Radiometric Phase Calibration

Phase Stability

Inhomogeneously distributed water vapor results in different electrical path lengths above the different antennas, or phase error. The phase errors scatter flux, limiting the dynamic range, and also cause decorrelation, which artificially decreases the source amplitude. Effects of phase errors on imaging has been studied for the MMA, and the intrinsic phase stability of the sites is being measured.

Since the intrinsic phase stability of even the best sites is often not sufficient for long baseline (>1000m) or high frequency (>200 GHz) observations, we are actively investigating several ways of correcting for the phase errors. The MDC Phase Stability Working Group Report outlines several strategies for calibrating the phase on very short timescales. The leading methods are fast switching phase calibration, traditional phase calibration performed on ~10 second cycles, and radiometric phase correction, which determines the amount of water vapor above each antenna radiometrically and calculates the phase by dead reckoning. Preliminary work suggests that either of these methods will permit 220 GHz observations on long baselines most of the time on the Mauna Kea or Chilean sites, but there are many details yet to be sorted out.

We expect much progress in this area as radiometric phase correction instrumentation becomes operational at each of the existing millimeter interferometers. Some of the ongoing radiometric calibration projects include:

Last modified 2 December 98

mma@nrao.edu