# 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
154.5376108  -174.06  -147.40   0.29  0.04    300     0
154.5410693  -138.32  -111.60   0.26  0.03    293     0
154.5445414  -105.56  -100.60   0.29  0.03    294     0
154.5480134  -105.61  -123.81   0.23  0.03    294     0
154.5514854  -131.71  -140.60   0.24  0.03    294     0
154.5549574  -140.23  -141.20   0.31  0.03    294     0
154.5584294  -139.88  -140.01   0.35  0.03    294     0
154.5619015  -139.97  -140.32   0.35  0.03    294     0
154.5653735  -139.99  -140.10   0.35  0.03    294     0
154.5688456  -140.45  -140.90   0.36  0.03    294     0
154.5723176  -143.21  -161.85   0.38  0.03    294     0
154.5757895  -177.03  -165.20   0.42  0.03    294     0
154.5792616  -171.59  -144.54   0.40  0.03    294     0
154.5827336  -143.10  -145.30   0.41  0.03    294     0
154.5862057  -141.27  -141.30   0.40  0.03    293     0
154.5896777  -140.18  -140.54   0.38  0.03    294     0
154.5931497  -142.27  -143.06   0.38  0.04    294     0
154.5966218  -143.92  -144.30   0.29  0.03    294     0
154.6000938  -134.44  -111.55   0.29  0.03    294     0
154.6035658  -106.74   -71.47   0.37  0.05    294     0
154.6070379    97.68   168.80   0.46  0.11    294     0
154.6105099   172.50   161.55   0.35  0.04    294     0
154.6139820  -108.28    23.96   0.43  0.05    294     0
154.6174540    22.07    -8.46   0.43  0.04    293     0
154.6209259   -88.12  -232.01   0.33  0.06    294     0
154.6243980    62.09     4.94   0.38  0.03    294     0
154.6278700  -132.76  -157.97   0.39  0.07    289     0
154.6313421   -98.66   -34.06   0.42  0.03    292     0
154.6348141    15.14   102.86   0.38  0.05    294     0
154.6382861  -124.46   -81.05   0.35  0.06    294     0
154.6417582   -73.15   -93.50   0.29  0.04    294     0