Remote Sens._**2015**, \(7\), 9431-9449; doi:10.3390/rs70709431  
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combining multiple images with one another, not only for inter-comparisons, but also to dramatically speed up applications, such as near-real-time disaster mapping. Accurate geolocation, dependent on radar system timing, permits multiple image products to be quickly combined, e.g., expediting SAR interferometry (InSAR) coregistration or layered with other data sources, such as digital elevation models (DEMs), cadastral maps, vegetation maps, forest maps, hydrological maps, _etc._ A good example of this type of layering is described in [1], whereby long time series can be quickly and automatically radiometrically terrain-corrected using a Digital Terrain Model (DTM), useful for, e.g., wet-snow monitoring. Sentinel-1A (S1A) provides a continual supply of all-weather, day-or-night imagery of the Earth's surface which can be used for time series analyses, or combined with data from other platforms or sensors. For these goals to be feasible, S1A products [2] are required to provide high and consistent geolocation accuracy, as specified in Tables 7-1 through 7-10 of [2]. During the S1A in-orbit commissioning phase, the SAR team at the Remote Sensing Laboratories, University of Zurich (UZH), Switzerland, performed mission performance monitoring for the S1A product geometric calibration and validation. Preliminary geolocation estimates were provided in [3]. S1A reached its designated reference orbit on 7 August 2014, providing the stable SAR imaging geometry required for performing highly accurate geolocation measurements. Since then, many more SAR acquisitions have been collected and analyzed. The Sentinel-1 SAR instrument operates at 5.405 GHz (C-band, corresponding to a radar wavelength of about 5.6 cm) and supports four exclusive imaging modes providing different resolution and coverage: Interferometric Wide Swath (IW), Extra Wide Swath (EW), StripMap (SM), and Wave (WV) [4]. Table 1 provides the product resolutions and sample spacings for the modes relevant to geometric calibration/validation. The IW mode is the main mode of operations for the Sentinel-1 mission, enabling the systematic monitoring of large land and coastal areas at a ground resolution of 5 m \(\times\) 20 m. The SM mode, due to its fine slant range and azimuth resolutions of between \(\sim\)2-5 m, is subject to more stringent requirements for geolocation accuracy. For this reason, this paper focuses on the calibration and validation of the geolocation accuracy for S1A SM Single Look Complex (SLC) and Ground Range Detected Fine-resolution (GRDF) image products. As described in [5], the Sentinel-1 SM mode is comprised of six beams, _i.e._, SM1-SM6, covering an incident angle range of 20\({}^{\circ}\)-43\({}^{\circ}\).