Inside the folder you will find the raw data file (.dat) along with a configuration file (.conf) USRP recording output log (.log) and output from their gnss-sdr run (.txt), Google Earth output (.kml) and NMEA messages (.nmea) 

Unfortunately the provided configuration file doesn't work with the current gnss-sdr. However the default configuration file provided with gnss-sdr works just fine. So do the following

  1. Copy gnss-sdr.conf from the installation location (typically /usr/local/share/gnss-sdr/conf/gnss-sdr.conf) to the directory containing the raw data file you downloaded
  2. Update the line SignalSource.filename to the full path filename of the data file
  3. Set GNSS-SDR.SUPL_read_gps_assistance_xml=false

Once you made those two tweaks you can execute the example by running
gnss-sdr --config_file=/path/to/my/gnss-sdr.conf

You should see similar but not identical output to the output provided with the data file. Once the run completes you should have a number of new files. The KML and NMEA files were already described, but you will also see some files named GSDR350w40.15N and GSDR350w40.15O. The file ending in "N" is the navigation solution and the file ending in "O" is the observables. Basically, the navigation solution is your 'fix' and the observables are what information you received from the satellite to generate that fix. Lastly, the gps_ephemeris.xml is an ephemeris which is a description of the satellites' orbit in space.

2 comments:

  1. Im getting thjis when I try to callibrate it:

    SignalSource.Osmosdr_Signal_Source: Undefined implementation for block

    ReplyDelete
  2. You need to run 'cmake' with the following parameter:

    cmake -DENABLE_OSMOSDR=ON ../

    prior to building

    ReplyDelete