RT_INS_Vel_NED_2: INS Down Velocity
RT_INS_Vel_NED_2: 142, 17 (0x8E1120 + unit id) (0x391)
Byte 0: Validity
Byte 1: Accuracy (D)
Bytes 2-4: INS velocity D (m/s)
NED velocities are velocities in the local North, East and Down directions at the current location.
Speed resolution is m/s * 1e-4.
NED velocity has components in the local North, East and Down directions. E.g. an NED velocity of 2, 2, 0 implies that the vehicle has a velocity component in the North direction of 2m/s and a velocity component in the East direction of 2m/s, and no vertical velocity component (the vehicle is travelling exactly on the flat). In this case, the vehicle is heading NE, and would have a speed of 2.83m/s (square root of 2*2 + 2*2 + 0*0).