Version 26 (modified by wja, 15 months ago) (diff)

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Verify GPS Base station position

When processing the aircraft navigation using differential GPS it is vitally important that the base station coordinates are added correctly. Sometimes it appears that the stated position can be out by the order of a few metres, so we should do static PPP processing on the base station data to get a fix on it's position. If we use as much data as available (48hrs appears max in GrafNav) then the solution can get to accuracy of a few cms.

Procedure

  • Open GrafNav 8.10 and do the PPP processing:
    • Create a new project. File<New Project<File name: To make sure that files are saved to the correct folder, save the project to posatt/basestation/ppp_result at the start before processing - use a filename such as YYYYJJJ_baseppp where YYYY is the year and JJJ is the julian day.
    • Click on the 'Convert GNSS Data' icon in the toolbar (to the right of the Print thumbnail). This brings up 'Convert Raw GNSS to GPB'. *'Get Folder': navigate to the select the basestation directory. This should populate the 'Source Files' list on the left to the window. *Click 'Auto Add All' to select the revent Leica format (*.m00).
    • Add the new GPB basestation file as remote file
      • Choose use advanced method option
      • Choose get info from file and use the default
      • If grafnav throws the error "Zero number of epochs in Default_9488_1008_054912.gpb" you will need to convert or split the basestation files (explained here)
    • Settings -> load from PPP
    • Download the precise clock and ephemeris data
      • Go to the settings menu > PPP Processing > Precise tab and click download (Check the date to make sure the correct day is being downloaded.)
      • When downloaded, click OK.
      • Save to the ppp_result directory
    • Check Datums in the General tab
    • Process->PPP
    • Save the project again.
  • When the solution is available left click the GPS point to query the lat/lon/hei and make a note of it in the ticket. You will probably need to zoom in substantially.
  • If there are multiple base stations to use for the navigation processing repeat for all base stations.
  • Exit from GrafNav

Make sure that you save the result in posatt/basestation/ppp_result as this will be used in automatic scripts to check solution. Saved filename should follow the usual conventions, which at the time of writing, is "YYYYJJJ_baseppp", e.g. "2013123a_baseppp".

Note that you should now either exit grafnav prior to processing the flight data or start a new project so that files are not overwritten.

PPP Processing

If the PPP processing method doesn't work and gives an error about not finding epochs, then try this:

  1. Check you are using the latest version of grafnav (8.1 at time of writing)
  2. If you are, then try downloading the latest manufacturer files:
    1. Go to website: http://www.novatel.com/support/info/documents/569
    2. Download latest manufacturer files
    3. Unzip and place contents in the C:\novatel\WaypntGPS810\