Positional Accuracy
The accuracy with which an object's position can be determined is limited by the atmospheric phase stability, the closeness of a suitable (astrometric) calibrator, and the calibrator-source cycle time. Under good conditions, in A configuration, accuracies of about 0.05 arcseconds can be obtained. Under more normal conditions, accuracies of perhaps 0.1 arcseconds can be expected. Under extraordinary conditions (probably attained only a few times per year on calm winter nights in A configuration when using rapid phase switching on a nearby astrometric calibrator - see Rapid Phase Calibration and the Atmospheric Phase Interferometer (API)), accuracies of 1 milliarcsecond have been attained with the VLA.
If highly accurate positions are desired, only "A" code (astrometric) calibrators from the VLA Calibrator List (http://www.vla.nrao.edu/astro/calib/manual/) should be used. The positions of these sources are taken from lists published by the United States Naval Observatory (USNO).
