## 2 Test Site and Dataset
### Dataset  
In light of all of this evidence, the use of both orbital and ground-based PSI techniques were planned in 2011. The reasons were two-fold. On the one hand, most of the geotechnical devices installed over the landslide surface did not work properly, providing unreliable measures, thus hindering the characterization of the landslide extension and dynamics. On the other hand, conventional geotechnical field measurements present lower densities and worse coverage compared with SAR techniques, which is a key factor in landslide characterization. In this framework, the remote sensing laboratory (RSLab), in collaboration with the Department of Geotechnical Engineering and Geosciences of the Universitat Politecnica de Catalunya (UPC), carried out a one-year monitoring work, from October 2010, to November 2011, employing SAR images acquired with the TerraSAR-X space-borne sensor in sliding spotlight mode and with the UPC's GB-SAR system, referred to as RiskSAR-X. The RiskSAR-X sensor focused on the urbanized area of the landslide, located at the lower part of the sliding mass where the geotechnical devices were installed, while the sliding spotlight TerraSAR-X images covered the whole landslide extension. Some important decisions, such as the orbit pass or the GB-SAR location, need to be further commented on at this point. These aspects are especially important in landslide monitoring applications that typically occur in steep topography scenarios, in which the acquisition geometry plays an important role. Minimizing the geometrical distortion effects of SAR imaging and maximizing the detection of the real ground displacement are key factors in order to improve the performance of PSI techniques. The former influences the area illuminated by the SAR sensor and, consequently, the quantity of useful information collected by it. The latter is related to the fact that SAR systems are only able to detect displacements in the line-of-sight (LOS) direction. Three geometric distortions are present in SAR imaging regardless of the platform nature: the foreshortening, the layover and the shadowing [4]. For the current polar-orbiting SAR orbital sensors, the look direction is either east or west, for the ascending or descending pass, respectively. For this reason, SAR sensors only are able to detect movements along slopes facing either east or west and are almost insensitive to movements in the north or south directions. Since the landslide of El Forn de Canillo is facing west, the descending pass was finally selected to avoid foreshortening and layover phenomena in the illuminated area. Moreover, it must be taken into account that SAR systems only have sensitivity to detect displacements in the LOS direction. For this reason, detected displacements are rarely the real ones, but a projection of them. In landslide applications, the more realistic assumption regarding the displacement direction consists of considering that it is produced along the steepest gradient of the slope. Again, the descending pass resulted in being more suited for this scenario, since the projection factor between the real displacement and the LOS directions was lower compared with the one provided by the ascending mode. Unlike space-borne SAR sensors, which are constrained by the orbit pass, GB-SAR sensors allow fitting the sensor location and illumination angle to the specific characteristics of the area of interest. In order to minimize the foreshortening and to maximize the detection of the ground displacements considering that the slide moves along the steepest gradient of the terrain slope, the RiskSAR-X sensor was finally located at the foot of the landslide, \(100\) m away from the slope. Furthermore, this emplacement also minimized the shaded areas in the SAR images collected. In order to ensure a millimetric repositioning of the instrument to avoid a later co-registration of the data, the system was mounted over a cement base reinforced with a lightweight metallic frame.