  LIGO, the Laser Interferometer Gravitational-wave Observatory, has been
designed and constructed to measure gravitational wave strain via differential
arm length. The LIGO 4-km Michelson arms with Fabry-Perot cavities have
auxiliary length control servos for suppressing Michelson motion of the
beam-splitter and arm cavity input mirrors, which degrades interferometer
sensitivity. We demonstrate how a post-facto pipeline (AMPS) improves a data
sample from LIGO Science Run 6 with feedforward subtraction. Dividing data into
1024-second windows, we numerically fit filter functions representing the
frequency-domain transfer functions from Michelson length channels into the
gravitational-wave strain data channel for each window, then subtract the
filtered Michelson channel noise (witness) from the strain channel (target). In
this paper we describe the algorithm, assess achievable improvements in
sensitivity to astrophysical sources, and consider relevance to future
interferometry.
