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The National Marine Electronics Association (NMEA) is dedicated to the education and advancement of the marine electronics industry and the market which it serves.
The NMEA (National Marine Electronics Association) standard defines an electrical interface and data protocol for communications between marine instrumentation.
Under the NMEA-0183 standard, all characters used are printable ASCII text (plus carriage return and line feed).
The data is transmitted in the form of "sentences". Each sentence starts with a “$”, a two letter "talker ID", a three letter "sentence ID", followed by a number of data fields separated by commas, and terminated by an optional checksum, and a carriage return/line feed. A sentence may contain up to 82 characters including the "$" and CR/LF (Carriage return/Line feed).
If data for a field is not available, the field is omitted, but the commas that would delimit it are still sent, with no space between them. Since some fields are variable width, or may be omitted as above, the receiver locates desired data fields by counting commas, rather than by character position within the sentence.
The optional checksum field consists of a "*" and two hex digits representing the exclusive OR of all characters between, but not including, the "$" and "*". A checksum is required on some sentences.
Trimble 4D Control Server supports the GGA (Latitude and Longitude, UTC of fix, Number of satellites, and Horizontal DOP, GPS quality identifier) and GGK code messages as input, and additionally the GGQ code for NMEA output.
For more information refer also to the NMEA-0183 standard.