## 2 Test Site and Dataset
### Dataset
The RiskSAR-X system was placed on a base at approximately \(30\) centimeters above the ground to elevate the rail and reduce the impact of the nearby vegetation. Figure 2 shows a Google Earth image indicating the TerraSAR-X coverage, the area of interest selected for the orbital PSI processing, the RiskSAR-X location and the area illuminated by the GB-SAR sensor. Figure 3a shows a panoramic view of El Forn de Canillo seen from the RiskSAR-X point of view, which is represented in Figure 3b. Regarding the RiskSAR-X characteristics, it is based on the usage of a digital direct synthesizer (DDS) chipset that generates a high rate linear frequency modulated continuous wave (LFM-CW) signal. With this architecture philosophy, the RiskSAR-X sensor is able to perform fast scans, minimizing the effect of troposphere disturbances and reducing the possible amplitude and phase distortions due to target instabilities during the scanning time. Each scan takes approximately from \(1\) to \(2.5\) min in single and  
Figure 3: **(a)** Panoramic view of El Forn de Canillo from **(b)** the RiskSAR-X point of view. Figure 2: Coverage of the TerraSAR-X and RiskSAR-X sensors over the area of El Forn de Canillo. The solid rectangle indicates the TerraSAR-X coverage: \(10\) km in range per \(5\) km in azimuth with a heading of \(9.8^{\circ}\) with respect to the north. Upper left (\(42.5896^{\circ}\) latitude, \(1.5450^{\circ}\) longitude), upper right (\(42.5746^{\circ}\) latitude, \(1.6649^{\circ}\) longitude), bottom left (\(42.5452^{\circ}\) latitude, \(1.5348^{\circ}\) longitude) and bottom right (\(42.5302^{\circ}\) latitude, \(1.6548^{\circ}\) longitude). The dashed rectangle indicates the area of interest selected for the orbital PSI processing: \(4.5\) km in range per \(2.5\) km in azimuth. The RiskSAR-X location is indicated with the white dot (\(42.5653^{\circ}\) latitude, \(1.5921^{\circ}\) longitude). The area illuminated by the GB-SAR sensor is indicated with the solid line within the dashed rectangle: \(500\) m in height, \(1600\) m in range and \(1000\) m in width, with a heading of \(32^{\circ}\) with respect to the north. fully-polarimetric modes, respectively. The range resolution of the RiskSAR-X sensor is \(1.25\) m, and as in all GB-SAR sensors, the azimuth or cross-range resolution (the term which is more often employed in GB-SAR geometry) is not constant, ranging from \(0.75\) m at a near range up to roughly \(5\) m at the far range of \(1500\) m. Table 1 presents the sensor setting parameters used during the measurements. The employment of TerraSAR-X images in sliding spotlight is commented on in the following. As stated above, the new family of X-band sensors are demonstrated to improve the monitoring capabilities over man-made structures, such as buildings, bridges, railways and highways, but also over natural corner reflectors, such as rocky areas or bare surfaces. This is directly related to their high spatial resolution, but also to their shorter revisit time, allowing a significant improvement in the temporal and spatial sampling for geo-hazard assessment. The use of the spotlight mode [30] offers a resolution improvement in the azimuth direction (up to \(1\) m) with respect to the conventional strip-map mode by sweeping the azimuth beam during image acquisition. As will be shown, this resolution improvement will reveal major details, allowing maximization of the detection of persistent scatters. The main characteristics of the TerraSAR-X system in spotlight mode are reported in Table 1.  
Since it is well-known that the landslide of El Forn de Canillo is quite stable nowadays, with only some residual movement of the order of \(1\)-\(3\) cm per year, continuous monitoring was considered unfruitful. For this reason, a total of \(10\) acquisitions collected at the X-band, with a temporal baseline of approximately one month, were finally carried out for both sensors; see Table 2. Notice that the temporal span during the winter is slightly greater.