Backyard GPS

​

NMEA sentences

​

NMEA consists of sentences, the first word of which, called a data type, defines the interpretation of the rest of the sentence. Each Data type would have its own unique interpretation and is defined in the NMEA standard. Whatever device or program that reads the data can watch for the data sentence that it is interested in and simply ignore other sentences that is doesn't care about.

 

More information: http://www.gpsinformation.org/dale/nmea.htm

​

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 remains blank (it contains no character before the next delimiter – see Sample file section below).

• 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).

• <CR><LF> ends the message.

 

There is no way to indicate anything back to the unit as to whether the sentence is being read correctly or to request a re-send of some data you didn't get. Instead the receiving unit just checks the checksum and ignores the data if the checksum is bad figuring the data will be sent again sometime later.

​