Advancing Forest Structure Retrieval through Multi-Frequency PolInSAR and TomoSAR: Leveraging ESA’s P-Band Biomass and NASA–ISRO NISAR Missions

Abstract. Accurate estimation of forest structural parameters, particularly canopy height and aboveground biomass (AGB), is essential for carbon accounting, biodiversity conservation, and climate change mitigation under frameworks such as the UNFCCC and REDD+. While traditional field measurements provide accurate data, they are often spatially limited and logistically challenging, especially in dense or remote forests. Advanced Synthetic Aperture Radar (SAR) techniques, particularly Polarimetric Interferometric SAR (PolInSAR) and Polarimetric Tomographic SAR (PolTomSAR), offer promising solutions. These methods combine interferometric phase information with polarimetric scattering characteristics, improving sensitivity to vegetation structure and vertical scattering profiles. This paper reviews recent literature on PolInSAR and PolTomSAR methodologies and aligns the discussion with the capabilities of two major upcoming missions: ESA’s P-band Biomass SAR mission, scheduled for launch on April 29, 2025, and NASA–ISRO’s dual-frequency L- and S-band SAR mission, NISAR, set to launch on July 30, 2025. The P-band technology provides deep canopy penetration and enhanced ground return visibility, leading to more accurate Random Volume over Ground (RVoG) inversions and tomographic reconstructions. It is projected to achieve a canopy height root mean square error (RMSE) of 1– 2 m and an AGB RMSE of 15–25 Mg/ha in dense tropical forests. NISAR’s L/S configuration complements the P-band by integrating the L-band's sensitivity to trunks and lower canopy with the S-band's responsiveness to upper canopy foliage. This combination improves coherence stability and seasonal monitoring capacity. A comparative synthesis indicates that L-band PolInSAR remains a reliable operational choice; however, the P-band significantly reduces height bias in dense forests. L/S fusion further extends the dynamic range of biomass retrieval. Combining P-band tomographic products with L/S PolInSAR observations is expected to deliver the highest performance, mitigating biomass saturation and enhancing retrieval robustness across various forest types. Anticipated AGB R² values are expected to exceed 0.75, with RMSE in the range of 15–22 Mg/ha. These advancements pave the way for global-scale, multi-frequency, multi-technique forest monitoring frameworks that can achieve unprecedented accuracy in mapping forest parameters. Such integration will enhance carbon stock assessments, REDD+ monitoring, and ecosystem modeling, setting a new benchmark for operational forest structure retrieval in the P-band and NISAR era.