# FLATSIM: The ForM@Ter LArge-Scale Multi-Temporal Sentinel-1 InterferoMetry Service
## 5 Results and Products Qualification
### Result Discussion in Tibet Area  
The LOS velocity products obtained separately for 15 ascending tracks segments in East Tibet are displayed in Figure 13. A simple velocity offset is added to each map to enhance visual continuity. However, a proper assembly of the 15 velocity maps should also take into account additional ramps in azimuth and range, varying incidence angles from near range to far range, and referencing using GNSS data. Despite these strong limitations, the figure shows a very good across-track consistency of the LOS velocity features (see Figure 13). It highlights interseismic deformation in Tibet along major faults, as the Altyn Tagh fault and the Kunlun fault [22]. The deformation gradient across the Kunlun fault is particularly sharp and strong, due to continuing post-seismic deformation after the 2001 Mw7.8 Kokoxili earthquake [53; 54]. A number of other geophysical signals can be seen, especially large subsidence patches due to permafrost degradation [15; 19] south of the Kunlun fault, and smaller-scale, strong, round-shaped subsidence patterns in the Qaidam basin of probable hydrological origin. In order to assess the quality of the velocity maps, we provide additional indicators either as maps (Figure 14 or Figure 15 as averaged values per date or interferograms). The unwrapping fraction (Figure 15a) is given for each interferogram. It allows to automatically discard interferograms with the lowest unwrapped fractions (here interferogram 285 for example), or for the users to detect possible problems with specific periods of time or interferogram durations. The RMS network misclosure (Figures 14a and 15b,c) combines effects due to phase noise and to unwrapping errors. Typical values of up to 2 mm (\(\sim\)0.4  radians) are good and typical of a negligible contribution of residual unwrapping errors. The variation is then associated with temporal decorrelation. It can be seen here (Figure 15c) that the temporal decorrelation has on average a semi-seasonal behavior, with peak decorrelations in winter and in summer, larger in the southern section than in the northern one, mostly desert, region. A few acquisitions in the southern part of the study area may be affected by snow. Interferograms with large RMS misclosure (e.g., numbers 415, 430, 521 of the southern section) are most probably affected by residual unwrapping errors covering a non negligible fraction of their surface. Pixels with RMS misclosures larger than 3.5 mm (in yellow on Figure 14a) denote areas with unwrapping ambiguities too frequent to be automatically and consistently corrected by network inversion. Their origin in the present case is due to freeze-thaw cycles that create strong phase gradient and phase ambiguities across basin borders [15]. Finally, the number of inverted interferograms per pixel (Figure 14b) is also useful to map lateral variations in the quality of the velocity map. It is strongly correlated with the temporal coherence (Figure 14c). Here, sand dunes in the Tarim basin for example result in a low coherence and a low fraction of unwrapped interferograms. As including long temporal baseline interferograms strongly  
Figure 12: Sentinel-1 tracks footprint of (**a**) the Afar case study, (**b**) the Tibetan case study. reduce possible bias due to fading signals [45], we cannot exclude a bias in areas with a significantly reduced number of interferograms. Therefore, combining the three quality maps shown in Figure 14, using different thresholds, allow to mask pixels for which we have less confidence, before performing a geophysical interpretation of their displacement behavior through time. Figure 13: LOS velocity maps provided by automated FLATSIM processing in eastern Tibet over 15 separated track segments.The color scale is in rad/yr, positive away from satellite. The color scale amplitude from minimum to maximum, of 2.8 rad/yr, corresponds 12.5 mm/yr. The maps are overlying each other, from north to south and from east to west, i.e., only the segment on the south-west corner is displayed in its entirety.