## 4 Results
In our case, such reduction determines a reduction of computed velocities in the range between 8.7% and 61.6% (see Table 3). Specifically, Figure 4 shows how much the real deformation along the landslide is variable for a fixed negatively oriented LOS vector \(n^{\prime}_{\mathrm{LOS}}\) (see Equation (3)). With a (X, Y, Z) reference increasing positively toward west, north and up directions, it illustrates the differences in orientation (e.g., for CR58) and magnitude (e.g., for CR53) of UG oriented negatively in the x-plan due to its azimuth. This representation visually highlights the extent to which the GPS-LOS displacements, represented by a \(n^{\prime}_{\mathrm{LOS}}\) vector, differs from the upstream (e.g., accumulation) or downstream (e.g., transit regime or subsidence) displacement representing the real landslide kinematics. Figure 4: _Cont._  
Regarding mean velocities recorded over the period April 2014 to July 2015, the differences between PS-MTI and GPS-LOS and GPS-3D are presented in Table 3. In general, similar movement rates were assessed by both GPS-LOS and PS-MTI. In detail, it results that PS-MTI techniques has been able to correctly retrieve up to the 93% of the mean velocities recorded by GPS-LOS which, however, corresponds to only the 46% of the real GPS-3D mean velocity module. For CR58, the GPS-LOS and PS-MTI mean velocity are extremely low with respect to the real GPS-3D mean velocity module. In this case, only 8.7% of the real GPS-3D mean velocity module can be retrieved by PS-MTI measurements as CR58 displacement direction is almost perpendicular to the LOS projection on the ground (see Figure 4). Better performances have been obtained for CR49 (61% of the GPS-3D mean velocity module) since it is moving almost parallel the LOS projection on the ground. In terms of cumulative displacements, the time series obtained by PS-MTI and GPS-LOS are presented in Figure 5. It can be observed that in the accumulation zone (CR4, CR6, CR8 and CR11) the cumulative PS-MTI displacement ranges from 71 mm to 250 mm while the GPS-LOS displacement ranges from 66 mm to almost 280 mm. In the track zone, where PS-MTI time series are available only for CR13 and CR53, cumulative displacements range between 400 mm and 1300 mm for both the PS-MTI and the GPS-LOS. In the source zone (CR23, CR25, CR28, CR49, CR57, and CR58), the PS-MTI results in movements of 28 mm to 380 mm, which are similar to the GPS-LOS displacement of 32 mm to 430 mm (Figure 5). \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Zone**} & \multirow{2}{*}{**CR\#**} & \multirow{2}{*}{**CC**} & \multirow{2}{*}{\(\mu\)/\(\sigma\)} & \multicolumn{4}{c}{**Mean Velocity**} & \multicolumn{2}{c}{**Velocity Comparison**} \\ \cline{4-9}  & & & & **PS-MTI** & **GPS-LOS** & **GPS-3D** & **\% VELOS** & **\% VEL3D** \\  & & & & **(cm/yr)** & **(cm/yr)** & **(cm/yr)** & **(\%)** & **(\%)** & **(\%)** \\ \hline \multirow{4}{*}{Accumulation zone} & CR4 & 0.639 & 13.78 & 8.5 & 9.1 & 19.8 & 92.5 & 42.8 \\  & CR6 & 0.619 & 8.78 & 5.6 & 6.1 & 24.6  
The comparison between cumulative displacements obtained by PS-MTI and GPS-LOS using Equations (6) and (7) resulted in the boxplots of Figure 6. In terms of relative offsets, the cumulative displacements obtained by PS-MTI are evidently underestimated with respect to the GPS-LOS validation dataset. The median values of the relative offset ranges from \(-36\%\) to \(-1\%\). Higher values of offset are obtained for CR23, CR57, CR58. While for CR23 and CR57 there is a clear difference in both time-series, for CR58 the high relative offset value can be ascribed to the oscillations between PS evaluations and GPS measurements (as for CR4 and CR6) as well as to the magnitude of displacements, which is very small compared to all other CRs. In terms of absolute offsets, the distribution is close to the GPS positioning mean error (shaded area of Figure 6b) for all the points with low cumulative displacements (CR4, CR6, CR8, CR13 and CR58).