Ground based SAR interferometry: a novel tool for Geoscience

The word Radar is the acronym of Radio detection and ranging. Radar is an active instrument, which measures the echo of scattering objects, surfaces and volumes illuminated by an electromagnetic wave internally generated belonging to the microwave portion of the electromagnetic spectrum. It was born just before the second world war for detecting and ranging target for non-civilian scopes. In this case the requested spatial resolution was not so challenging for the technology available that time. The opening of new technological frontiers in the fifties, including the satellites and the space vehicles, demanded a better spatial resolution for application in geosciences remote sensing (RS). Synthetic aperture radar (SAR) technique was invented to overcome resolution restrictions encountered in radar observations from space and generally to improve the spatial resolution of radar images. Thanks to the development of this peculiar technique, the radar observations have been successfully refined, offering the opportunity of a microwave vision of several natural media. Nowadays SAR instruments can produce microwave images of the earth from space with resolution comparable to or better than optical systems and these images of natural media disclosed the potentials of microwave remote sensing in the study of the earth surfaces. The unique feature of this radar is that it uses the forward motion of the spacecraft to synthesize a much longer antenna, which in turn, provides a high ground resolution. The satellite SEASAT launched in 1978 was the first satellite with an imaging SAR system used as a scientific sensor and it opened the road to the following missions: ERS, Radarsat, ENVISAT, JERS till the recent TerraSARX and Cosmo-SkyMED. The measurement and interpretation of backscattered signal is used to extract physical information from its scattering properties. Since a SAR system is coherent, i.e. transmits and receive complex signals with high frequency and phase stability, it is possible to use SAR images in an interferometric mode. The top benefit from microwave observations is their independence from clouds and sunlight but this capability can weaken by using interferometric techniques. Among the several applications of SAR images aimed at the earth surface monitoring, in the last decades interferometry has been playing a main role. In particular, it allows the detection, with high precision, of the displacement component along the sensor–target line of sight. The feasibility and the effectiveness of radar interferometry from satellite for monitoring ground displacements at a regional scale due to subsidence (Ferretti et al., 2001), 1