USING THE ANGULAR DEPENDENCE OF THE POLARIZATION RATIO TO ANALYZE AIRCRAFT PASSIVE MICROWAVE MEASUREMENTS OVER LAND

R. De Jeu¹, S. Steele-Dunne², T. Lang³, C. Amiot⁴, M. Klein⁵, M. Eble⁵, D. Miralles⁶

¹Transmissivity BV, ²Delft University of Technology, ³NASA Marshall Space Flight Center, ⁴The University of Alabama, ⁵Boulder Environmental Sciences and Technology, ⁶Ghent University

The interpretation of passive microwave measurements over land has always been a challenge. Soil conditions, land cover, and the atmosphere affect the measurements to different degrees, and it is difficult to isolate individual components. In this study, we explore whether multi-angle observations can improve our understanding of these measurements and support us in deriving the different contributions. Between October and November 2024, a series of airborne flights carrying the Advanced Microwave Precipitation Radiometer (AMPR) were conducted over the United States. Three land-based flights with multi-angle observations from 0 to 45 degrees and dual-polarized measurements at 10.7, 19.35, and 37.1 GHz were analyzed. The data showed a strong linear relationship between the microwave polarization ratio and the incidence angles within the 25 to 45 degree range (e.g., R2 > 0.9 for 79.8% of all flight scans

analyzed at 10.7 GHz). This linearity cannot be captured by a ω−τ based radiative transfer model. The observed linearity was further evaluated with satellite observations from the Advanced Microwave Scanning Radiometer (AMSR2). This evaluation analysis confirmed the observed linearity for the three frequencies. The slope between the polarization ratio and the incidence angle was calculated for each multi-angle flight scan to characterize the linearity and used as input in regression models to derive soil moisture. A model solely based on 10.7 GHz data revealed a strong correlation (R2 = 0.81) with Level-3 soil moisture from the Soil Moisture Active Passive (SMAP) mission, exemplifying the added value of multi-angle observations.