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Session

📌Poster Session and Networking Aperitivo 🍷
Location: Lower Lobby

Presentation 11

Dual-polarimetric scattering information for AGB modeling using NISAR Simulated data

Unmesh Khati¹, Bhavya Jaiman¹, Abhinav Verma², Avik Bhattacharya²

1: Indian Institute of Technology Indore, India; 2: Indian Institute of Technology Bombay, India

Accurate estimation of forest above-ground biomass (AGB) is essential for modeling global carbon cycle. Synthetic Aperture Radar (SAR) remote sensing is extensively used for forest AGB estimation. The role of polarimetric and time-series SAR data in improving estimates of AGB have been tested using limited airborne and spaceborne data. With the launch of NASA-ISRO SAR (NISAR) mission, we would soon have access to dual-polarimetric SAR data with global coverage at 12-day temporal resolution. In this work, we explore the utility of the dual-polarimetric scattering information from NISAR simulated data and its application for forest AGB modeling.

The NISAR simulated data is obtained from the UAVSAR AM-PM campaign with the raw SAR data reprocessed to NISAR-like 20 MHz bandwidth, added noise and a spatial resolution coarser than 6 m. The NISAR simulated data is acquired over Lenoir Landing site in Alabama, USA which is a temperate deciduous forest with mean and maximum field measured AGB of 125 Mg/ha and 373 Mg/ha respectively. Field validation is obtained from 41 plots of 1600 sq.m and 10 plots of 400 sq.m along with 1-m airborne lidar CHM. For Lenoir Landing site, six SAR acquisitions in 2019 and 2022 are used in this work. The NISAR simulated data is provided in HDF5 format and it is processed using the ISCE3 toolbox to obtain the dual-polarimetric (HH, HV) geocoded covariance matrix (C2).

The scattering information is obtained from dual-polarimetric C2 matrix using Scattering Power Factorization Framework (SPFF) to derive the dihedral-like (Pd), surface-like (Ps) and residual (Pr) scattering power components. These components are analyzed across the time-series as well as across the biomass range. The reference AGB is binned at 1 Mg/ha intervals and the behavior of scattering powers analyzed. We observe that the Ps increases initially till 20 Mg/ha and then declines and stabilizes around 60 Mg/ha. This would most likely be due to change in surface scattering from ground and smaller vegetated regions. The Pd and Pr both increase with AGB with the saturation characteristics showing time-dependence. These scattering powers are used for modeling AGB using semi-emperical models like the modified water cloud model (WCM) and the six-component model used in NISAR biomass ATBD. When the WCM is modeled using the HV-backscatter, a component due to dihedral-scattering reduces the sensitivity to AGB. This is removed when we use the Pd and Pr as inputs to WCM. We observe that the modified WCM model with the input of Pr component leads to a RMSE better than 25 Mg/ha across the six NISAR simulated acquisitions. The three components vary temporally as well as with changes in surface soil moisture and precipitation conditions.

The use of dual-pol scattering powers is likely to improve sensitivity to AGB changes as well as extend the inversion range due to higher saturation level. In our extended work, we will have an in-depth discussion on how the use of dual-polarimetric scattering power improves AGB retrieval accuracy, compare it with WCM and 6-component ATBD model, as well as try to understand the forest-type and temporal changes in SAR scattering over the temperate forest site.