Craig Walker
National Radio Astronomy Observatory
Shami Chatterjee
Cornell University
March 30, 2000
Corrections for the effects of the ionosphere on VLBI phases have been tested using a geodetic project and a phase referencing test observation. Dual frequency S/X measurements on strong sources with very good positions are treated as ``truth'', and calibrations derived from GPS based ionospheric models are compared to them. All of the GPS models appear capable of reducing the effects of the ionosphere by a factor of roughly 2 to 5 and so provide a valuable aid for phase referencing observations. The SATLOC model does not appear to have advantages over the global models archived at CDDIS, despite a finer grid in space and time. Therefore it does not appear to be worth the effort to establish an in-house SATLOC archive.