Quick initial check on NMEA data correctness

Owned by Kim Jansen (Unlicensed)
Last updated: Jun 15, 2017
2 min read

Problem

In case you suspect any issues with the incoming data without an obvious reason, eg. the fault condition does not happen regularly please follow this procedure.

Solution

  1. Download and install Notepad++
  2. Download / install Putty
  3. Kill any application that may be listening on the COM-ports (eg. Collector)
  4. Launch the Putty executable
  5. Change 'Session' to 'Serial'-mode and set the appropriate baudrate and COM-port
  6. Goto 'Logging' and enable "All session output", pick a nice location for the log-file and disable flush of log file
  7. You will end up with a possibly large log file, rightclick and edit with Notepad++
  8. Now first have a brief look at the data and have a look for sentences that look out of the ordinary, correct sentences look similar to:

    1. Note: NMEA starts with $ or !, followed by ASCII characters for the message delimited by commas, and ends with a * followed by a checksum
    2. All sentences can be reasonably well read by humans, only AIS (VDM+VDO) needs decrypting but are still normal characters
  9. If you quickly discover malformed data please resort to the other articles for a solution
  10. If you do not find any doubtful data please continue for a more analytical step
  11. In Notepad++ goto 'Search'→"Find characters in range..."
  12. Select "Non-ASCII Characters (128-255)" and press 'Find'
    1. In case you prefer an alternative texteditor, please make sure that it supports regular expressions and search for '[^\x00-\x7F]+'
  13. As with the above example it may be that one or more lines are found with non-ASCII characters, these indicate an issue with one of the following
    1. NMEA Distribution (cabling, wiring, polarity, interference, overflow)
    2. Data source that sends incorrect data
    3. NMEA Input on the system failed
  14. Please check 13a..13c, in case 13c seems to be the case contact our support team