From AMS Glossary
In general, an automatic feedback control system in which the controlled variable is mechanical position or any of its derivatives. In radar, it is a method for controlling the positioning of an antenna.
The servo loop control system, or servomechanism, employs an alignment error voltage to command the antenna to move to a position that will reduce the alignment error to zero. The system consists of a computer, amplifiers, filters, and motors that move the antenna about its vertical and horizontal axes.