## 2 Materials, Methods, and Discussion  
### Sentinel-1 Archive Data  
ESA is developing the Sentinels, a new family of six missions, each based on a constellation of two satellites to fulfil revisit and coverage requirements, providing robust datasets for Copernicus Services. The Sentinel-1 constellation ensures the continuity of the ERS and ENVISAT missions and has been specifically designed to provide systematic and regular SAR observations with high temporal (six-day revisit time) and spatial (\(\sim\)250 km swath and \(\sim\)165 km azimuth width) coverage. The Terrain Observation by Progressive scans SAR (TOPSAR) of the Interferometric Wide Swath (IWS) is the main acquisition mode over land [23], in addition to Stripmap and Extra Wide Swath (EWS) products. Sentinel-1 SAR products are freely available for both research and commercial applications, thus providing the scientific community, as well as public and private companies with more opportunities to explore. Currently, Sentinel-1 products are available through the following platforms:  
* Sentinels Scientific Data Hub ([[https://scihub.copernicus.eu/](https://scihub.copernicus.eu/)]([https://scihub.copernicus.eu/](https://scihub.copernicus.eu/))) operated by ESA, the most completed and updated (within 24 hours for systematically archived data) archive where SAR data are identified according to relative orbit and slice number. In 2015, alone, more than 1000 out of the 15,243 users registered from the UK [24]. * Alaska Satellite Facility [25], as part of an agreement between the U.S. State Department and the European Commission, where products are identified from their path and frame. * Sentinel Data Access Service [26] operated by the Satellite Applications Catapult in association with the UK Space Agency where SLC IWS SAR data are identified according to the Absolute Orbit number. Starting from October 2014, when just two IW SLC images were available over Great Britain (Figure 2), and tracking the status of the archive, we found that there were a total \(\sim\)11,200 SLC Sentinel-1A images as of the end of December 2016. The analysis shows that the average number of scenes acquired per month for each acquisition geometry (ascending and descending) increased from a few tens in 2014 to over 100-200 in 2015 and up to \(\sim\)400 at the end of 2016. The scenes are almost equally distributed among the ascending and descending orbits, as opposed to the observed preponderance of ERS and ENVISAT descending frames with respect to ascending ones from 1991 to 2010 [10]. The current acquisition rate has been achieved after the completion of the commissioning phase of the second satellite of the constellation, Sentinel-1B (launched in April 2016 and orbiting Earth 180\({}^{\circ}\) apart from its twin), when further products have been made available from mid-September 2016 and the number of scenes acquired per month over the landmass has increased to \(\sim\)400. A short gap occurs between 16 June 2016 and 28 June 2016 due to the persistence of the onboard anomaly of the SAR antenna [27]. The Sentinel-1A IW coverage of Great Britain is achieved by 46 different ascending frames (from 158 to 200) and 41 different descending frames (from 392 to 428) distributed along a total of six paths in each geometry (Figure 3). Among the possible path-frame combinations, a full coverage of Great Britain can be achieved with just 15 IW frames. However, it becomes apparent that the opportunity to use always the same frames to cover the same area has been affected by some inconsistency in the Sentinel-1 acquisition as shown in the example of Figure 4. In this case, the Sentinel-1A ascending stack along  
Figure 3: Satellite SAR image availability over Great Britain: spatial distribution of Sentinel-1A frames for the (**a**) ascending and (**b**) descending modes. Along the same path a step of 5 in frame ID is required to cover the next or previous adjacent area.