# GPS Time Series Analysis from Aboa the Finnish Antarctic Research Station
## 3 Results  
### PPP-Derived Coordinate Time Series  
This section reports on the quality of the PPP-derived coordinate time series at Aboa in terms of internal precision, accuracy and postfit observation residuals. Figure 7 illustrates the topocentric coordinate time series at Aboa from the RTKLIB and GIPSY processing, respectively. The origin of the topocentric system is set to the mean coordinates over the entire period. Whereas the GIPSY solution shows no noticeable outlier, the RTKLIB solution exhibits a few obvious outliers. In addition, the RTKLIB solution appears somehow noisier. Nevertheless, both time series appear to follow a similar trend. Figure 8 depicts the standard deviation of the estimated position as given by the processing engine. The median standard deviation along all the directions is below 5 mm. In the horizontal direction, the median precision is around 2 mm for RTKLIB and less than 1 mm for the GIPSY solution, respectively. In the vertical direction, both solutions exhibit about 4 mm median standard deviation. The GIPSY solution appears a little more robust for the entire interval period, whereas the RTKLIB solution exhibits larger standard deviations in the beginning of the time series. A significant improvement is visible in 2004. However, this may not be attributed to the processing engine but perhaps to the data quality or the difference in the precise products used by the two solutions. One may suspect a weaker performance of the products, perhaps in the Antarctic region, before April 2004. After that, the standard deviations appear rather consistent despite episodic mild jumps for a few days. Although these small differences, we can conclude that both time series are robust. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Service** & **Reference** & **Link** \\ \hline International Mass Loading Service & [69] & [[http://massloading.net/](http://massloading.net/)]([http://massloading.net/](http://massloading.net/)) \\ EOST loading service & [70] & [[http://loading.u-strasbg.fr/index.php](http://loading.u-strasbg.fr/index.php)]([http://loading.u-strasbg.fr/index.php](http://loading.u-strasbg.fr/index.php)) \\ GFZ service & [71] & [[https://isdc.gfz-potsdam.de/esmdata/loading/](https://isdc.gfz-potsdam.de/esmdata/loading/)]([https://isdc.gfz-potsdam.de/esmdata/loading/](https://isdc.gfz-potsdam.de/esmdata/loading/)) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Environmental loading services. Figure 9 illustrates the accuracy of the RTKLIB solution with respect to the GIPSY solution as error distribution histograms of the local offsets. The accuracy validation requires that both series are expressed in the same frame. Thus, the RTKLIB coordinates were first transformed from ITRF2008 to ITRF2014. Then, the differences in Cartesian coordinates were calculated. These differences were converted to local NEU offsets for a more meaningful interpretation. Next, the outliers and trend were removed using a reiterative linear regression algorithm combined with the IQR (inter quartile range) robust statistic and a threshold of 2.2 for outlier removal. This threshold is roughly equivalent to the 3-sigma and it works well in our experiences. This process was carried out individually for each axis. The detrended offsets show peak-to-peak variations of less than \(\pm 8\) mm for the North and East directions. The variations on the Up direction are twice as large and account for \(\pm 18\) mm. Overall, 2.6 mm rms (root mean square) is obtained along the horizontal directions, whereas 6.0 mm rms is computed for the vertical direction. Table 4 summarises the conventional and robust statistics. These statistics confirm an excellent agreement between the two solutions. Figure 8: Internal precision in terms of standard deviation of the two solutions: RTKLIB (**left**) and GIPSY (**right**) [unit: mm].