GPS Link

Data sent from a serial GPS interface is formatted to form a "sentence". Each line/sentence is self-contained and independent from other sentences. All NMEA Sentences start with an ASCII dollar sign ($), to delimit the start of the frame (known as a sentence in the NMEA standard). The next five characters identify the talker (two characters, GP for GPS). This is followed by the type of message (three characters).

The sequence $GP forms the synchronisation marker for decoding the frames sent by a NMEA GPS receiver.

All data fields that follow are comma-delimited and represented in ASCII text. This can include information such as the PVT (position, velocity, time) computed by the GPS receiver. The data may vary in the amount of precision contained in a message. For example, time may be represented in decimal parts of a second or location may be show with 3 or even 4 digits after the decimal point. Where data is unavailable, the corresponding field remains blank (it contains no character before the next comma).

The first character that immediately follows the last data field character is usually asterisk this indicates presence of a longitudinal frame parity checksum, represented as a two-digit hexadecimal number. The checksum is the bit wise exclusive OR of ASCII codes of all characters between the $ and *, excluding these two characters. This checksum is optional for most data sentences, but is compulsory for RMA, RMB, and RMC (among others).

Each frame end is followed the characters <CR><LF>, simplifying display on a simple serial terminal.

Examples include:

See also

Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland.(2014)