Customer Service Center

• PosApp (SimGEN)
• GPS L1C, L2C or L5 navigation data

1. Go to menu [Options-->General Options]
2. Enable "Dump navigation data"
3. Run the scenario (normal speed or turbo speed)
4. For GPS modernised CNAV navigation data, PosApp creates a binary files called
       - L1C = "nav_data_bits_1.L1_cnv" (pre FEC) & "nav_data_fec_1.L1_cnv" (post FEC)
       - L2C = "l2c_nav_data_1.cnv" (pre FEC) & "l2c_nav_data_fec_1.fec_cnv" (post FEC)
       - L5 = "l5_nav_data_1.cnv" (pre FEC) & "l5_nav_data_fec_1.fec_cnv" (post FEC)
    
5. You can decode the binary files using the following utilities via menu [Tools-->General Utilities]
       - L1C = "GPS L1C Nav Data Extract"
       - L2C & L5 = "GPS L2C/L5 Nav Data Extract"

But to decode the binary files for yourself, the following explanations are of benefit:-

• First 12 bytes (0 -11) are the characters “GSS CNAVDATA”
• Next byte (12)  is major file version (at the time of writing, the current value is 0)
• Next byte (13) is minor file version number (at the time of writing, the current value is 2)
• Next byte (14)  is file type:
  • Value 0  = L1C or L2C bits (pre-fec)
  • Value 2  = L1C or L2C or L5C symbols (post-fec)
  • Value 64 = L5C bits (pre-fec)
• Next byte (15) is spare
• Then there follows repeated blocks of bits or symbols, block length depends on file type.
• The MSB of the first byte contains the first bit/symbol of the message and the LSB contains the eighth bit/symbol.
• Where the message length is not a multiple of 8 then the unused least significant bits of the last byte should be discarded. For example
  • L2C pre-FEC nav data message block is 300 bits long, written as 38  bytes. 38*8 = 304 bits, so the 4 LSBs of the last byte are not part of the data and should be discarded.
  • L2C post-FEC symbol data message block is 600 symbols long, written as 75 bytes. 75*8 = 600 symbols exactly, so no bits are discarded.
     
• Message lengths for the various file types are given below:
  • L1C Pre-FEC Binary File = "nav_data_bits_1.L1_cnv"
        - Bytes to Read = 113
        - Bits to Read = 900
        - 113 * 8 = 904,  so discard last 4 LSBs of last byte
     
  • L1C Post-FEC Binary File = "nav_data_fec_1.L1_cnv"
        - FEC Bytes to Read = 225
        - FEC bits to Read = 1800
        - 225 * 8 = 1800, so no bits to discard
     
  • L2C Pre-FEC Binary File = "l2c_nav_data_1.cnv" & L5C Pre-FEC Binary File = "l5_nav_data_1.cnv"
        - Bytes to Read = 38
        - Bits to Read = 300
        - 38 * 8 = 304, so discard last 4 LSBs of last byte
     
  • L2C Post-FEC Binary File = "l2c_nav_data_fec_1.fec_cnv" & L5C Post-FEC Binary File = "l5_nav_data_fec_1.fec_cnv"
        - FEC Bytes to Read = 75
        - FEC bits to Read = 600
        - 75 * 8 = 600, so no bits to discard