NMEA Data with some faulty characters

Owned by Kim Jansen (Unlicensed)
Last updated: Jun 08, 2017
2 min read

Problem

The application shows incorrect data or does not update its readings. The frequency in occurrence of this fault condition can be very low, it may even happen only once and cause an anomaly in historic data.

It is essential to note that at least parts of the NMEA0183 message format can be distinguished (eg. "$GPGGA,11573###").

Conditions:

  • Data is recognizable as NMEA0183
  • May miss characters or newline
  • May include additional characters
  • May have unexpected end of lines

Known causes:

  • Wrong wiring
  • Bad cable
  • Overflow at some point in the NMEA distribution
  • Interference

Our Collector software will ignore data that it is unable to parse, this will mean some or all data is no longer updated. Furthermore the Collector can verify whether the checksum (last two characters) correspond to the sentence, however it is impossible to detect multiple bit errors and will mean some sentences are considered valid while the data is actually faulty.

Example (Putty):

Solution

The problem description indicate that the unit itself is properly configured and that the data providing systems and infrastructure need to be investigated.

Possible causes and solutions in order of likelyhood
  1. RS422 signals terminated as RS232 or vice versa, check and restore NMEA distribution pinouts and wiring
  2. Use a USB to serial converter and Putty to check if source device (eg. GPS or gyro) provides reliable data
  3. Check for proper grounding of wires, make sure at least one end is grounded

Please review related articles. In case this leads to the conclusion that shipwide cabling or other hard to troubleshoot or fix parts of the onboard installation may be faulty it is advised to place a MOXA NPort close to the source device(s) and configure the Collector to read NMEA over network.