Performance of BDS B1 frequency standard point positioning during the main phase of different classes of geomagnetic storms in China and its surrounding area

. Geomagnetic storms are one of the space weather events. The radio signals transmitted by modern navigation systems suffer from the effects of storms which can degrade the performance of the whole system. In this study, the performance of BeiDou Navigation Satellite System (BDS) B1 frequency standard point positioning in China and its surrounding area during different classes of storms is investigated for the ﬁrst time. The analysis of the results revealed that BDS B1 frequency standard point positioning accuracy was deteriorated during the storms. The probability of the extrema in the statistics of positioning 5 errors during strong storms is the largest, followed by moderate and weak storms. The positioning accuracy for storms of a similar class is found not to be at the same level. The root mean square error (RMSE) in position for the different classes of storms could be at least tens of centimeters in the East, North and Up directions.

The basic strategy for selecting storms is that the Dst should be as minimum as possible and the duration of each storm 55 should be more than 12 hours. To ensure that each storm was independent and not influenced by another storm, a condition was applied that the Dst index for ten days before and after the main phase day must be greater than the minimum value for each individual class of storms. Finally, five cases were chosen for each class of storms from 2015 to 2018(Xue et al., 2020. The principal characteristic of a geomagnetic storm is its main phase (Loewe and Prölss, 1997). The main phase of storms including the related Dst values, the start and end epoch, and the duration is presented in Table 2 (see (Xue et al., 2020)). TYPE interval is 30s. The related information, such as geodetic coordinates, receiver and antenna version, is shown in Table 3. The last two columns show the dates when the hardware like receiver or antenna was changed or updated.
The data were processed in the kinematic mode of standard point positioning (SPP) using BDS single frequency pseudorange observations. Considering the dispersive nature of the ionosphere, only B1 pseudorange was used here. The pseudorange observation equation is illustrated as follows.
where, B 1 is BDS B1 pseudorange observation, ρ is the geometric range, dt r is the receiver clock error, dt s is the satellite clock error, T is the tropospheric delay, I 1 is the ionospheric delay, db r1 is the receiver differential code biase (DCB), db s1 is the satellite DCB, ε is the noise error.
A conventional option was set for the standard point positioning program. The satellite orbit and clock were computed from 75 IGS navigation data. The tropospheric delays were derived using the Saastamoinen model. The ionospheric delays were calculated by the broadcasted BDS navigation ionospheric model. Time group delays in the broadcast ephemeris were extracted, converted and utilized to compute the satellite DCB. For each epoch, station coordinates and its receiver clock error were Wherein, <> is the average of the variable, P OS ref,i is the precise solutions in SINEX files, P OS est,i is the solutions obtained in this processing, n is the total number of samples.  The reason for this could be attributed to the receiver issues under the storm. In addition, it can be seen that the change in PDOP values vary and most values were greater than 5 during the whole storm period, thus suggesting that the influence of the storm on PDOP is strong.  Table 3). There were also changes in the positioning errors along the ENU directions during the recovery phase.    MIN was up to -93.21 meters in the N direction of HKWS. The corresponding BIAS and RMSE were also large. It is supposed that this irregular behaviour might be related to the fact that the ionospheric activity at the low latitudes is more intense than  effect. The issues on the specific receiver (LEICA version, see Table 3)  Dst during that period. These imply that the storm activity became complicated during this period. After removing these three stations, the final statistics are illustrated in

Conclusions
In this study, the performance of BDS B1 frequency SPP during the main phase of different classes of storms in China and its surrounding area was investigated. From the results, it was observed that the positioning accuracy is affected to different levels during the storms. Some relevant conclusions can be drawn from the analyses. Firstly, the probability of the extrema in 170 the statistics of positioning errors during strong storms is greatest, followed by moderate and weak storms. Secondly, during the same class of storms the positioning accuracy may vary. Thirdly, the positioning accuracy may be influenced even in the recovery phase of storms.
The findings in this study will contribute to the prediction of BDS positioning accuracy under different strengths of geomagnetic storms. Besides, the influence of storms could be comparable to other GNSS systems like GPS. Thus, the findings 175 could also be beneficial to those systems. However, since the study period was in the descending phase of solar cycle 24, the influence of space weather events on the ionospheric activity was not intense. Therefore, the effects on the positioning accuracy might not be entirely apparent. The study needs to be extended with the arrival of solar cycle 25 and with the addition of more