As mentioned in the previous "Simulations 101" section, the usual ("2GC") approach to calibration involves determining corrections for the complex gains for each receiver in the array (generally for each pair of feeds at each element in the array). The gains of the instrument drift over the course of an observation generally due to the electronics and the atmosphere. These drifts can be solved for if the ideal response of the instrument to the sky is known. The response to a bright point source is a trivial prediction which is why such such sources are often used as phase calibrators in the case where the science target is either unknown or faint. Observations of the target are alternated with visits to a strong point source on a switching timescale that is short enough to track the expected drifts in the complex gains.

(The phase variations induced by the troposphere become more severe with increasing frequency, which is why a fast swtiching scheme is often needed at frequencies above ~15 GHz.)

This simulation demonstrates the calibration principle in a very simple way. We'll simulate an observation of a central point source, then fake some gain variations on the observation and examine the difference. Finally we'll solve for these gain drifts and remove them.