First Demonstration of Forest Structure Monitoring with Early P-Band BIOMASS Pol-InSAR Data

Abstract

Monitoring forest structure is a key objective of the recently launched ESA’s BIOMASS mission [1]. It operates at P-band (70 cm wavelength) to retrieve forest height, biomass, and disturbance at global scale. The long wavelength provides sensitivity to the full canopy depth, offering unique opportunities to characterize forest scattering mechanisms from canopy to ground. This study presents first analyses of forest structure mapping using early BIOMASS P-band fully polarimetric and interferometric data. Natural forest scenes are complex radar scattering environments, where surface, dihedral, and volume contributions coexist within a single resolution cell. While polarimetric SAR (PolSAR) techniques can reveal scattering diversity, they cannot resolve its vertical distribution. Conversely, interferometric SAR (InSAR) provides height sensitivity but is influenced by temporal and environmental decorrelation. The combination of both by means of Pol-InSAR [2] techniques offers the potential to jointly exploit scattering diversity and height information for structural mapping. To this end, this work proposes to first separate the fully polarimetric response into simpler scattering contributions by means of interferometry, i.e., ground and volume [3]. The ground response is then modelled as a mixture of elementary surface (Bragg) and dihedral (Fresnel) mechanisms, simplifying the interpretation of their polarimetric signatures [4]. Preliminary results using fully polarimetric COM-1 acquisitions already demonstrate the excellent radiometric and polarimetric quality of the BIOMASS data. The scattering entropy can be employed as a measure of polarimetric coherence and spatial variability. The proposed decomposition effectively distinguishes forest areas with open and closed canopies, highlighting the mission’s sensitivity to structural diversity even prior to full calibration. These results contribute to the ESA’s BioTomEx project and support the development of BIOMASS-derived forest disturbance products. Data from the GABON-X campaign that will take place in November 2025 will help consolidating proposed Pol-InSAR approaches for forest structure monitoring by comparison with BIOMASS data. [1] S. Quegan et al., “The European Space Agency BIOMASS mission: Measuring Forest above-ground biomass from space,” Rem. Sens. Environment, vol. 227, pp. 44-60, June 2019. [2] K. P. Papathanassiou and S. R. Cloude, “Single-baseline polarimetric SAR interferometry,” IEEE Trans. Geosci. and Remote Sens., vol. 39, no. 11, pp. 2352-2363, Nov. 2001. [3] Alonso-González, A., Papathanassiou, K. “Polarimetric Change Analysis in Forest using PolInSAR Ground and Volume separation techniques,” EUSAR 2021; 13th European Conference on Synthetic Aperture Radar, pp. 1-6. VDE, 2021. [4] A. Freeman and S. L. Durden, “A three-component scattering model for polarimetric SAR data,” IEEE Trans. Geosci. and Remote Sens., vol. 36, no. 3, pp. 963-973, May 1998.