Multidimensional SAR tomography: Methods and applications

3-D SAR tomography is an active radio remote sensing technique, stemming from synthetic aperture radar (SAR) interferometry, allowing the advanced functionality of fully three-dimensional imaging. Typical application is to scenarios with multiple height-distributed point-like scatterers, or with volumetric scatterers, allowing the solution of layover in urban or infrastructure areas, and the reconstruction of the vertical structure of forest or ice layers. This new 3-D radar imaging and information extraction ability is very important in complex remote sensing areas such as biosphere and cryosphere monitoring, related to critical environmental problems of global warming and climate change. After first theoretical and experimental investigations started around the 2000, thanks to many developments from several SAR remote sensing groups SAR tomography is now also being applied experimentally to new dedicated spaceborne SAR missions. The 3-D tomographic concept has been also extended to higher output dimension in the more recent differential tomography methods, producing 4-D (3-D + time) images, so bridging the gap between the mature differential interferometry and the advanced SAR tomography. Typical application in this case is to urban or complex infrastructure scenarios for improved deformation monitoring, in terms of density of the measurements, accuracy, and coverage of specific zones for single building application where layover phenomena can impair full deployment of differential interferometry. Application of 4-D differential tomography to forest layers with temporal decorrelation has also been proposed. This paper present an overview of the main state-of-the-art processing methods and typical experimental applications for three-dimensional and higher-dimensional tomographic SAR imaging, developed during the last decade or more, as an extension and updating of previous review papers. New trends for future developments are also pointed out.