Phase Referencing and Narrow-Angle Astrometry in Current and Future   Interferometers
  Atmospheric turbulence is a serious problem for ground-based interferometers. It places tight limits on both sensitivity and measurement precision. Phase referencing is a method to overcome these limitations via the use of a bright reference star. The Palomar Testbed Interferometer was designed to use phase referencing and so can provide a pair of phase-stabilized starlight beams to a second (science) beam combiner. We have used this capability for several interesting studies, including very narrow angle astrometry. For close (1-arcsecond) pairs of stars we are able to achieve a differential astrometric precision in the range 20-30 micro-arcseconds. 
Benjamin F. Lane, Matthew W. Muterspaugh (MIT)