Operational Modes for Sentinel-1 Next Generation C-Band Synthetic Aperture Radar

Abstract

In April 2014 was launched the first satellite of the Sentinel-1 (S1) mission, and two years later, with the launch of the second one, the constellation was completed. Each S1 satellite is expected to operate for at least seven years, and will be replaced by a second satellite, in order to guarantee a further continuation of the service supported by the mission. Though the relative youthfulness of the mission, the preparations for the next generation (NG) of the S1 mission (S1-NG) are ongoing, and the European Space Agency (ESA) is currently supporting a phase 0 study with the aim to define a baseline system concept and architecture for the space segment. Given the user-driven nature of the mission, the design of S1-NG satellite is strictly related to the requirements imposed by the final users of the S1 data. Specifically, main applications of S1 data include: monitoring of the European marine environment, of the Arctic and see-ice zones, of forests and crop; monitoring and assessment of land surface motion risks; water, forest fire, and flood management; surveillance of the open ocean. A remarkable feature of the S1 mission, key of its success within the user community, is the use of a reduced number of operational modes, each dedicated to systematic acquisitions over specific areas. In particular, the interferometric wide swath (IW) mode is the main mode for acquisitions over land and costal areas, and the extra wide swath (EW) mode for see-ice monitoring. The corresponding Synthetic Aperture Radar (SAR) images are acquired with a mission repeat interval of six days, in single and dual polarization. Moreover, they are characterized by a spatial resolution of 5 m x 20 m over swath of about 250 km in IW mode, and by 20 m x 40 m over a 400 km swath in EW mode. With respect to S1, S1-NG is expected to improve significantly the answer to the user needs, in terms of SAR data quality. In fact, the SAR images will be characterized by a spatial resolution of 5 m x 5 m, over a swath width of 400 km. For see-ice applications, the coverage could even reach 600 km. Moreover, the mission repeat interval will be reduced to four days, and the instrument will be provided by fully polarimetric capability. The necessity to guarantee the continuity of the service, i.e. to preserve as much as possible compatibility between S1 and S1-NG on one side, and that of improving the quality of the delivered data pose severe constraints on the design of S1-NG operational modes. In this work possible options will be discussed; the achievable imaging performance analyzed; the challenges and the opportunities that they offer discussed, both from the point of view of the applications and the SAR system.