Atmospheric distortions of spaceborne SAR polarimetric signatures at X and Ka-band

In recent years operational missions and many investigations have assessed the potentialities of spaceborne Synthetic Aperture Radars (SARs) operating at X band and above to monitoring the Earth surface. The most appealing aspect of these instruments is the capability of observing the Earth at very high spatial resolution (of the order of meters) and with a well-known sensitivity to water content, ground roughness, ground displacement (i.e., interferometry applications) and so on. These characteristics make this kind of instrument very suitable for monitoring floods, earthquakes, volcanoes, urban areas, infrastructures, land use and marine surfaces and to produce Digital Elevation Models (DEMs) with a resolution of few meters (e.g. COSMO-SkyMed, TerraSAR-X/TanDEM-X systems). Moreover, fully-polarimetric SAR systems allow the complete characterization of target scattering properties to be measured, improving target classification capabilities. The experience gained throughout the mentioned missions has stimulated the development of SAR systems operating at higher frequencies, such as Ka-Band (around 35 GHz) and above, subject of many investigations. Nevertheless, sensitivity to atmospheric effects is a major concern. Numerous works in the last years have assessed how the atmosphere can influence spaceborne SARs operating at frequencies above C band. In particular, atmospheric water-based particles have demonstrated to condition SAR response echoes, both in amplitude and in phase, and in some extent the ground resolution, due to their turbulent motion in presence of precipitations. Such phenomenon are complex, and even if studied in the last decades, several issues are still open, especially respect polarimetric effects. Actually, the SAR slant-view echoes, are given by the ground surface backscattering, weighted by the two-way path attenuation, and significantly influenced by a volume contribution determined by the match water-based particles, characterized by their spatial distribution and electromagnetic properties (e.g. raindrops, snowflakes or ice droplets); moreover SAR band frequency and polarization differentiate the importance of such effects. To allow the comprehension of atmosphere on SAR received signal, we have developed a forward response model, where a known scenario is used to simulate the SAR slant echoes. The developed model is fully polarimetric and allows characterizing the SAR signature (in both amplitude and phase) in presence of a water-based particles distribution (e.g. precipitating cloud, such as cumulonimbus, or also non-precipitating one, such as cirrus), given a reference ground surface. In particular, we have model the Normalized Radar Cross Section (NRCS) and the complex correlation coefficient among copolarized returns, useful to characterize the signal differential phase shift (i.e., between horizontal and vertical polarizations: these represent the most significant elements of the measured covariance matrix, given by the one of the surface target with the superimposed scattering atmosphere. The polarimetric NRCS is model as combination of the clear-sky ground surface NRCS, weighted by the two-way atmospheric path attenuation, and by a volume contribution determined by the water-based particles reflectivity and extinction, still weighted by the two-way path attenuation. On the other hand, the observed complex correlation coefficient is modelled through the correlation coefficient of the ground target and hydrometeors reflectivity and specific differential phase, both weighted by the two-way path attenuation. The atmospheric scenario can be modeled through both synthetic scenarios, realized by stratified distributions of homogeneous particles (e.g. raindrops and ice) within a basic shape (e.g. rectangle or Gaussian) and realistic ones, produced by high resolution Cloud Resolving Models. The first scenarios are unrealistic but allow a better understating of the phenomenon and analysis of the SAR sensitivity range. On the other side, the second kind of target allows to establish a statistics of the applicability limits of the SAR atmospheric effects. In this work, realistic scenarios have been simulated using the System for Atmospheric Modeling (SAM) mesoscale model, that we have used to simulate high-resolution (250 m ground, in altitude ranging from 250 m to 800 m at 30 km height) tridimensional volumes (64 x 64 km2 and 30 km height) of water-based particles. In particular SAM simulates water content (g/cm3) distributions of precipitating (rain, snow, graupel) and non-precipitating (cloud ice and cloud liquid water) particles; on the other hand, their scattering properties have been characterized using T-Matrix simulation through the Hydrometeor-Ash Particle Ensemble Scattering Simulator (HAPESS). More exactly the SAR simulation datasets is composed by a significant number of vertical sections (each of them representing a simulation case study) has been selected among the available SAM 3-D structures, as representative of the variety of clouds systems (e.g. convective or stratiform). The ground surface scattering model completes the scenario. The framework can be easily configured to simulate bare soils, using for instance Semi-Empirical Models, as well as surface canonical targets, such as dihedrals or volume scattering, for an ease analysis of the effect of the atmosphere on the polarimetric signature and a comparison with literature works. In this work, we will use this model framework to analyze the distortions introduced by atmospheric effects, both in amplitude and phase, so focusing our attention not only on the most evident one, but also on other, less evident but still present, as phase distortions. This analysis will be performed on both X-band and Ka frequency bands. A verification of these results analysis will arise by the study of real COSMO-SkyMed and TerraSAR-X acquisitions.