“…Range cell realignment is considered to be routine and is based upon, for instance, the correlation method (see Chen and Andrews [1]) or the minimum-entropy method (see Wang and Bao [2]). Phase autofocus is more stringent in its requirements and many nonparametric methods have been proposed, most of which track the phase history of an isolated dominant scatterer (prominent point processing (PPP), see Steinberg [3]) or the centroid of multiple well-isolated scatterers (multiple scatterer algorithm (MSA), see Carrara et al [4], Haywood and Evans [5], Wu et al [6], Attia [7]). The phase-gradient algorithm (PGA, see Wahl et al [8]) is another popular nonparametric technique, which iteratively estimates the residual phase by integrating over range an estimate of its derivative (gradient).…”