One of the dominant sources of phase noise in millimeter-wave interferometric observations is fluctuations in the amount of atmospheric water vapor. While standard astronomical calibration techniques can remove slow, large scale atmospheric-induced phase drifts that occur over several minutes, these techniques are not able to remove the small scale fluctuations that occur on time scales of seconds. As current and future interferometers expand to longer baselines, it is becoming increasingly important to correct data for these rapid fluctuations in order to routinely achieve the anticipated sub-arcsecond resolution images.
The Owens Valley Radio Observatory (OVRO) millimeter-wave interferometer is investigating one method to correct for rapid phase fluctuations by monitoring the 22.2 GHz atmospheric water vapor line along the line of sight of each telescope. By calibrating the fluctuations in the amount of water vapor to the corresponding change in astronomical phase, the astronomical phases can in principle be corrected on time scales of seconds. We summarize here the most recent results, and in particular, the recent success in improving the coherence of standard astronomical observations at OVRO. Future work will concentrate on making full phase corrections to astronomical data.