Performance analysis of multi-temporal and multi-frequency SAR interferometry

Nowadays, several satellite missions are available providing interferometric SAR data at different wavelengths, spatial resolutions, and revisit time. A new interesting opportunity is provided by Sentinel-1, which has a spatial resolution comparable to previous ESA C-band sensors, and revisit times improved up to 6 days. According to these different SAR space-borne missions, the present work discusses current and future opportunities of Multi-temporal InSAR (MTI) applications to ground instability monitoring. The mission parameters, which impact on the quality of the MTI products, are the wavelength of the SAR signal (decreasing from L, ~23 cm, to C, ~5.5 cm, to X band, ~3.1 cm), the revisit time, the spatial resolution, and the orbital tube. The figures that allow estimating the quality of the displacement maps are: the number of measurable coherent targets on the grounds (e.g. the PS density), the maximum detectable displacement velocity, the minimum number of processed images or the observation time span needed to derive reliable products, and the noise affecting both the final displacement and residual height estimation. Issues related to coherent target detection, mean velocity precision and product geo-location are addressed through a simple theoretical model assuming backscattering mechanisms related to point scatterers. In particular, a comparative analysis is carried out, aimed at addressing specific advantages and limitations of different satellite missions. This performance analysis may provide support for the SAR data selection, or for the definition of the geometrical and radiometrical configurations of future missions. For instance, high resolution data are found to increase the density of coherent targets, thus improving the monitoring of local scale events. Short X-band wavelengths improve the sensitivity to displacements. Short revisit times allow collecting large data stacks in short times, and improve the temporal sampling, thus increasing the chances to catch pre-failure signals (high-rate, nonlinear signals). The precision of the displacement rate detection depends on the number of images and on the phase noise, while the precision of the residual height depends also on the orbital tube size. These relations have been formalized in a conceptual model, by assuming typical values for target characteristics such as persistent scatterer shape and signal to clutter ratio. The model allows to derive quantitative, although indicative predictions for the performance of several missions (present and future), based on the above mentioned mission parameters. In particular, for example, Sentinel-1 (S1), which will provide SAR data for the next years with short revisit times, results able to provide reliable displacement estimations at large scale, and in quite limited observation time spans (accuracy of about 1 mm/y for time series covering 1 year). However, the narrow orbital tube size, may limit the height precision, and consequently the geo-location quality (e.g. vertical precision above 20 cm with observation time span below 12 months, assuming S1- A/B 12 days repetitivity). X-band missions as COSMO-SkyMed (CSK) or TerraSAR-X, show the best performance in terms of precision on the estimated mean velocity and height, especially for limited observation time spans (e.g. precision of 2 mm/y for velocity and 3 cm for height, with 12 months of observation time and 1 acquisition per month). By considering the forthcoming new SAR missions (e.g. SAOCOM, Tandem-L, NISAR, RADARSAT Constellation Mission, PAZ, COSMOSkyMED- SG, TerraSAR-X-NG), the resolutions and revisit times will be able to further improve the interferometric performances. The paper also presents examples of multi-sensor ground instability investigation. More than 20 years of MTI SAR data have been processed, coming from both legacy ERS and ENVISAT missions, and last-generation RADARSAT-2, COSMO-SkyMed, and Sentinel-1 sensors. Moreover, a comparison has been performed between the outcomes of the model developed for assessing the MTI product precision, and the results obtained by processing the real datasets from ERS, ENVISAT, RADARSAT-2, COSMO-SkyMed and Sentinel-1 missions available for a real test case.

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