A launch vehicle's inertial measurement unit (IMU) is dependent on its precise knowledge of its initial position, velocity, and attitude. Small inaccuracies in the initial latitude can cause large initial velocity and attitude errors. Therefore, the location of a launch site must be carefully surveyed.
With the launch site location well known and the rocket on the pad, alignment can begin. Alignment is the process of precisely determining the IMU attitude. Generally, three techniques can be used: optical alignment, leveling, and gyro-compassing.
Optical alignment uses lines of sight to determine the initial twist, pitch, and yaw of the IMU. Leveling uses the accelerometers in the IMU to sense the direction of the plumb-line gravity vector. Gyro-compassing uses the gyros in the IMU to sense the direction of the Earth rotation vector. Optical alignment can yield highly accurate results, but requires human involvement, which is not always practical. Leveling and gyro-compassing can be performed automatically, but neither can fully determine IMU attitude. In practice, different techniques are combined, such as leveling with optical azimuth determination or leveling and gyro-compassing.
In addition to alignment, an IMU must undergo calibration to identify errors associated with the accelerometers and gyros. Before leaving the factory, the IMU is made to measure different known accelerations and spin rates. These measurements reveal any sensor errors (i.e., variations from the known values) so that the flight computer can compensate for them.
Vibrations, temperature changes, and mounting inaccuracies can cause the sensor errors to drift from their factory-calibrated values. When possible, the IMU is calibrated on the launchpad using the known Earth rotation rate and plumb-line gravity vectors as references.