NMEA VTG datagram format
The NMEA VTG datagram contains the actual course and speed relative to the ground.
Format
$--VTG,x.x,T,y.y,M,z.z,N,g.g,K,a*hh<CR><LF>
Description
1 $—Talker identifier
2 VTGDatagram identifier
3 x.x,TCourse over ground (DegreesTrue)
4 y.y,MCourse over ground (DegreesMagnetic)
5 z.z,NSpeed over ground (knots)
6 g.g,KSpeed over ground (km/h)
7 aMode indicator
•  A = Autonomous
•  D = Differential
•  N = The data are not valid.
8 *hhChecksum