# dh is 'reliable' (usually a gyro) minus 'accurate'
# reliable instrument, message = gyro,hdg
# accurate instrument, message = posmv,pmv
# dday      mean_hdg last_hdg mean_dh std_dh num badflag
110.0745231   -71.96   -73.49  -1.29  0.22    119     0
110.0759112   -72.36   -71.86  -1.32  0.22    119     0
110.0772992   -69.57   -53.47  -1.22  0.24    119     0