Phase Calibration of Repeat-Pass Monostatic and Bistatic Airborne SAR Tomographic Data: A Case Study From the TomoSense Campaign

The European Space Agency (ESA) TomoSense campaign was designed to advance forest parameter retrieval in preparation for future spaceborne synthetic aperture radar (SAR) missions. The campaign was flown in 2020-2021 at the Kermeter area in the Eifel Park, Germany, to acquire monostatic and bistatic SAR data over a temperate forest in hilly topographic conditions. The dataset includes multibaseline, full-polarimetric L-band airborne SAR data acquired in a bistatic configuration by flying two aircraft in close formation, to enable tomographic imaging of the vegetation. As largely discussed in the literature, achieving high-quality tomographic imaging requires accurate compensation of any phase terms not directly associated with the scene geometry, which includes platform position errors in navigational data and, in the case of bistatic acquisitions, clock synchronization errors. Building on the distinct features of navigation and clock synchronization errors, this article proposes a four-step calibration approach that progressively refines the data to achieve accurate tomographic imaging. In the first step, clock mismatches are retrieved and corrected based on a direct analysis of monostatic and bistatic data acquired in the same flight. In the second step, single-pass time-varying baseline errors between platforms are iteratively inverted and corrected. The third step jointly analyses the data from all flights to fully restore the interferometric coherence in all interferograms. The last step produces the final trajectory correction by a bistatic implementation of the phase center double localization (PCDL) algorithm. The experimental results demonstrate that the proposed approach produces high-quality tomographic imaging for both the monostatic and the bistatic datasets. Finally, we demonstrate the possibility of tomographic imaging by coherently combining the two datasets.