Defining a Trade-off Between Spatial and Temporal Resolution of a Geosynchronous SAR Mission for Soil Moisture Monitoring

Abstract

The next generation of synthetic aperture radar (SAR) systems could foresee satellite missions based on a geosynchronous orbit (GEO SAR). These systems are able to provide radar images with an unprecedented combination of spatial (≤1 km) and temporal (≤12 h) resolutions. This paper investigates the GEO SAR potentialities for soil moisture (SM) mapping finalized to hydrological applications, and defines the best compromise, in terms of image spatio-temporal resolution, for SM monitoring. A synthetic soil moisture–data assimilation (SM-DA) experiment was thus set up to evaluate the impact of the hydrological assimilation of different GEO SAR-like SM products, characterized by diverse spatio-temporal resolutions. The experiment was also designed to understand if GEO SAR-like SM maps could provide an added value with respect to SM products retrieved from SAR images acquired from satellites flying on a quasi-polar orbit, like Sentinel-1 (POLAR SAR). Findings showed that GEO SAR systems provide a valuable contribution for hydrological applications, especially if the possibility to generate many sub-daily observations is sacrificed in favor of higher spatial resolution. In the experiment, it was found that the assimilation of two GEO SAR-like observations a day, with a spatial resolution of 100 m, maximized the performances of the hydrological predictions, for both streamflow and SM state forecasts. Such improvements of the model performances were found to be 45% higher than the ones obtained by assimilating POLAR SAR-like SM maps.

Meteorological OSSEs for New Zenith Total Delay Observations: Impact Assessment for the Hydroterra Geosynchronous Satellite on the October 2019 Genoa Event

“…In fact, the MCSs which form over land (e.g., in the Sahel where they are responsible for the majority of annual rainfall [94]) are known to be controlled by the surface properties [95]. The added value of the Hydroterra soil moisture observation in the hydrological framework have been discussed in [29]. Future works are needed to assess the impact of these new observations (ZTD and soil moisture) in a complete hydro-meteorological framework that is very important to forecast high impact weather events over areas with complex terrain, such as the Mediterranean region.…”

“…Concerning soil moisture, the added value of Hydroterra-derived estimates has been discussed in Cenci et al [29]. To the best of our knowledge, a similar kind of analysis has never been carried out for ZTD estimates from Hydroterra observations and their impacts on the predictive capability of severe hydro-meteorological events.…”

Novel Weight-Based Approach for Soil Moisture Content Estimation via Synthetic Aperture Radar, Multispectral and Thermal Infrared Data Fusion

“…In addition to SMC levels, a variety of other factors contribute to the backscattered radar signal, from surface characteristics like the surface roughness profile, the mineralogical composition of the soil, and the dielectric features of the soil to radar characteristics like the incidence angle (from nadir), the operating frequency of the radar sensor, and polarisation [ 24 ]. Moreover, the soil dielectric features or dielectric constant ( εs ) exhibit a polynomial relationship with SMC, and other dependencies have been found between SMC and the mineralogical composition of soil and SAR frequency [ 18 , 34 ].…”

“…However, the authors of [ 16 , 17 ] proposed semi-empirical calibrations of RMS height and correlation length to improve the characterisation of surface roughness parameters, which produced better results. The revisit time of high-resolution SARs is still inadequate, which makes tracking SMC temporal variations difficult [ 18 ]. Susceptibility to medium-to-thick vegetation cover, which consequently causes volume scattering, has a direct and negative impact on the accuracy of SMC retrieval [ 19 ].…”

Soil Moisture Remote Sensing across Scales

“…Cenci et al [14] compared a SAR in geostationary orbit to a more traditional SAR in polar orbit, in terms of spatial and temporal resolution, by means of data assimilation into hydrological models. They found that a geostationary SAR can be a valuable contribution to hydrological applications if the spatial resolution is kept high (∼100 m).…”