A comparative study of tomographic SAR focusing methods

SAR tomography is a remote sensing technique, extending SAR interferometry, that allows three-dimensional imaging of volumetric targets. It allows, for example, to analyse the vertical structure of vegetation layers or other targets with significant penetration of the sensor´s radiation (dry soil, ice layers, etc.). The ability to monitor the 3-D inner structure of volumetric targets and to extract information about the nature and location of ongoing scattering processes promises a break-through in key environmental problems. Indeed, structural parameters of volume scatterers in the biosphere and cryosphere, such as vegetation height and moisture content, forest vertical structure and biomass, or snow / ice depth and layering are critical inputs for ecological process modelling and enable effective monitoring of ecosystem change. Due to its unique capabilities and potential, SAR tomography has gained significant attention during the last years and become an established technique for analysis of all kinds of three-dimensional backscattering scenarios. In particular, for future space-borne SAR sensors operating in longer wavelength, like BIOMASS, SAOCOM-CS or Tandem-L, it is planned to employ 3-D tomographic imaging modes on an operational basis. This paper reviews the different state-of-the-art signal processing methods for three-dimensional SAR imaging. It thereby focuses on techniques suitable for imaging of distributed scenarios, i.e. natural targets which possess a continuous distribution of scatterers along the vertical axis (e.g. vegetation) and which are not only composed of few prominent point scatterers (e.g. layover scenarios). The requirements, advantages and disadvantages of different techniques are assessed and their imaging capabilities are demonstrated using air-borne SAR data.