Baseline calibration is a needed step in all applications of SAR interferometry and differential interferometry. A new approach for baseline calibration is proposed, based on the idea of maximizing the correlation between the original complex interferogram and reference values of it obtained from ground control points. The main advantage with respect to traditional techniques is that the method does not require the phase to be unwrapped in advance, and therefore the results are not affected by possible unwrap errors. In addition, successive phase unwrap is facilitated by the better phase flattening possible after baseline calibration. The method is computationally more demanding than traditional techniques, though the requested computational time is comparable with that of other processing steps of SAR interferometry. Tests performed on real ERS SAR images confirm the validity of the proposed approach.