  Two homodyne Michelson interferferometers aboard the LISA Pathfinder
spacecraft will measure the the positions of two free-floating test masses, as
part of the NASA ST7 mission. The interferometer is required to measure the
separation between the test masses with sensitivity of 30 pm/sqrt(Hz) at 10
mHz. The readout scheme is described, error sources are analyzed, and
experimental results are presented.
