Forest Vertical Structure Inversion Based on Baseline Optimization InSAR Phase Histogram Technique

Objectives: The primary tasks include: (1) Proposing multi-baseline optimized phase histogram (PH) technique to explore the 3D imaging capabilities of low-frequency airborne synthetic aperture radar （SAR） data for forested areas and to estimate canopy height model (CHM); (2) using a simplified physical model to simulate and explain the principles and limitations of the PH technique. Methods: Introducing a novel technique, the PH method, for estimating the 3D vertical structure and canopy height of forested areas.The PH technique leverages the phase-to-height relationship derived from interferometric phase and vertical interferometric wavenumber to assign each pixel to a specific horizontal height layer. By accumulating the magnitude of all pixels within a given spatial window at the same height layer, it approximates the 3D backscatter profile of the forest. To address the limitations of single-baseline observations and the significant variation in vertical wavenumber caused by unstable airborne platforms, a full-baseline-based optimized multi-baseline interferometric combination strategy is proposed. This strategy restricts the height am biguity range to obtain phase histograms that completely cover the experimental area. Results: The results indicate that, under appropriate interferometric baseline length, the PH technique can retrieve an approximate coarse-resolution vertical structure of the forests, with the 3D backscatter power profiles characterizing the dominant scatterers. Additionally, the proposed method can provide quite good forest height estimates. Specifically, taking LiDAR forest height as the reference value, the average root mean square error of estimated forest height using P-band and L-band data are 4.6 m and 5.2 m, respectively. Conclusions: Although the PH technique cannot precisely separate ground and canopy signal, it can still characterize, to some extent, the distribution of scatterer density and scattering energy across horizontal height layers within a spatial window. This relationship holds potential for further exploration in forest height inversion and biomass estimation. Overall, as a novel method for retrieving forest vertical structure with a limited number of interferograms, the PH technique could serve as a viable alternative in future spaceborne SAR satellite missions for forest monitoring.