A quick example on what $GPVTG messages are. Keep in mind that the sentence Identifier can also be displayed as $GNVTG. The message is the same but some GNSS receivers use this format.
Track Made Good and Ground Speed.
054.7,T True track made good 034.4,M Magnetic track made good 005.5,N Ground speed, knots 010.2,K Ground speed, Kilometers per hour
1 = Track made good
2 = Fixed text ‘T’ indicates that track made good is relative to true north
3 = not used
4 = not used
5 = Speed over ground in knots
6 = Fixed text ‘N’ indicates that speed over ground in in knots
7 = Speed over ground in kilometers/hour
8 = Fixed text ‘K’ indicates that speed over ground is in kilometers/hour
9 = Checksum
The actual track made good and speed relative to the ground.
x.x,T = Track, degrees True
x.x,M = Track, degrees Magnetic
x.x,N = Speed, knots
x.x,K = Speed, Km/hr