In principle, baseline calibration was done before the observations. Under special circumstances, however, the operator and on-duty astronomer at Plateau de Bure may decide to postpone baseline determination until later. In such a case, the data must be corrected for the new baseline values before any instrumental phase calibration can be performed, because a wrong baseline implies a wrong prediction of the calibrator and source phases as a function of time.
This modification can be done on compressed data using the MODIFY BASELINE command.