NMEA-0183 Sentence Format
All NMEA 0183 data is sent in the form of text sentences, each beginning with a ‘$’ (or ‘!’) symbol, and use commas to separate each part of the sentence. Maximum sentence length, including the ‘$’ and is 82 bytes. NMEA sentences are plain printable ASCII text, and have the following format:
- A 2-letter ‘YY’ code follows, giving the talker id (e.g. ‘GP’ for GPS, and for a depth sounder this should be ‘SD’) and a 3-letter ‘XXX’ code follows, giving the sentence data type (e.g. ‘RMC’ is a “Recommended Minimum Navigation Information” sentence, and ‘DBT’ is a “Depth Below Transducer” sentence).
- A comma follows, then the contents of the sentence data, which changes depending on the data type and the current values of what is being monitored.
- ‘hh’ is checksum and preceded by the ‘*’ character and is calculated by taking the 8-bit exclusive-OR of all characters in the sentence, including ‘,’ delimiters, between but not including the ‘$’ (or ‘!’) and ‘*’ delimiters.
- The string always ends with a carriage return and linefeed combination (Hex 0x0D 0x0A, ASCII ‘rn’ ()).
Our solutions support the standard sentence ($SDDBT) from the NMEA-0183 for easy integration with common flight controllers (on Ardupilot and PX4 autopilot) and many other data acquisition systems. Additionally, other formats can be added upon request.
$SDDBT has the following format: $SDDBT,FEET,f,METRES,M,FATHOMS,F*hh
- DBT – Depth Below Transducer
- FEET – Water depth, fee
- METRES – Water depth, meters
- FATHOMS – Water depth, Fathoms
For examples: $SDDBT,7.8,f,2.4,M,1.3,F*0D