On the retrieval of the Campi Flegrei caldera (Italy) 3D displacements by exploiting Capella Space Mid-Inclination Orbits DInSAR measurements: first results

Differential Synthetic Aperture Radar (SAR) Interferometry (DInSAR) is a powerful remote sensing technique that allows us to measure surface displacements with centimetric to millimetric accuracy. This approach exploits the phase difference between pairs of complex SAR images, relevant to radar acquisitions collected at different times, to retrieve the surface displacements measured along the radar line-of-sight (LOS). Initially developed to analyze single deformation events, the DInSAR methodology has advanced to study the evolution of the detected displacements over time through multi-temporal (or advanced) DInSAR techniques. These advanced methods enable the generation of displacement time series, revealing spatial and temporal deformation patterns. Among these approaches, the Small Baseline Subset (SBAS) technique permits the generation of LOS displacement time series and the corresponding velocity maps by utilizing SAR data pairs with small spatial and temporal baselines, which help to reduce noise decorrelation effects and increase the number of coherent points. Traditionally, the satellite SAR systems used for DInSAR applications are positioned in a sun-synchronous orbit (SSO), meaning they repeat (nearly) the same orbit at the same local time. This orbital configuration is particularly suitable for remote sensing applications because it allows global coverage, the same illumination geometry, and relatively stable environmental conditions among successive temporal passes.

However, SSO-DInSAR has limitations when measuring the North-South component. Indeed, the LOS direction of a sun-synchronous SAR satellite is primarily oriented towards the East-West direction (the orbits have a quasi-polar direction), making it mainly sensitive to Vertical and East-West displacements. As a result, retrieving accurate North-South displacement information from SSO-DInSAR data can be challenging, especially for slow-moving deformation processes. Conversely, when focusing on mid- to low-latitude regions, Mid-Inclination Orbits (MIOs) may offer an effective solution for retrieving the three-dimensional (3D) field of the occurring displacements. Indeed, MIOs provide advantageous geometries for measuring the North-South displacements.

However, using MIOs is not straightforward due to challenges such as the lack of access to polar ground stations and variations in local time across the Area of Interest (AoI), which increases the temporal variability of the atmospheric DInSAR phase component.

In this work, we present the first results achieved by processing, through the Parallel SBAS processing chain, three different DInSAR datasets generated from the 45° MIO SAR data experimentally collected by Capella Space over the Campi Flegrei caldera (Italy), where the bradyseism phenomenon, restarted in 2005, is still ongoing. To our knowledge, the results described in this work represent the first application to fully retrieve the 3D deformation field with multi-angle/multi-temporal DInSAR data, thus demonstrating the feasibility of the MIO satellite configurations for such DInSAR purposes.