# Analysis of GNSS Displacements in Europe and Their Comparison with Hydrological Loading Models
## 4 Discussion
### Common Mode Estimation in GNSS  
Since Europe has had weak tectonic activity and a stable climate over the years without great meteorological events such as El Nino or ice melting, we expected a quite low interannual signal. The PCA decomposition of GNSS in Figure 10a confirms this hypothesis since the PCs seem to be dominated by noise. Loading models have a much lower noise level than the GNSS observations and exhibit significant differences due to different estimates of continental water storage variations. By analysing the second and third modes together in Figure 10, we can find similarities between the EOFs and PCs of (i), (f), (g), and (l), on the one hand, and (j), (k), and (h), on the other hand. However, considering the discrepancies between the models themselves, it seems inadequate to indicate the superiority of any model over another. The choice of one model rather than another in the comparison with GNSS should then be particularly justified, especially if the interpretation is only based on PCA results. The difference in the total variance fraction of the first mode between GNSS and the loading models is most likely due to the difference in noise content (Figure 11). We qualitatively compared our PCA results with those of [44] who performed PCA of the UNR/NGL (University of Nevada Reno/Nevada Geodetic Laboratory) GNSS solutions [82] over the same region and the same time span as in our study and removed the seasonal signal with the same method as the one presented in Section 3.4. We note that the second mode of MG3 GNSS (Figure 10e) does not appear in [44], and we can speculate about the origin of such a uniform signal over Great Britain associated with this singular time signature. This mode could be associated with the correlated noise of the MG3 time series. As this noise component could differ from one GNSS analysis centre to another, it could be very different for NGL products. Moreover, we would like to emphasise the importance of the choice of the GNSS network for performing PCA [40]. The choice of a homogeneous network is wise in order to equally distribute the signal across the entire region. However, the stations selected for the PCA also need to meet a completeness criterion during a time interval, which, most of the time, results in an inhomogeneous network. There are then two options. The first is to keep this inhomogeneous network unchanged, knowing that PCA could over-represent the regions with a denser station distribution. This could be the reason why the modes that had the largest variance fraction in our study corresponded to an important signal over the most dense regions (north of Spain, Great Britain, and France). The second option is to select specific stations in order to produce a homogeneous network, asking for subjective criterion selection. For example, what would be the criterion for choosing between two stations, both reaching the completeness criterion, being 50 km away but showing different time series? The choice of particular stations for producing the homogeneous network should thus also impact the PCA results. In conclusion, as the choice of the network has such an impact on the PCA results, and as the GNSS noise could be specific to each GNSS solution, the discrepancies or similarities of the interannual signal given by PCA between models and GNSSs should then be considered and interpreted very carefully. **Author Contributions:** This study was designed by all the authors. [PERSON] and [PERSON] were responsible for the GNSS MG3 products and GINS software. [PERSON]-[PERSON] provided the loading models computed from hydrology models and GRACE. [PERSON] computed the GNSS solutions. [PERSON]-[PERSON] ran the CATS treatments. [PERSON] and [PERSON] analysed the results. [PERSON] wrote the preliminary draft, and [PERSON] and [PERSON] created the figures. All the authors contributed to the original draft preparation. All authors have read and agreed to the published version of the manuscript.