First of all,the inertial measurement unit is composed of three accelerometers and three gyroscopes, to be able to measure the orientation of the three axes.
These accelerometers and gyroscopes input along the space of the three-axis, that is, three mutually perpendicular direction axes. By such a distribution in the design, it can get any movement or angular motion of the motion carrier in the space. But because the gyroscope measures the positional change of the relative space and the vector to be measured which requires the reference object of the system. Coordinate transformation is needed,through that way we can measure the data.