TThe SIM Interferometry Test Bed 3 (STB3) is the spaceborne-stellar-interferometer simulator for SIM, built and operating at JPL. Its construction details and performance are described elsewhere in this conference. The test bed consists of an interferometer system built on a large optical table, and a star simulator built on another large optical table placed directly across it. The optical tables float on
independent, air-filled suspension legs simulating the SIM spacecraft and the distant stars it is to observe. In order to demonstrate the performance requirements, a novel attitude control system (ACS) has been built and installed on the STB3. In this paper, the details of the design, construction and performance of the attitude control system are presented. The attitude control system has been used to meet certain SIM requirements. An example of this performance test is also included.