COSMO-Skymed constellation control: hardware and software implications

COSMO-SkyMed (COnstellation of Satellites for Mediterranean basin Observation) is an Earth Observation space program funded by the Italian Ministry of Research and Ministry of Defence and conducted by the ASI in conjunction with Italian MoD. The System is dedicated to Earth remote sensing and data exploitation through radar (SAR) imaging, serving a large variety of Customers including Institutional (civilian and Defense) and Commercial Users at national and international level. The COSMO-SkyMed constellation, in its full configuration, consists of four orbiting satellites each carrying a X-band Synthetic Aperture Radar (SAR), in Low Earth Orbit (LEO) suitable for night/daylight and all weather observation capabilities. The satellites are equipped with on-board GPS receiver and Precise Orbit Determination SW, necessary to provide a powerful autonomous navigation control, and to ensure high accuracy in geo-location of the acquired radar images. The Constellation geometry and characteristics are fundamental for achieving and maintaining the COSMO-SkyMed key performances throughout the whole system lifetime, in terms of global access and short revisit time to any target sites on Earth surface, global system response time, and imaging accuracy. As such, COSMO-SkyMed Constellation control functionality conceives suitable strategies for managing the Constellation itself in all its configurations and operative states, from the initial deployment throughout the end of life phase. Fundamentally, the COSMO-SkyMed Constellation is capable to operate in nominal or in interferometric orbital configurations, as briefly outlined hereafter: The "nominal Constellation configuration" (full sized) consists of four satellites, equi-phased at 90° each other in the same orbital plane. This configuration achieves a revisit time and a global access to any Earth sites in the order of few hours. In this framework, "revisit" means the capability to fly again over a given geographical site and to image it in different conditions (e.g. varying the incidence angle). Thus, in order to maintain the performance characteristics, it is indispensable to control within strict tolerance limits the Constellation geometry, such as the relative phasing among satellites, and the repeatability of the satellites ground tracks,. These requirements determine one of the fundamental characteristics of the COSMO-SkyMed Constellation Control. The "interferometric Constellation configuration" allows to produce three-dimensional SAR images by combining two radar measurements of the same point on the Ground obtained from slightly different incidence angles. The control requirement for such interferometry configuration is expressed in terms of keeping the mutual relative positions among satellites (i.e. the "interferometric baseline") with an accuracy of tens of meters. This high accuracy in mutual satellite positioning guarantees the requested image quality, taking into account the SAR instrument access capabilities, and the Constellation orbit characteristics. Thus, in interferometric configuration, the Constellation control is twofold, to concern the Ground-track absolute control box (analogous to the nominal configuration), and the maintenance of the "interferometric baseline". In both the configurations delineated above, a Constellation Control strategy is applied, involving: Ground Segment, which characterises the capabilities of orbit determination, prediction, and generation of manoeuvre plans to be applied to one or more satellites in a co-ordinated way. Space Segment, which concerns the autonomous execution of commands and manoeuvres on-board, according to the plans received from the GS. Starting from the specific Constellation mission requirements delineated above, this paper presents an efficient and economically valid approach for designing and implementing the COSMO-SkyMed Constellation management functionality. The paper also delineates how this control is carried out, in terms of command & control operations, guidance navigation, satellite autonomy, mission safety, operational availability, and provision of services to Users.