Fringe Fitting
After correlation and application of the pulse calibration, the phases on a VLBA target source still can exhibit high residual fringe rates and delays. Before imaging, these residuals should be removed to permit data averaging in time and, for a continuum source, in frequency. The process of finding these residuals is referred to as fringe fitting. Before fringe fitting, it is recommended to edit the data based on the a priori edit information provided for VLBA stations. Such editing data are delivered in the FG table. The old baseline-based fringe search methods have been replaced by more powerful global fringe search techniques (Cotton 1995a; Diamond 1995). Global fringe fitting is simply a generalization of the phase self-calibration technique, as during a global fringe fit the difference between model phases and measured phases are minimized by solving for the station-based instrumental phase, its time slope (the fringe rate), and its frequency slope (the delay). Global fringe fitting in AIPS is done with the program FRING or associated procedures. If the VLBA target source is a spectral line source or is too weak to fringe fit on itself, then residual fringe rates and delays can be found on an adjacent strong continuum source and applied to the VLBA target source in a phase-referencing technique.
VLBA delays do tend to be very stable (1 to a few ns) during observations at higher frequencies where the ionospheric variations are limited. Thus one, or a small number, of high SNR fringe fits on strong fringe finders may provide superior results for delay, over trying to fit weaker sources. The phases will still need to be corrected, but that can be done with self-calibration or a phase-only fringe fit.
