Press Coverage

www.geospatialworld.net | May-June 2020 48 www.geospatialworld.net | Mar-Apr 2020 48 Short guide to understanding spatial resolution, noise level, and radiometric resolution By Davide Castelletti & Gordon Farquharson T he number and variety of applications that exploit Synthetic Aperture Radar (SAR) data are growing. ese applications include environmental monitoring, surveillance, emergency response, infrastructure monitoring, urban planning, and food security. With the growing demand for SAR images, the community of SAR users is also transforming. Radar images are now processed by GIS users, soware developers, and computer vision and machine learning engineers, and are increasingly interpreted by non-radar specialists. In this article, we describe key characteristics of SAR imagery, with the goal of providing a short guide to understanding SAR products. We pres- ent the aspects of spatial resolution that are particular to SAR , and cover concepts such as noise level and radiometric resolution that are related to radar design and image formation. We will see that resolution is not the only parameter one should consider when evaluating SAR image quality, but instead that a set of different parameters should be assessed collectively to select the best data for each specific application. SAR acquisition geometry In contrast to optical imagers, synthetic aperture radar systems only acquire imagery from the side of the scene (Figure 1). 1 A SAR image is formed from data collected by a coherent radar that transmits pulses of radio frequency energy toward the ground and measures the strength of the reflected signal as a function of distance ("time of flight") from the radar. In addition, the platform carries the SAR antenna along a track, and by this action, the ground is "scanned" in two dimensions. In the ranging ("range") dimension, objects are placed according to their distance from the radar. e second dimension is the "along-track" (or "cross-range" or "azimuth") dimension. In this dimension, the ground is scanned by the beam moving across the ground at a rate equal to the speed of the platform, and objects are placed in this dimension according to their position along the track. An image is built up from the reflected signals in both dimen- sions. As a result of this acquisition methodology, the resolution of a SAR image has two components: a range resolution and an azimuth resolution. Spatial resolution: range and azimuth resolution 1 Note that for simplicity, we will restrict this discussion to side-looking SAR systems, and not deal with squinted-SAR systems in this post. Figure 1: (a) SAR acquisition geometry and (b) mapping om slant plane to ground plane. Figure 1: (a) SAR acquisition geometry and (b) mapping om slant plane to ground plane. SAR IMAGERY AND QUALITY METRICS WIDE ANGLE

• Cover