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Positioning Application: What is the format of the GLONASS navigation data dump file, so that I can decode it?

Environment/Versions
  • PosApp (SimGEN / SimREPLAYplus / SimREPLAY)
  • GLONASS
  • Navigation data messages
Answer

NOTE: Spirent provide a GLONASS nav data decoder
                - Go to the PosApp directory (C:\Program Files\Spirent Communications\Positioning Application)
                        OR
                - Go to PosApp menu [Tools-->General Utilities]

 

FILE STRUCTURE

  • XML format
  • Open with any standard text editor, like Notepad.
  • One 'string' of nav data looks like
            <string svid="2" framenum="5" stringnum="8">
                            6A965 96955 665A9 69956 6555A 55996 55555 5556A 567E3 75096 
            </string>
     
  • XML header identifies the related SVID, Frame and String.
  • There are 50 hex characters
  • Each character represents 4 nav data symbols = 200 symbols total
  • The 200 symbols contain 170 symbols of nav message + 30 bits (time mark)
  • The 170 symbols of nav message is the Modulo-2 addition of
        - The wanted 85 bits of nav data (20ms period)
        - A meander sequence (10ms period)

DECODING THE DATA

  • Convert the symbols to bits
  • Remove 30 bits for the time mark
        - This is always "111110001101110101000010010110" = "E375096" (in hex)
        - Notice this sequence on the end of each string
  • Reverse the Modulo-2 process for the remaining bits
        - The meander is a repeating 10101010 sequence
        - Align the nav message bits with the meander
        - Take 2 bits at a time
        - A "1" in the nav message identifies the meander bit that becomes the output
     
  • The following example takes the first 5 hex characters from the above nav data extract
    The first few bit pairs are colour-coded to show how output bits are selected

        - 6A965 96955 = 01 10 10 10 10 01 01 10 01 01 10 01 01 10 10 01 01 01 01 01 ... (10ms period)
        - Meander       = 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 ... (10ms period)
        - Output         =  0   1   1   1  1   0   0  1   0   0   1   0  0   1   1   0   0  0   0   0  ... (20ms period)

 


Product : Nav Data Decoder,SimREPLAYplus,SimREPLAY,PosApp,Navigation Data,GLONASS