Data Hacking

Chapter 9 — Examining the Data
BWC (Bearing and Distance to Waypoint Using a Great Circle Route)
This represents time, distance, bearing to, and location of a specified waypoint from the present
position calculated along the great circle path. The great circle path means along the surface of
the earth as opposed to a straight-line path.
$GPBWC,hhmmss,llll.ll,e,yyyyy.yy,f,xxx,T,yyy,M,zzz,N,###*hh
hhmmss UTC time
1111.11 Destination waypoint latitude
e N or S (for latitude)
yyyyy.yy Destination waypoint longitude
f E or W (for longitude)
xxx Bearing to waypoint (degrees True)
T Degrees True
yyy Bearing to waypoint (degrees Magnetic)
M Degrees Magnetic
zzz Distance to waypoint (measured in nautical miles)
N Nautical miles
### Waypoint ID
*hh Checksum
NMEA Checksum
You may have noticed the checksum that appears at the end of the NMEA sentences. If you
are wondering what this is and how it is calculated, it’s not that tricky, and it generally only
applies to mathematicians and programmers.
The checksum is always a two-character hexadecimal number. It is determined by looking at all
the characters that fall between the $ symbol and the * symbol, converting them to byte values
and then performing an exclusive OR or XOR on the bytes and then converting the final byte
checksum to hexadecimal.
You can find numerous examples of code on the web that will show you how to check this
value in a variety of languages. Using your favorite search engine, enter the words “NMEA
checksum” and you will find hundreds of code samples in several languages.
This might seem rather academic, but if you plan to write applications that interface with GPS
receivers and work on the NMEA sentences, then checking the checksum is vital.
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