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The NMEA Serial Data Standard

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NMEA-0183

The NMEA-0183 is a serial communications standard for the encoding and sending of GPS and other navigation information between devices.

Under the NMEA-0183 standard, all characters used are printable ASCII text (plus carriage return and line feed). NMEA-0183 data is sent at 4800 baud.

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.

If data for a field is not available, the field is simply 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 should locate 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.

The standard allows individual manufacturers to define proprietary sentence formats. These sentences start with "$P", then a 3 letter manufacturer ID, followed by whatever data the manufacturer wishes, following the general format of the standard sentences.

Some common talker IDs are:

Garmin GPS-38, NMEA-0183 V. 1.5 mode Standard: GLL, RMB, RMC, WPL, BOD, XTE, VTG, BWC Proprietary: PGRMM (map datum), PGRMZ (altitude), PSLIB (dgps ctrl)

Garmin GPS-38, NMEA-0183 V. 2.0 mode Standard: GLL, RMB, RMC, WPL, BOD, GSA, GSV, RTE, GGA Proprietary: PGRME (estimated error), PGRMM, PGRMZ, PSLIB

Garmin GPS-45 (and probably GPS-40 and GPS-90) Standard: BOD, GLL, RTE, RMB, RMC, GGA, GSA, GSV Proprietary: PGRME, PGRMM, PGRMZ

Garmin GPS-65 (and probably GPS-75) Standard: BWC, GLL, RMB, RMC, R00, WPL, XTE, VTG Proprietary: PGRMM, PGRMZ, PSLIB

Magellan Trailblazer Standard: APB, BWC, GGA, GLL, RMB, RMC, VTG Trimble Ensign XL Standard: APA, BWC, BWR, GGA, GLL, RMB

Sample Sentences Dissected:

A talker typically sends a group of sentences at intervals determined by the unit's update rate, but generally not more often than once per second. Characters following the "*" are a checksum. Checksums are optional for most sentences, according to the standard.

Here list a listing of a few of the sentences generated by many of the various talkers.

GGA Global Positioning System Fix Data

GGA,123519,4807.038,N,01131.324,E,1,08,0.9,545.4,M,46.9,M, , *42

GLL - Geographic position, Latitude and Longitude GLL,4916.45,N,12311.12,W,225444,A

GSA GPS DOP and active satellites GSA,A,3,04,05,,09,12,,,24,,,,,2.5,1.3,2.1*39

GSV - Satellites in view GSV,2,1,08,01,40,083,46,02,17,308,41,12,07,344,39,14,22,228,45*75

RMC - Recommended minimum specific GPS/Transit data RMC,225446,A,4916.45,N,12311.12,W,000.5,054.7,191194,020.3,E*68