NMEA 0183 - Application Layer Protocol Rules

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.

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