## 3 Results and Discussion
### MOT Effect on the Spectral Characteristics of Global IGS Stations  
From Figure 2, we observe that the loading displacement caused by MOT exhibits some periodic characteristics with period as about two years, in particular for island station KOKB, but the MOT induced 24-hour GPS position time series does not show obvious periodic features (see Figure 4). To better investigate the impact of MOT on the periodic motion of global IGS reference stations, we calculate the power spectrum density (PSD) of the 109 stations in both Experiment A and B through using CATS software [49]. Figure 10 gives an example of the PSD result for station BAHR and ARTU (Arti, Russia). Black and red curves in the figure represent the PSD before and after MOT correction  
Figure 9: Percentage of stations with the WRMS variation rate inside certain range. Different color indicates different WRMS variation rate in unit of %. (**Left**: N component, **middle**: E component, **right**: U component). respectively. Vertical black and green dash lines denote the harmonics of 1 _cpy_ and 1.04 _cpy_ respectively. We observe that MOT could only cause small periodic difference, although the magnitude for coastal station BAHR is slightly bigger than inland station ARTU. We find that strong annual signal exists for each component of the two stations both before and after MOT correction, which also exhibit an increase except the East component of station BAHR, in particular for the N and E components of station ARTU, which increased by 23% and 10% respectively. However, there is no regular rule to describe the characteristics of the other periodic signals caused by MOT. From Figure 10, we could also see the existence of the anomalous harmonics of 1.04 _cpy_\(\pm\) 0.008 _cpy_, which is now known as the GPS draconitic year [29], for example, the 4 th draconitic signal in the U and N components of station BAHR, the 1 st draconitic signal in the N and E components of station ARTU, no matter MOT correction is applied or not. Moreover, the 1 st and 4 th draconitic signal in the U component of BAHR decreased by about 7% and 55% respectively after considering the impact of  
Figure 10: PSD results for the U, N, and E components of station BAHR (top panels) and ARTU (bottom panels) before and after MOT correction. Vertical black and green dash lines represent harmonics of 1 _cpy_ and 1.04 _cpy_. MOT, while the 1 st draconitic signal in the E component and the 2 nd draconitic signal in the N component of station ARTU also decreased by 9% and 18%. Until now, the well-known origin of the above anomalous harmonics comes from the GPS technique error, such as long-period GPS satellite orbit modeling error, the repeating geometry of the satellite constellation with respect to the tracking stations, _etc_. [29]. Our PSD results from these two stations indicate that MOT could also be a possible origin that caused the draconitic signal. Figure 10 only represents the PSD results for single stations. Since there is no big difference in the PSD characteristics between coastal/island stations and inland stations, here we stack and filter the PSD time series of the selected 109 stations for the N, E, and U components respectively, so as to have  
Figure 11: Top: Global stacking PSD results for the filtered U, N, and E component before and after MOT correction. Bottom: PSD difference caused by MOT for each corresponding component. The vertical black and green dash lines in the figure have the same meaning as that in Figure 10.  
a global view of the PSD difference caused by MOT. For the implementation of the PSD stacking and filtering, please refer to [29] for detail. Figure 11 shows the final filtered PSD stacking results for each component before and after MOT correction (top panels), together with their differences (bottom panels). In the bottom panels of Figure 11, positive value means that the PSD amplitude is decreased after correcting MOT effects.