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Chapter 8: GPS and a Real- T ime Situational Displa y 203

There are a large variety of messages available in the NMEA standard; however, the
following subset is applicable to the GPS environment and is shown in Table 8.3. All GPS
messages start with the letters “GP.”

Latitude and Longitude Formats
The two digits immediately to the left of the decimal point are whole minutes; those to the
right are decimals of minutes. The remaining digits to the left of the whole minutes are
whole degrees.

Message Prefix Meaning
AAM Waypoint arrival alarm
ALM Almanac data
APA Auto pilot A sentence
APB Auto pilot B sentence
BOD Bearing origin to destination
BWC Bearing using great circle route
DTM Datum being used
GGA Fix information
GLL Lat/Lon data
GRS GPS range residuals
GSA Overall satellite data
GST GPS psuedorange noise statistics
GSV Detailed satellite data
MSK Send control for a beacon receiver
MSS Beacon receiver status information
RMA Recommended Loran data
RMB Recommended navigation data for GPS
RMC Recommended minimum data for GPS
RTE Route message
TRF Transit-fix data
STN Multiple data id
VBW Dual ground/water speed
VTG Vector track a speed over the ground
WCV Waypoint closure velocity (velocity made good)
WPL Waypoint location information
XTC Cross-track error
XTE Measured cross-track error
ZTG Zulu (UTC) time and time to go (to destination)
ZDA Date and time

Table 8.3 NMEA GPS Message Types