Although this section requires some mathematics skills, it may be important to spend some time to fully understand how navigation measurements (position, and velocities) and orientations are represented.
You will learn how they are represented, what is a reference frames and which one are used.
An inertial frame is a frame in which Newton's laws of motion apply. It is therefore not accelerating, but can be in uniform linear motion. All Inertial sensors (accelerometers, gyroscopes) produce measurements relative to an inertial frame.