A Comparison Between SAR Tomography and the Phase Histogram Technique for Remote Sensing of Forested Areas

Abstract This paper compares two approaches for estimating forest height and vertical structure using synthetic aperture radar (SAR) data, namely the well-known SAR tomography (TomoSAR) technique and the recently introduced phase histogram (PH) technique. By jointly exploiting multiple SAR images, TomoSAR allows for direct imaging of the three-dimensional structure of the vegetation, from which biophysical parameters such as forest height and terrain topography can be extracted. The PH technique assigns each pixel in a SAR interferogram to a specific height bin based on the value of the corresponding interferometric phase, thus allowing for the estimation of the forest vertical structure by accumulating pixels magnitudes within a given spatial window. The two techniques are here compared on an experimental basis using L-Band tomographic data from the ESA campaign TomoSense, flown in 2020 at the Eifel Park in North West Germany. The analysis we present considers a characterization of forest scattering by 3D imaging and forest height retrieval. Experimental results indicate that the PH technique can only loosely approximate the vertical structure produced by SAR tomography. Still, it can produce a reasonably good estimate of forest height. In particular, TomoSAR and the PH technique are observed to have an average root mean square error (RMSE) with respect to Lidar estimate of 2.63 m and 4.35 m in North-West heading data, and 1.84 m and 5.46 m in South-East heading data, respectively. The observed results are interpreted in light of a simple physical model to characterize phase histograms depending on the number of scatterers within the SAR resolution cell, on which basis we derive analytical expressions to predict height dispersion in phase histograms. The conclusion from both experimental and theoretical results is that phase histograms cannot correctly reproduce forest structure, unless the distribution of scatterers within the SAR resolution cell is characterized by a single dominant scatterer. Consistently, we conclude that the PH technique is inherently best suited for the analysis of high- or very-high resolution data, which suggests its use in the context of higher frequency SAR Missions.