Data Hacking

198 Part III — Data Hacking
String Type Description
$GPSTN Multiple Data ID
$GPTRF Transit Fix Data
$GPVBW Dual Ground/Water Speed
$GPVDR Set and Drift
$GPVLW Distance Traveled through the Water
$GPVPW Speed, Measured Parallel to Wind
$GPVTG *Track Made Good and Ground Speed ( Course over ground and ground speed )
$GPWCV Waypoint Closure Velocity
$GPWNC Distance, Waypoint to Waypoint
$GPWPL Waypoint Location
$GPXTE Cross-Track Error, Measured
$GPXTR Cross-Track Error, Dead Reckoning
$GPZDA UTC Date/Time and Local Time Zone Offset
$GPZFO UTC & Time from Origin Waypoint
$GPZTG UTC & Time to Destination Waypoint
NMEA consists of sentences; and for each of these sentences. the first word, called a data type,
defines how the rest of the sentence is interpreted. Each data has its own unique interpretation,
as defined by the NMEA standard.
For example, take the GGA sentence shown here:
$GPGGA,180432.00,4027.027912,N,08704.857070, W,2,07,1.0,212.15,M,-
33.81,M,4.2,0555*73
This NMEA sentence shows an example that provides fix data information.
Sentences vary in the information they contain. Some sentences will repeat some of the information
already provided, while others provide new data. Devices attached to the GPS can look
for the NMEA sentence they want and choose to ignore others.
Normally, there are very few ways to control which NMEA sentences are sent from a GPS and
which aren’t. The usual state of affairs is that each GPS receiver simply transmits all of the data
and lets the attached device pick and choose what to read and what to ignore. Some receivers
can be set to send only certain types of sentences, but there is little benefit to doing this. There
is no way for the receiving device to acknowledge receipt of the data or to request that the data
be retransmitted because it was lost or garbled — if the receiving unit checks the checksum on
a sentence and finds it is corrupted, the receiver must wait until the next time a similar NMEA
sentence is transmitted.