RT_CFG_Update_Start_Marker_1: Update start marker heading
RT_CFG_Update_Start_Marker_1: 159, 17 (0x9F1120 + unit ID) (0x7D1)
Bytes 0-1: Start Marker Heading
Bytes 2-7: Unused
Heading is unsigned, in the range 0..360 degrees.
Heading resolution is degrees/100