## 1 Introduction  
The GNSS (global navigation satellite system) precise kinematic positioning over the polar regions of Earth has always been an important topic, as its trajectory is essential information for any observing and monitoring system via mobile platforms, for example, for airborne gravimetry [1]. Usually, the traditional relative kinematic positioning (RKP), which requires reference stations nearby, is applied in order to remove most of the common biases. An alternative approach is the precise point positioning (PPP) using precise orbit and clock products and observations of a stand-alone receiver. This system usually takes about 30 min for initialization to achieve centimeter-level positioning accuracy and can be significantly improved by the carrier phase ambiguity resolution (AR) technique [2; 3; 4]. Compared with the positioning in other regions, there are quite few International GNSS Service (IGS) stations. It is also difficult to set up a reference station because of the harsh weather and the adamant soil layers. Therefore, RKP does not always work well over the polar regions. Moreover, satellites are usually tracked at lower elevation and over a shorter continuous period, due to the GNSS constellation with inclined orbit planes. Both observations of short period and at lower elevation bring difficulties in integer ambiguity resolution on one hand, on the other hand they also degrade the accuracy in height components. Therefore, further investigation is still required for PPP and its integer ambiguity resolution applied to the polar regions, although they have been well demonstrated elsewhere. Nowadays, with the construction of global coverage of the Galileo and Beidou navigation satellite system (BDS), the multi-GNSS, including GPS, GLONASS (Russian GLObal Navigation Satellite System), Galileo and BDS, can provide a great number of satellites for positioning. That means that the poor accuracy of the vertical position due to the lower tracking elevation over Antarctica can be improved considerably. The accuracy of GPS, GLONASS and Galileo final orbit and clock products turns out to be at the same level, since their MGEX (Multi-GNSS Experiment) tracking stations are globally and evenly distributed, whereas there are only about 71 stations with Beidou observations, mainly distributed in Europe and Asia-pacific areas, and only two stations named CAS1 and DAV1 in Antarctica. Because of the insufficiently well distributed stations and a poorly developed solar radiation pressure model [5], the accuracy of the Beidou satellite orbits is relatively low compared to that of the other GNSS satellites, particularly for the BDS Geostationary Earth Orbit (GEO) satellites. The positioning and ambiguity fixing of BDS PPP will be greatly affected since it is dependent of the orbit and clock products. [PERSON] et al. [6] and [PERSON] et al. [7] illustrated that the impact of BDS GEO orbit errors is similar for all stations in a regional network as the directions of the satellite to all receivers are almost the same and thus can be assimilated into the uncalibrated phase delays (UPDs). However, the impact difference gets larger for a global network from station to station and is regarded as a main obstacle for ambiguity fixing. In order to get rid of the disadvantage of RKP and PPP, a method named precise orbit positioning (POP) [8] is applied to process the Antarctica data. This is, in principle, a network solution with fixed satellite orbits, where satellite clock offsets are estimated with a global or large regional reference network instead of a few nearby reference stations. According to [PERSON] [9], 96% of the orbit errors in the radial direction can be compensated by the satellite clock in such a network processing mode. Including more stations will improve the clock estimation, especially when the distribution of the tracking stations is rather sparse over the interesting region.