Forest Vertical Structure Inversion and Evaluation with Spaceborne X-band Multi-static Synthetic Aperture Radar

Abstract

Forest vertical structure is important for forest resource monitoring and biomass estimation. Tomographic imaging and phase histogram (PH) are two typical methods for reconstructing forest vertical scattering profiles. The new spaceborne X-band multi-static synthetic aperture radar system, HT-1, can acquire multi-baseline data in a single pass, providing a common data foundation for applying and evaluating both methods. However, the limited penetration of X-band signals produces weak below-canopy ground responses in forest areas, making it difficult to obtain a reliable terrain reference directly from the reconstructed profiles. Under such conditions, the relative performance of the two methods remains unclear. This study develops a unified inversion and comparative evaluation framework for both methods in terms of vertical structure representation, forest height accuracy, and computational efficiency. The Saihanba area is selected as the test site. ICESat-2 data and an XGBoost model are used to correct the external digital elevation model (DEM), providing a common terrain reference for both methods, with airborne lidar data used for validation. The results show that DEM correction reduces DEM root mean square error (RMSE) from 10.06 to 3.47 m. With the corrected DEM, the forest height RMSEs of Capon and PH are 4.30 and 4.50 m. Profile analysis reveals that Capon can reveal double-layer scattering structures in some sparse forest and forest-edge areas, but such structure occurs only locally and cannot serve as a ground reference across the full scene. After baseline selection, PH achieves accuracy close to Capon with much lower computational cost. These findings indicate that PH can balance accuracy and efficiency when an accurate DEM and a suitable baseline are available, making it a practical solution for rapid large-scale forest height inversion under HT-1 data conditions.