Application Layer Protocol Rules
- Each message's starting character is a dollar sign.
- The next five characters identify the talker (two characters) and the type of message (three characters).
- All data fields that follow are comma-delimited.
- Where data is unavailable, the corresponding field contains NUL bytes (e.g., in "123,,456", The double comma between 3 and 4 is telling the listener the second field's data is unavailable).
- The first character that immediately follows the last data field character is an asterisk, but it is only included if a checksum is supplied.
- The asterisk is immediately followed by a checksum represented as a two-digit hexadecimal number. The checksum is the bitwise exclusive OR of ASCII codes of all characters between the $ and *. According to the official specification, the checksum is optional for most data sentences, but is compulsory for RMA, RMB, and RMC (among others).
ends the message.
As an example, a waypoint arrival alarm has the form:
- $GPAAM,A,A,0.10,N,WPTNME*32
where:
| GP | Talker ID (GP for a GPS unit, GL for a GLONASS) |
| AAM | Arrival alarm |
| A | Arrival circle entered |
| A | Perpendicular passed |
| 0.10 | Circle radius |
| N | Nautical miles |
| WPTNME | Waypoint name |
| *32 | Checksum data |
The new standard, NMEA 2000, accommodates several talkers at a higher baud rate, without using a central hub, or round-robin packet buffering.
The NMEA standard is proprietary and sells for at least US$ 325 as of June 2010. However, much of it has been reverse-engineered from public sources and is available in references like gpsd and Dale DePriest's.
Read more about this topic: NMEA 0183
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