DInSAR performance of the NIMBUS-SAR medium inclination orbit mission for 3D surface displacements retrieval in natural hazards scenarios

Differential Synthetic Aperture Radar (SAR) Interferometry (DInSAR) is a well-established remote sensing technique that enables to measure Earth surface displacements with centimeter-to-millimeter accuracy. In particular, this technique exploits the phase differences between two SAR images relevant to acquisition pairs carried out over the same area in different epochs, with (nearly) the same illumination geometry. DInSAR was initially developed to analyze single deformation events, such as earthquakes and volcanic unrests. More recently, multi-temporal DInSAR techniques have been developed to track the temporal evolution of detected surface deformation by retrieving displacement time series.

The large availability of spaceborne SAR systems is characterized by dawn-dusk, sun-synchronous systems. In this orbital design, the interferometric performance may exhibit some drawbacks related to the revisit time and/or the spatial coverage. Moreover, the low sensitivity to the North-South deformation component typical of sun-synchronous DInSAR systems is a limitation for investigating deformation phenomena.

In this context, constellations of small SAR satellites are increasingly becoming an effective solution. Compared with “conventional” SAR spaceborne systems, small satellites have reduced design, engineering, and management costs. Moreover, the possibility of launching multiple satellites on the same vector allows space agencies to deploy constellations in a single mission. However, due to their reduced size and weight, such systems have limited imaging performance, which could jeopardize their coverage capability and imaging performance. Accordingly, effective exploitation requires innovative mission configurations.

This work provides an update on the NIMBUS-SAR mission, part of the SAR component of the Italian IRIDE program, which will include two batches of 6 high-resolution X-band small satellites each, operating at altitudes between 490-550 km.

To achieve the goal of covering the Italian territory with high spatial resolution and short interferometric revisit time, the mission will employ a Medium Inclination Orbit (MIO) solution. This will allow to effectively cover the whole Italian territory in 6 days and, through the DInSAR exploitation, to measure also the North-South deformation component, thus permitting us to investigate the three-dimensional behavior of the detected displacements. More specifically, the NIMBUS-SAR constellation will be deployed in 49° right-looking (batch 1, to be launched at the end of 2026) and 43° left-looking (batch 2, to be launched at the end of 2027) inclination orbits.

In this contribution, we first provide an update on the expected DInSAR performance of the NIMBUS-SAR mission, with emphasis on the retrieval capability of the North-South deformation component. Moreover, to fully assess the MIO configuration DInSAR performance in a natural hazard scenario, we also present some results obtained as output of an experimental campaign conducted by Capella Space over the Campi Flegrei caldera (Italy), which is characterized by renewed uplift phenomena since 2005. Specifically, four Stripmap SAR datasets were collected through ascending and descending orbits and right- and left-looking directions, exploiting complementary satellite heading angles. The presented results demonstrate the feasibility of using MIO DInSAR data to high accurately retrieve 3D displacements, particularly of the North-South component, in an active volcano monitoring scenario.