RT_GPS_Speed GPS 2D and 3D speed
RT_GPS_Speed: 140, 16 (0x8C1020 + unit id) (0x310)
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Byte 0: Validity
Byte 1: Accuracy (m/s)
Bytes 2-4: GPS speed 2D (m/s)
Bytes 5-7: GPS speed 3D (m/s)
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This message outputs GPS-only scalar speed.
Accuracy resolution is m/s * 0.1
Speed resolution is m/s * 1e-4