The Radiometric Measurement Quantity for SAR Images

A synthetic aperture radar (SAR) system measures among other quantities the terrain radar reflectivity. After image calibration, the pixel intensities are commonly expressed in terms of radar cross sections (RCSs) (for point targets) or as backscatter coefficients (for distributed targets), which are directly related. This paper argues that pixel intensities are not generally proportional to RCS or derived physical quantities. This paper further proposes to replace the inaccurate term RCS by equivalent RCS as the radiometric measurement quantity for SAR images. The measurement procedure as such remains unchanged. Two problems are seen with using RCS as the radiometric measurement quantity for point targets. First, the RCS is frequency and/or angle dependent for many targets, whereas a SAR system operates not at a single set but through a range of frequencies and aspect angles. This begs the question of which RCS within this 2-D range should actually be taken as the measurement result. Second, the pixel intensities seen in SAR images are the output of a filtering process, which is affected by the complex transfer function of the point target. RCS, on the other hand, does not depend on phase. The equivalent RCS expresses the reflectivity in terms of the RCS of an equivalent conducting sphere which would result in the same pixel intensity as the one observed if the sphere were to replace the actual target in the scene. This distinction is crucial in defining the measurand for radiometric SAR measurements in principal, and hence, it is important for radiometric SAR calibration. It is furthermore of particular practical importance for current and emerging wideband, high-resolution, and high-accuracy SAR systems.