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At this point, the source model and observed data exist for all calibrators at all uv-points, and it is possible to begin solving for the gains. It is almost invariably the case that the instrumental and atmospheric amplitude and phase gains (hereafter ``complex gains'') are the dominant calibration error in VLA data. Therefore we solve for this factor first.
For the standard cross-calibration scheme described here, only one term (G) will be used to describe the net complex gain (and so it will include atmospheric effects). One complex gain factor, as a function of time, will be determined for each field, spectral window, polarization, and antenna in the dataset.
Note: Well-behaved antennas that are located near the center of the array are chosen as reference antennas for each dataset. For non-polarization datasets, reference antennas need not be specified although you can if you want to. If no reference antenna is specified, an effective phase reference that is an average over the data will be calculated and used. For data that requires polarization calibration, you must choose a reference antenna that has a constant phase difference between the right and left polarizations (i.e., no phase jumps or drifts). If no reference antenna (or a poor one) is specified, the phase reference may have jumps in the R-L phase, and the resulting polarization position angle response will vary during the observation, thus corrupting the polarization imaging.
To solve for G, first choose a solution interval which samples the time scale over which you think the instrument or atmosphere is changing. Time scales depend on the array configuration and the observed frequency. Here, a time scale of 60 sec is chosen for the 6 cm continuum polarimetry data while scan-based solutions (one solution per integration) are chosen for the 21 cm spectral line data.