The KydroSAR project is aimed at exploiting a Synthetic Aperture Radar (SAR) at Ku and Ka band for mapping, detecting and estimating fundamental variables of the hydrometeorological cycle, such as atmospheric precipitation, water vapour and flooded areas. The proposal is a feasibility study of a dual-frequency lightweight small SAR whose baseline foresees dual-polarization capability, large swath, high orbit duty cycle and ground resolution going from few to hundreds of meters, depending on the observation mode. A simulation-based approach, based on physical-electromagnetic modelling of the spaceborne SAR response, is foreseen in order to develop and test the retrieval techniques of geophysical variables.
KydroSAR is the key element of the KydroSAT mission concept, proposed within a H2020 recent call, based on a formation of two near-polar low-Earth-orbit (LEO) mini-satellites both carrying a KydroSAR payload in order to provide a wider swath using small SARs. The KydroSAT observation capability is enhanced by the convoy with the LEO spaceborne multiband SIASGE, composed by COSMOSkyMed X-band SAR plus SAOCOM L-band SAR. In this way the same hydrometeorological scene is observed at L, X, Ku and Ka bands, representing a unique opportunity to improve high-resolution atmospheric, cryospheric and hydrologic products.
The goal of the KydroSAR proposal is to reinforce and keep the momentum of the Sapienza group expertise in the exploitation of the emerging Ku/Ka-band SAR science and technology. Innovation and impact of KydroSAR are expected from scientific, technological and application point of view: 1) precipitation and integrated water vapour maps will be characterized from space together with flood delineation and impact of atmospheric turbulence; 2) hydrology and meteorology communities will explore sub-pixel inhomogeneity, coverage delineation and data assimilation, especially in mountainous areas, small catchments, open ocean and polar regions.
Innovation and impact of KydroSAR concept are expected from a scientific and application point of view: i) high-resolution snowfall fields in high-latitude regions will be characterized from space together with mid-latitude/tropical rainfall; flood delineation will be improved by multi-frequency probing together with high resolution land and sea-ice snow water equivalent estimation; ii) hydrology, cryology and meteorology communities will explore high resolution data products, addressing sub-pixel inhomogeneity which affects current coarse-scale passive microwave retrievals from space (especially in mountainous areas and small catchments as well as over open ocean and polar regions) as well as test KydroSAR data assimilation within hydrometeorological numerical forecast models.
In each of the indicated fields, the proposed system appears particularly innovative and ambitious. The use of high-frequency satellite SAR for precipitations has become a concrete possibility only in recent years, as assessed by several works (both model and experimental) mainly at X-band. The proposed architecture benefits of the increased resolution and water-particle sensitivity offered by Ku and Ka bands. Still more innovative is the use of SAR for water vapour observations, actually only object of theoretical studies. Also the systematic use of dual polarization data, always including the cross one, represents a near absolute novelty. Finally, the innovative concept of multi-band, multi-polarization satellite SAR convoy of the KydroSAT-SIASGE system, offer the possibility of having better insights in volumetric hydrological and cryosphere applications.
The trend in SAR imaging applications pushes for increased performances (geometric resolution and/or extended swath coverage), sensible reduction of payload mass (ultra-small instrument), and providing detection/non imaging functionalities. To cover the different classes of operational application, significantly different technical requirements have to be fulfilled by the different SAR system designs. Consequently, future space-borne SAR systems, either mono-static or multi-static, will be characterized by a high degree of flexibility (multi-mode systems) and increased digitalization. This implies in most of cases, the adoption within the radar design of the digital beam forming technique, which relies on the combination of advanced multi-channel radar front-end architectures with novel operational modes. Several applications push for very high range geometric resolution. Achieving such a resolution in range turns into very larger bandwidth requirements, which poses a significant challenge to both transmitting and receiving chains. On the receive side, a technique largely used in radars to cope with wide signal bandwidth is the de-ramping technique, whose major limitation is to constrain the swath width. To overcome these contrasting needs is the use of multi-channel receiving systems, each receiver¿s channel dealing with a fraction of the overall bandwidth. On the azimuth direction, the resolution increasing deals with increased azimuth scanning range which, for active antennae, turns into the need for an increased number of radiating elements pushing for higher levels of integration. Capability to synthesize several beams in the azimuth direction by means of digital beam-forming could allow to couple the high azimuth resolution with the continuous coverage.
Ground-breaking objective of the KydroSAR project is, as already mentioned, to prove that: i) high-frequency SARs can be exploited for hydrometeorological observation and modelling; ii) high-frequency miniaturised SARs can offer small technological implementation with advanced high-performance solutions. Currently the nearest available mission is given by COSMO-SkyMed, TerraSAR-X ones, that are focused on ground applications, operate at X-Band and whose data are mainly single-polarization. The convoy concept of the KydroSAT-SIASGE system, even if already experienced (e.g. the TerraSAR-X and TanDEM-X system), still represents an innovative and promising concepts, as assessed by several mission projects now in development (e.g. SAOCOM ¿ SAOCOM-CS, SIASGE, TanDEM-L). The multi-band multipolarization concept of KydroSAT plus SIASGE system, together with the possibility of improving existing geophysical products (e.g. precipitation mapping), could open the way towards the development of new ones.