Requirements and prospects of landslide analysis through SAR interferometry and recent satellite missions

Thanks to the all-weather, day-night capability to detect and measure small ground surface deformations, multi-temporal Synthetic Aperture Radar (SAR) Interferometry (InSAR) techniques are attractive for landslide investigations. This Multi-temporal InSAR (MTI) application poses challenges related to the limited spatial extent of the phenomenon, the complexity of the ground deformations, and the occurrence in mountainous and vegetated areas that cause visibility problems. Nowadays, several satellite missions are available providing interferometric SAR data at different wavelengths, spatial resolutions, and revisit time. High-resolution X-Band SAR sensors, such as the COSMO-SkyMed constellation, have recently provided data with spatial resolution reaching metric values, and revisit time up to few days leading to increase the density of the measurable targets as well as to improve the detection of non linear movement. Medium resolution C-band SAR data have been thoroughly exploited in the last two decades thanks to the ERS-1/2, ENVISAT-ASAR and Radarsat missions. A new interesting opportunity is recently provided by Sentinel-1 mission, which has just started to deliver data to the user community. Although the spatial resolution is comparable to previous ESA C-band missions the revisit time is reduced to 12 and 6 days, by considering, respectively, one or two satellites. It is envisioned that by offering regular globe-scale coverage, improved temporal resolution and freely available imagery, Sentinel-1 will guarantee an increasing use of MTI in landslide investigations. These background missions are necessary for long-term, systematic mapping of unstable unstable slopes and regional scale assessment of landslide processes. According to these different SAR space-borne missions, the present work discusses current and future opportunities of MTI applications to slope instability monitoring by addressing, through a simple theoretical model, issues related to slope visibility, sensitivity to ground displacement, coherent target detection, mean velocity precision. In particular a comparative analysis is carried out aimed at investigating specific advantages of different satellite missions with respect to wavelength, resolution, revisit time, and clutter backscatter. According to this analysis, we show that a reliable estimates of the displacement rates are possible in a narrower time span by using X-band than C-band, but Sentinel-1 constellation will improve the performance of C-band. Moreover, for a certain time span the expected velocity STD is better for X-band than for C-band but, again, the Sentinel-1 constellation will improve the performance of C-band. This impacts on the monitoring capabilities in high risk situations by providing results in a shorter time, and allows monitoring mountainous areas during short period avoiding snow coverage, thus maximizing the chance to detect coherent targets. Finally, the work provides examples of multi-sensor and multi-scale slope instability investigations obtained by processing real data from both C-band medium resolution and X-band high resolution SAR data. In particular, the differences between displacement maps in terms number of images, observation time span, number of coherent targets, and estimated displacement precision, are used to support the outcomes of previous model.

Work supported by the project “APULIA SPACE” (PON03PE_00067_6), PON Ricerca e competitività 2007-2013.