Special issue on Advances in Interferometric Synthetic Aperture Radar Processing: Editorial

Synthetic aperture radar (SAR) is a powerful mature technique providing unique high-resolution two-dimensional radio reflectivity images, representing the electrical and geometrical properties of a surface in nearly all weather nightand- day conditions. Range resolution is obtained through signal modulation, while platform motion and synthetic aperture processing allow azimuth resolution. Accuratemeasurement of radio reflectivity is very useful for vegetation and snow mapping, forestry, land-use monitoring, agriculture, soil moisture determination, mineral exploration, and for oceanography, hydrology, and geophysics. Interferometric methods, based on further information extraction from the phase difference among at least two complex-valued SAR images, have successfully expanded in the last two decades the remote sensing capabilities of SAR. Depending on the acquisition configuration, SAR interferometry can provide efficient operational topographic mapping or displacement monitoring tools for land and ice applications. Sensitivity to topography with decimeter (from aircraft) to meter (from satellite) accuracy is obtained by the cross-track interferometric configuration, where the SAR acquisitions are separated by a baseline orthogonal to the platform flight path. Cross-track interferometry is finding many applications in remote sensing fields where digital elevation maps (DEMs) are useful, for example, for topographic and urban mapping, geophysics, forestry, hydrology, glaciology, siting for cell phones, and flight simulators. Sensitivity to small surface displacements, of the order of a few millimeters, is obtained by the configuration of differential interferometry, exploiting time acquisition diversity on long (days to years) time scales. Differential interferometry is an established technique for analyzing ground displacements of tectonic nature, for monitoring volcanic areas and slope instabilities, as well as for capturing precursor displacements to building collapses. Another promising SAR interferometry mode allows also ocean surface (or moving vehicle) velocity sensing. This mode is termed along-track interferometry, since it exploits SAR acquisitions separated by a baseline aligned with the platform flight path. It can be regarded as a differential interferometry technique acting on short (fraction of second) time scales to measure decimeter-to-meterper- second velocities. Further details on the various interferometric SAR techniques and their applications can be found in the tutorials papers [1, 2]. The need for advanced signal processing techniques within the interferometric SAR processing field is continuously increasing, for improving existing interferometric functionalities, producing novel parameter extraction capabilities, and fully exploiting the rich SAR data archives and the potentials originated by new experimented and planned interferometric SAR sensors (multi-baseline airborne and minisatellite cluster systems). Observing this trend, we have put together this special issue. All of the submitted papers went through peer reviews to ensure their correctness, technical significance, and relevance to the special issue. It consists of ten papers on the development of advanced models and new signal processing algorithms in the interferometric SAR field, with an approach mainly oriented towards the exploitation of statistical methods and of baseline/time/frequency/polarization acquisition diversity, to face the challenges of an accurate, reliable, and fully capable interferometric radar remote sensing and to deal with increasingly various and difficult scenarios. In particular, the papers concern the fertilization with, and application of, methods and concepts from the areas of filtering, parameter estimation, spectral estimation, array processing, coherent data fusion, model inversion, detection, and physicalbased statistical modeling. System performance analysis and fuzzy signal processing are also tackled. The papers are categorized into four interferometric topics: cross-track interferometry, differential interferometry, polarimetric interferometry, and along-track interferometry.