  The Advanced Photon Source (APS) was commissioned in 1995 as a
third-generation x-ray user facility. At that time orbit control was performed
exclusively with broadband rf beam position monitors (BPMs). Since then,
emphasis has been placed on incorporating x-ray beam position monitors into the
orbit control algorithms. This has resulted in an order of magnitude
improvement in long-term beam stability vertically, using x-ray BPMs (X-BPMs)
on bending magnet beamlines. Additional processing will allow similar
improvements horizontally, once systematic effects associated with variable
insertion device (ID) x-ray beams are properly compensated. Progress to date
and upgrade plans will be presented, with an emphasis on the details of the
required digital signal processing.
