NMEA
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NMEA 0183 (or NMEA for short) is a combined electrical and data specification for communication between marine electronics and also, more generally, GPS receivers.
The NMEA 0183 protocol is a means by which marine instruments and also most GPS receivers can communicate with each other. It has been defined by, and is controlled by, the US based National Marine Electronics Association (http://www.nmea.org/).
The 0183 standard, uses a simple serial protocol transmitting a "sentence" from one "talker" to one or more "listeners".
Example of sentence:
Waypoint Arrival Alarm
$GPAAM,A,A,0.10,N,WPTNME*43
Where:
AAM Arrival Alarm A Arrival circle entered A Perpendicular passed 0.10 Circle radius N Nautical miles WPTNME Waypoint name *43 Checksum data
The new standard, NMEA 2000, accommodates several "talkers" at a higher baud rate, without using a central hub.
The NMEA standard is proprietary and expensive. However, much of it has been reverse-engineered from public sources and is available in references like this one (http://gpsd.berlios.de/NMEA.txt).