Estimation of Synthetic Aperture Resolution by Measuring Point Scatterer Responses

The actual image resolution achieved by synthetic aperture systems often differs from the theoretical performance, and it is advantageous to automatically measure the realized resolution of these image products to monitor their quality. This article describes a novel image processing algorithm for measuring resolution based on the detection of prominent point scatterers. The algorithm proceeds in the following three main steps. 1) Identify candidate point scatterers via connected-component labeling and dual thresholding. 2) Prune the candidate scatterers and measure the size of the accepted detections by fitting an ellipse to the half-power contour traced in an oversampled image snippet. 3) Calculate the mode of the ellipse semiaxes lengths from a collection of scatterers to represent the local resolution estimate. The fitting of an ellipse to the half-power contour is a new contribution to resolution estimation that facilitates both detection and measurement, and it provides estimates of resolution in any direction, instead of only the range and along-track directions, which is important for images with excessive sensor motion. The accuracy and robustness of the algorithm are tested by applying it to simulated imagery with a series of parameter sweeps. Finally, the algorithm is applied to collected synthetic aperture sonar data and shown to be insensitive to seafloor textures.