CISESS Seed Grant: High-Resolution Soil Moisture from GNSS-Reflectometry Observations

Figure 1: Data Availability for the 9-km gridded CYGNSS reflectivity from 2018 to 2023.

© Proposal Team

Jifu Yin, Associate Research Scientist, UMD/CISESS/ESSIC

Soil Moisture (SM) is a vital state variable of land surface in hydrological, meteorological and climatological studies as it impacts energy, carbon and water interactions in the boundary layer. Reliable SM observations are essential for users involved with weather and water forecasting products to make the best decisions on calibrating model physics and parameters and conducting data assimilations. Unlike in situ measurements, microwave remote sensing offers SM observations with sufficient coverage and consistency at regional and global scales. Many national and international agencies have launched satellite missions/sensors to observe SM. However, the traditional microwave satellite missions offer coarser than 25 km SM products with costing upward of a billion dollars. With receiving the transmitted Global Navigation Satellite System (GNSS) signals, a GNSS-Reflectometry (GNSS-R) receiver onboard a small satellite can provide the L-band reflections, which could be used to retrieve SM with finer resolution. Relatively, small satellites cost hundreds of millions of dollars less than the traditional satellites. The theoretical footprint of a reflected GNSS signal is at the finer resolution, making the observations sensitive to small-scale surface features and thus capable of providing approximately daily data with 9 km spatial sensitivity (Figure 1). However, the current GNSS-R SM retrievals suffer from low spatial coverage, because of their narrow swaths (Figure 2). Given the SPIRE and TRITON were successfully launched in 2023 and the Cyclone Global Navigation Satellite System (CYGNSS) mission has offered data since 2018, it is anticipated to develop a 9 km soil moisture product with better spatial coverage through integrating all available GNSS-R Observations. This CISESS Seed Grant Project is to develop high-resolution satellite soil moisture retrievals through integrating CYGNSS, TRITON and SPIRE reflectivity and signal-to-noise observations.

Figure 2. Daily example of the 9 km CYGNSS reflectivity for 18 August 2023

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