Data Hacking

Examining NMEA Sentences
Chapter 9 — Examining the Data
Many applications enable you to look at, examine, and even save NMEA sentences from your
GPS because this is the closest, purest way to work with the GPS data. However, most people
don’t give them much thought because the sentences themselves seem to complex. Little information
is provided as to what they contain and how the data is structured.
This is a real shame — with a bit of information and a little practice, reading NMEA can
become quite easy as you learn what to look for and where to look for it.
The following sections examine some of the common NMEA sentences that you will come
across if you view NMEA data. There are the main sentences, the ones that relate to navigation
and signal quality.
RMB (Recommended Minimum Navigation Information)
RMB is the “recommended minimum navigation” sentence, and it is sent whenever a route
or a goto (such as going to a waypoint) is set as active. Some systems are set to transmit this
all the time, transmitting null data if no goto is selected, while others only send it when
required.
$GPRMB,A,x.x,a,c--c,d--d,llll.ll,e,yyyyy.yy,f,g.g,h.h,i.i,j*kk
A Data status (A = OK, V = Void)
x.x Crosstrack error (measured in nautical miles, maximum 9.99)
a Direction to steer to correct crosstrack error (L = left, R = Right)
c--c Origin waypoint ID#
d--d Destination waypoint ID#
1111.11 Destination waypoint latitude
e N or S (for latitude)
yyyyy.yy Destination waypoint longitude
f E or W (for longitude)
g.g Range to destination (measured in nautical miles, maximum 999.9)
h.h Bearing to destination, True degrees
i.i Velocity towards destination (measured in nautical miles)
j Arrival status (A = arrived, V = not arrived)
*kk Checksum
RMC (Recommended Minimum Specific GPS/TRANSIT Data)
This is the NMEA equivalent of PVT (Position, Velocity, Time) data.
$GPRMC,hhmmss.ss,A,llll.ll,e,yyyyy.yy,f,x.x,y.y,ddmmyy,z.z,a*hh
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