Source position

The most accurate possible source positions are needed for generating the proper correlator models for data processing. NRAO maintains a list of milliarcsecond positions for strong sources that appear in astrometric VLBI or connected-element interferometer catalogs. Positions generally are taken from the schedule file, so it is essential that the schedule file have the most accurate possible source positions. To keep fringe rate decorrelation low, source positions for correlation should be accurate to:

where tint is the correlator integration time in seconds and νobs is the observing frequency in GHz. However, it is desired that positions be better than this by a factor of at least 3-5, to provide the best results. When phase-referencing (see below), and even more so for astrometry, the source position errors are a larger problem and should be kept as low as possible (fraction of a mas is best). The correlator model is very detailed, and used to best advantage when source positions are as accurate as possible.

An accurate source position service is also available to obtain positions accurate enough for correlation.