Mid-Inclination Orbits for Small Satellite SAR Constellations and their Interferometric Exploitation: the IRIDE case study

Differential Synthetic Aperture Radar (SAR) Interferometry (DInSAR) plays nowadays a crucial role in studying ground deformations with centimeter-to-millimeter accuracy. Initially exploited to investigate individual deformation events, such as earthquakes and volcanic unrests, DInSAR has evolved in the last two decades thanks to the accessibility to large multi-temporal SAR data archives. This evolution has led to the development of advanced (also referred to as multi-temporal) DInSAR techniques, enabling to follow the temporal evolution of the detected surface displacements through the retrieval of deformation time series.

Despite the wide availability of spaceborne SAR systems with different characteristics (i.e., spatial coverage, spatial resolution, revisit time, orbital tube, etc.), the DInSAR community increasingly demands better coverage performance and improved imaging capabilities to address the latest emerging needs. For instance, short revisit time and high spatial coverage and resolution are usually needed to study fast deformation phenomena. Moreover, most SAR constellations exploit single plane, dawn-dusk, sun-synchronous orbits because this simplifies the satellite design across all subsystems, resulting in cost savings. However, in this traditional orbital design, the interferometric revisit time becomes considerable, thus representing a limiting factor. Furthermore, the poor sensitivity to the North-South deformation component that characterizes the sun-synchronous DInSAR systems represents a fundamental limitation in investigating the deformation phenomena.

In this scenario, the use of small SAR satellites is gaining traction, thanks to the simplified design and manufacturing processes. Additionally, the ability to launch multiple satellites, by using the same vehicle, enables the deployment of an entire constellation in a single mission. However, these systems, being smaller and lighter, have constraints on their imaging performance, potentially compromising coverage capabilities. Consequently, innovative mission configurations are necessary for their effective use.

This work focuses on a SAR component of the Italian IRIDE program, which will be implemented for the Italian government and completed by 2026 under the management of the European Space Agency, with the support of the Italian Space Agency. This SAR component, called NIMBUS, is expected to include, in its first batch and its preliminary design, 6 high-resolution X-band small satellites operating at altitudes between 490-550 km and in various operating modes including a StripMap one with a swath extension that is not designed to be extremely wide (25-30 km).

To cover the Italian territory with high spatial resolution and the shortest interferometric revisit time, we investigate a Mid Inclination Orbit solution that, through the DInSAR exploitation, can effectively measure the North-South deformation component, thus permitting us to investigate the three-dimensional behavior of the retrieved displacements.

Our simulations show that the analyzed IRIDE SAR component, through the preliminary setup in a 49° inclination orbit, permits covering nearly all the Italian territory with a 6-day revisit time in a right-looking acquisition mode. Moreover, we show that the simulated configuration would provide an excellent DInSAR retrieval capability for the North-South deformation component. Indeed, with such an orbital configuration, more than 40% of this component contributes to the SAR Line of sight projection, significantly better than what is typically achievable with sun-synchronous systems.