Precipitation observations from polarimetric SARs of the COSMO- SkyMed satellite constellation

In Synthetic Aperture Radar (SAR) images, precipitation results in both propagation and backscatter effects that modify surface returns. These effects are quite evident for SAR operating at frequencies of X band. Several examples concerning images of X-band SAR systems currently in orbit, have been shown in the literature. A further effect induced by the inherent Doppler spectrum of precipitation is the degradation of nominal resolution, which however remains better than that provided by current space borne precipitation radars. However, the major limitations of the SAR images for the study of precipitation are due to oblique viewing geometry that makes it difficult to interpret signatures of precipitation and sensitivity. Due to the anisotropy of many hydrometeors, propagation and backscattering effects depend on polarization and therefore different precipitation signatures can be detected by polarimetric SAR especially if local incidence angle are far from vertical. Although precipitation monitoring is not among the main objectives of current SAR missions, analysis of SAR images in precipitation can support the investigation of potential of SAR technology for precipitation studies. This paper focuses on the analysis of images collected by the SARs of the COSMO SkyMed constellations in the presence of precipitation. COSMO SkyMed (Constellation of Small Satellites for Mediterranean basin Observation) is a mission of the Italian Space Agency based on a constellation of four satellites equipped with a SAR at X-band with polarimetric capabilities. An attractive feature of the full constellation is the short revisiting time (that is the capability to image a given area also using different conditions such as varying SAR look angle) which is of the order of a few hours. Of particular interests are the images collected in the Ping Ping HH VV polarimetric mode. In this mode, SAR operates in HH for a fraction of the synthetic aperture, and then VV is used. Therefore HH and VV returns are not coherent with each other and copolar returns can be analyzed. Images were collected in 2010, when the constellation was composed of three satellites. Satellites operated in the “Tandem Like” interferometric configuration: two satellites were at 67.5° each other allowing acquisitions of the same scene with the same viewing geometry and a decorrelation time of one day. Polarimetric SAR observations were scheduled on regions where operational weather or research radars were available: Piedmont (Italy) and Tampa Bay (Florida, US) where the Bric della Croce C-band dual polarization radar and two WSR-88D S-band Doppler radars (in Tampa Bay and Melbourne, respectively) are running and in Emasalo (Finland) where a specifically designed sector scanning for Kumpula research radar have been performed. At the moment, the dataset of CSK images collected in precipitation is composed by one image in Piedmont and 4 images in Tampa Bay. Quasi coincident volumetric observations from ground based radar can be used to estimate the components due precipitation (namely attenuation and precipitation backscattering) that determine SAR returns in precipitation, thus providing a valuable tool to support interpretation of SAR images with precipitation. The image below is an RGB composite obtained using red = |S_HH|², green = |S_VV|² and blue = 2^-0.5|S_HH - S_VV|² where S_HH and S_VV are the received copolar amplitude, and a multilooking process applied to a 5x5 window, that reduces the spatial resolution from 10x10 m² to 50x50 m². The original image was collected by the CSK2 satellite of the constellation on 26-06-2010 over the Tampa Bay region when convective precipitation was occurring. The figure summarizes quite well several effects of precipitation, namely a) attenuation effects (the dark spots that were found corresponding to peaks of reflectivity maps collected by the KTBW radar) b) enhancement of returns due to the backscattering of hydrometeor in regions closer to the satellite ground track (in the figure they are on the left of dark spots), and c), some artifacts of the Ping Pong mode resulting in periodical strips that must be properly processed. The paper analyzes, using the support of coincident observations from ground based weather radars, some effects of precipitation on SAR polarimetric images. In addition to the effects listed above, signal extinction, degradation of spatial resolution, and, differential effects of precipitation on HH and VV returns is described. Relations between SAR and measurements from ground based radars are also analyzed.