CACAL is the program used to calibrate the delay, phase and gain of the antennas. Only the delay calibration is essential: bad delays cause decorrelation across the passband, resulting in reduced S/N. The calibration of phase and gain is done mainly to aid in the on-line interpretation of the data (VIS display) and diagnostics of faults (ASSISTANCE). The off-line calibration using AIPS or MIRIAD will redetermine the phases and gains.