NMEA HDT datagram format
The NMEA HDT datagram provides the true vessel heading. The information is normally provided by a course gyro.
Format
$--HDT,x.x,T*hh<CR><LF>
Description
1 $—Talker identifier
2 HDTDatagram identifier
3 x.x,T:Heading  (Degrees True)
4 *hh: Checksum