 Each unit contains two gyroscopes that allow the telescope to point itself.The first unit, RSU 2, was replaced without problems, but when they attempted to replace the second unit, RSU 3, the unit would not align onto the guide pins, and they could not seat it into the equipment bay.Managers decided to put the unit originally intended for the RSU 1 bay into the RSU 3 bay, and it was installed without problems.