Inertial measurement unit (IMU) is a device that measures accelerations and angular velocities. Some IMUs can also measure an external magnetic field with a magnetometer.
In an Inertial Navigation System (INS), IMU measurement data are used to calculate a unit’s orientation and relative changes in its velocity and position.
For navigation, they have to be properly initialized with absolute position and velocity. As velocity and position measurements are based on IMU measurement integration without corrections the INS solution drifts over time depending on IMU performance. Usually, GNSS measurements are used for correction in GNSS aided INS.