The first operation is to select in the current index the calibrator
observations performed during the time period when calibration is needed,
and to plot the visibilities after having selected the baselines, side
bands and/or subbands needed. This is done via `SET BASELINES`,
`SET BANDS`, `SET SUBBANDS` commands. Time should be specified
as X axis and Phase as Y axis (in `SET MODE TIME`, of course). `SET
PHASE CONTINUOUS` should also be specified, otherwise fitting a smooth
curve will be difficult.

Then a calibration curve is determined by command `SOLVE PHASE`. By
default this command will fit a cubic spline into each measured phase or
amplitude. With option `/PLOT`, the curve will be plotted with the data
points. The calibration curve is then in memory.

The time interval between knots of the cubic splines is available as a
parameter through the `SET STEP` command. Its default value is 3
hours. Additional knots may be introduced with the option `/BREAK`, to
match discontinuities or angular points.

Alternatively, with option `/POLYNOMIAL`, a polynomial fit will be
made. The degree of the polynomial is automatically chosen, using the
statistical errors bars for each data point.