## 1 Introduction  
China has been developing its own independent satellite navigation system for decades. Now the COMPASS system, also known as BeiDou, is emerging and gaining more and more attention in the worldwide GNSS communities. The system is designed as a global system, but with special concern for service in China and its surroundings [1]. Its development is scheduled into three phases: the demonstration system, the regional system, and the global system. Presumably, such a special schedule \"from regional to global\" will result in the utilization of Geostationary Earth Orbit (GEO) and Inclined Geosynchronous Earth Orbit (IGSO) satellites as a new feature of the BeiDou system. The demonstrational system was established as the BeiDou-1, with three GEO satellites providing positioning and short message communication services. The regional system comprises five GEOs, five IGSOs and four MEO satellites and provides positioning services for users in China and its surroundings. Afterwards, the whole constellation of the global system, which will consist of five GEOs, three IGSOs and 27 MEO satellites, is expected to be completed by the end of 2020 [1; 2; 3]. Up to now, the constellation of the regional system is completed and its operational service was officially announced by the BeiDou authorities at the end of 2012. Due to the similar signal structure and analogous frequencies of BeiDou with respect to that of the American GPS and the European Galileo systems, BeiDou-capable multi-GNSS receivers were already developed by US and European manufacturers even before the Interface Control Document (ICD) was publicly disclosed. This enabled a number of investigations being carried out since the first experimental satellite M1 (C30) was launched in 2007. These research projects addressed the aspects of the signal decoding method [4], receiver hardware and software analysis [5], satellite visibility and dilution of position precision [6], precise relative positioning, and measurement quality analysis [3]. Precise orbit determination (POD) and precise clock determination (PCD) are essential functions of any global satellite navigation system. Their performance in terms of accuracy and time latency decides the capacity of the system services to some extent. Hence, POD and PCD of BeiDou are also hot topics for GNSS scientists as well. In general, a long orbit arc is needed for BeiDou POD using its own phase and range observations in order to obtain a stable solution because of the special constellation and the corresponding regional tracking network. The detailed dynamic and observation models are almost the same as for the GPS system except the phase center corrections of receivers and satellites are unknown, and satellite attitude control mechanics is not yet clear. There are mainly two different strategies for POD data processing: (1) simultaneous observations from other systems or their derived products are strongly involved [2; 7; 8; 9], and (2) only BeiDou data are employed [10; 11]. In the former one, data from the other system, typically GPS, put very strong constraints on receiver clocks and tropospheric delays besides station coordinates, whereas the latter one is able to demonstrate the capacity of BeiDou as an independent navigation system. Using the first strategy [PERSON] _et al._[9] presented POD results with an accuracy of a few meters for GEOs and 10 to 20 cm for IGSO satellites with data from an Asian-Pacific regional tracking network comprising six stations out of the Chinese territory. In their study, the impacts of both data arc length and parameterization of radiation pressure force model are also investigated. As most of the scientists pay more attention on the performance of BeiDou system alone as an independent system, POD and PCD are carried out using only BeiDou data from a regional tracking network consisting of about twelve  
Among the 16 satellites, G2 is drifting unstably and unusable, and M1 was for signal testing and validation only and is no longer used because of its clock problem [13].