A calibration apparatus for calibrating an inertial measurement unit, IMU, having a sense axis and producing an output representative of an orientation of the sense axis, the calibration apparatus comprising: a rotational hub configured to rotate the IMU about a first axis of rotation, and characterised by: an elevation hub configured to rotate the IMU about a second axis of rotation perpendicular to the first axis of rotation, the orientation of the sense axis relative to the first axis being controllable by rotation of the IMU about the second axis; and a controller system configured in combination with the rotational hub and elevation hub to: rotate the IMU about the first axis of rotation at a known rate of rotation with the IMU positioned at a first elevation relative to the second axis of rotation; rotate the IMU about the first axis of rotation at a known rate of rotation with the IMU positioned at a second elevation relative to the second axis of rotation, the second elevation being different from the first elevation; and based on the output of the IMU during the rotation of the IMU at the first elevation and the output of the IMU during the rotation of the IMU at the second elevation, determining an IMU correction factor for calibrating the output of the IMU. a rotational hub configured to rotate the IMU about a first axis of rotation, and characterised by: an elevation hub configured to rotate the IMU about a second axis of rotation perpendicular to the first axis of rotation, the orientation of the sense axis relative to the first axis being controllable by rotation of the IMU about the second axis; and a controller system configured in combination with the rotational hub and elevation hub to: rotate the IMU about the first axis of rotation at a known rate of rotation with the IMU positioned at a first elevation relative to the second axis of rotation; rotate the IMU about the first axis of rotation at a known rate of rotation with the IMU positioned at a second elevation relative to the second axis of rotation, the second elevation being different from the first elevation; and based on the output of the IMU during the rotation of the IMU at the first elevation and the output of the IMU during the rotation of the IMU at the second elevation, determining an IMU correction factor for calibrating the output of the IMU. an elevation hub configured to rotate the IMU about a second axis of rotation perpendicular to the first axis of rotation, the orientation of the sense axis relative to the first axis being controllable by rotation of the IMU about the second axis; and a controller system configured in combination with the rotational hub and elevation hub to: rotate the IMU about the first axis of rotation at a known rate of rotation with the IMU positioned at a first elevation relative to the second axis of rotation; rotate the IMU about the first axis of rotation at a known rate of rotation with the IMU positioned at a second elevation relative to the second axis of rotation, the second elevation being different from the first elevation; and based on the output of the IMU during the rotation of the IMU at the first elevation and the output of the IMU during the rotation of the IMU at the second elevation, determining an IMU correction factor for calibrating the output of the IMU. rotate the IMU about the first axis of rotation at a known rate of rotation with the IMU positioned at a first elevation relative to the second axis of rotation; rotate the IMU about the first axis of rotation at a known rate of rotation with the IMU positioned at a second elevation relative to the second axis of rotation, the second elevation being different from the first elevation; and based on the output of the IMU during the rotation of the IMU at the first elevation and the output of the IMU during the rotation of the IMU at the second elevation, determining an IMU correction factor for calibrating the output of the IMU.