A fundamental requirement for coherent processing of repeat pass SAR data stacks is to have precise knowledge of the relative position of each track. Indeed, sub-wavelength position errors give rise to residual phase screens among different passes, which hinder coherent applications. In this paper we describe an approach to estimate and remove phase screens by exploiting distributed targets, based on the concept of equivalent phase center. The proposed approach is demonstrated through numerical simulations and using campaign data. A cross-check of the results from simultaneous P- and L-Band acquisitions indicates that the dispersion of the retrieved flight trajectories is limited to a few millimeters. Preliminary results indicate that Capon-based tomographic imaging is more sensitive to phase errors, potentially resulting in artifacts that do not appear in Fourier-based approaches.