###### Abstract  
The geometric accuracy of synthetic aperture radar (SAR) data is usually derived from level-1 products using accurately surveyed corner reflector positions. This paper introduces a novel approach that derives the range delay and azimuth shift from acquired SAR raw data (level-0 products). Therefore, the propagation path is completely retrieved from SAR pulse transmission up to the reception of the point target's backscatter. The procedure includes simple pulse compression in range and azimuth instead of full SAR data processing. By applying this method, the geometric accuracy of ESA's Sentinel-1 SAR satellites (Sentinel-1A and Sentinel-1B) is derived for each satellite overpass by using corner reflectors with precisely surveyed GPS positions. The results show that the azimuth bias of about 2 m found in level-1 products for Stripmap acquisitions is reduced to about 15 cm. This indicates an artificial bias arising from operational Sentinel-1 SAR data processing. The remaining range bias of about 1.0 m, observed in L0-products, is interpreted as the offset between the SAR antenna phase center and the spacecraft's center of gravity. The relative pixel localization accuracy derived with the proposed method is about 12 cm for the evaluated acquisitions. Compared to the full processed level-1 SAR data products, this accuracy is similar in the range direction, but, for the azimuth direction, it is improved by about 50% with the proposed method. synthetic aperture radar; Sentinel-1; geometric accuracy; point target evaluation 11.15.  
that the two satellite constellation offers a 6-day repeat cycle over a given observation area. Both SAR instruments operate at C-band (with a center frequency of 5.405 GHz) and carry a right-looking active phased array antenna. Dual polarization operation (HH+HV, VV+VH) is realized by one transmit chain (switchable to H or V) and two parallel receive chains, one for H and one for V polarization. Sentinel-1 provides different operation modes: Stripmap, Interferometric Wide swath (IW), Extra-wide swath (EW) and Wave mode. The six available Stripmap beams have a swath width of 80 km each with a spatial image resolution of 5 m by 5 m. For IW and EW acquisitions, the instrument is operated in the Terrain Observation with Progressive Scan (TOPS) mode [3] by steering the antenna electronically from backwards to forwards for each burst. Compared to Stripmap, this technique enables the observation of a larger swath width of 250 km for IW mode and of 400 km for EW mode; the spatial resolution is accordingly reduced to 5 m by 20 m and 20 m by 40 m, respectively [4; 5]. The geolocation accuracy of space-borne SAR instruments, also known as pixel localization accuracy, has to be determined using the range-Doppler equations [6] and verified by analyzing SAR images [7; 8; 9]. To achieve and verify a high spatial accuracy of below one meter, the related geometric conditions have to be known very accurately, e.g., the spacecraft orbit, the SAR antenna geometry related to the spacecraft's center of gravity and reference points precisely surveyed on the ground. Moreover, SAR data processing has to consider all relevant timing issues that are relevant for focusing the SAR image in range and azimuth [10], which could likewise arise from the instrument internal delay or from the used interpolation, approximation and signal filtering technique. Furthermore, the wave propagation is affected by the atmosphere; considerable delays are induced by the troposphere and ionosphere [11] and need to be considered accordingly. In particular for TerraSAR-X, a geospatial accuracy of SAR images in the centimeter domain, below a tenth of the resolution cell, was reported [7; 8; 12; 13]. The pixel localization accuracy of both S-1A and S-1B was analyzed during their respective commissioning and routine operation phases within several studies [14; 15; 16; 17] by evaluating the SAR images (L1 products) processed with Sentinel-1's operational SAR Instrument Processing Facility (IPF).