## 6 Conclusions  
This paper proposes a novel and easily applied method to verify the geometric accuracy of SAR data products (L1). The method is based on the evaluation of the range delay offset and the azimuth shift derived from SAR raw data (L0 products). The geometric accuracy of ESA's Sentinel-1 satellites (S-1A and S-1B) is determined from acquisitions over the DLR calibration field using rotatable, remote controlled corner reflectors with precise GPS surveyed phase center positions. By applying the proposed method, the propagation path of each SAR pulse is completely retrieved: from transmitting the SAR pulses up to receiving the backscatter power from the point target. To estimate the range delay offset and the azimuth shift, a simple pulse compression is applied in range and a phase correlation in azimuth. As the proposed method does not perform full SAR data processing, the geometric offsets based on L1 and L0 products are completely independently derived. An additional benefit is obtained by their comparison: it allows us to verify if the differences between the geometric offsets of L0 and L1 products are artificially introduced by a processing bias or caused by geometric related offsets like the distance between SAR antenna phase center and spacecraft's center of gravity. Figure 7: Azimuth offset derived from corner reflector responses for S-1A (**a**) and S-1B (**b**) obtained using both product types: L0 products (blue) and L1 products (black). \begin{table}
\begin{tabular}{c c c} \hline  & **S-1A** & **S-1B** \\ \hline L0 products—Range offset (m) & \(-0.99\pm 0.12\) & \(-1.05\pm 0.07\) \\ L1 products—Range offset (m) & \(0.35\pm 0.12\) & \((0.18\pm 0.07)\)* \\ L0 products—Azimuth offset (m) & \(-0.11\pm 0.12\) & \(-0.14\pm 0.12\) \\ L1 products—Azimuth offset (m) & \(-2.26\pm 0.25\) & \(-2.36\pm 0.22\) \\ \hline \end{tabular}
\end{table} 
Table 3: Mean value and standard deviation (\(\mu\,\pm\,\sigma\)) for estimated range and azimuth offset using L0 data with the proposed method and L1 products for both S-1A and S-1B. Note that, for the S-1B range offset using L1 products, the outliers from Stripmap beam S1 are excluded (*). The results show that the azimuth bias of \(-2.3\) m obtained from the analysis of L1-products is reduced to about 15 cm using L0 data and the proposed method. This indicates an artificial bias introduced by the current Sentinel-1 SAR processor. The remaining range offset of \(-1.0\) m derived from the L0 data is interpreted as the offset between the phase center of the SAR antenna and the spacecraft's center of gravity. Assuming that such an offset is already considered during SAR data processing, the remaining range offset derived from the L1 products is 20 cm, except for the S-1B S1 beam that has an additional range bias of 1.6 m.  
The pixel localization accuracy has been estimated for both Sentinels by the evaluation of Stripmap mode data acquired within two different observation periods. The relative pixel localization accuracy in azimuth is 12 cm for both satellites and in range 12 cm for S-1A and 7 cm for S-1B, while, for the range direction, a similar accuracy is estimated from L1 products, and the standard deviation in the azimuth direction derived from full processed SAR images is higher, namely 25 cm for S-1A and 22 cm for S-1B. It has to be noted that the Sentinel-1 geometric requirements are already fulfilled by the L1 product results, but the current method nevertheless helps to verify and improve the geometric accuracy. The work in this paper was partially funded by the ESA Contract No. 400019795/17/I-BG. The authors want to thank [PERSON] from ESA-ESRIN for helpful discussions and comments. [PERSON] investigated the proposed method and analyzed the data; [PERSON] conceived and performed the point target's phase center estimation; [PERSON] calculated the internal delay of the SAR instrument from the calibration pulses; [PERSON], [PERSON] and [PERSON] commented on the manuscript and made useful suggestions; [PERSON] wrote the paper.