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202 Bu il d Y o ur O w n Q u a d c o p t e r

Figure 8.12 Propeller Serial Terminal screen capture of a GPS data stream.

start using the GPS receiver, but first I will discuss the NMEA protocol and the messages that
are being generated from the PMB-688 GPS receiver.

NMEA Protocol

As noted previously, NMEA is the acronym for the National Marine Electronics Association,
but nobody refers to it by its formal name. NMEA is the originator and continuing sponsor
of the NMEA 0183 standard that defines, among other things, the electrical and physical
standards to be used in GPS receivers. This standard specifies a series of message types that
receivers use to create messages that conform to the following rules, also known as the
Application Layer Protocol Rules:

• The starting character in each message is the dollar sign.
• The next five characters are composed of the talker id (first two characters) and the
message type (last three characters).
• All data fields that follow are delimited by commas.
• Unavailable data is designated by only the delimiting comma.
• The asterisk character immediately follows the last data field, but only if a checksum
is applied.
• The checksum is a two-digit hexadecimal number that is calculated using a bitwise
exclusive OR algorithm on all the data between the starting ‘$’ character and the
ending ‘*’ character as well as including those characters.