## 1 Introduction  
As a global navigation satellite system (GNSS) compatible with other worldwide navigation satellite systems, the BeiDou System (BDS) was independently established and operated by China. High accuracy and high reliability positioning, navigation and timing (PNT) services are expected to be provided to all kinds of users in any time, all-weather and anywhere in the World by 2020. Until the end of 2012, the constellation of BeiDou regional navigation satellite system consists of 16 satellites including six GEOs, five IGSOs and five MEOs with 14 satellites including five GEOs, five IGSOs and four MEOs operating well. The test version of Interface Control Document (ICD) for the signal-in-space of the BeiDou system was announced on December 27, 2011. This indicated that the BeiDou system started to provide Initial Operational Service. The formal version of ICD was announced and the Full Operational Service was provided for China and its surrounding areas on December 27, 2012. This indicated that the regional navigation satellite system had been set up. Domestic and foreign enterprises are being encouraged to participate in the research and development of BeiDou system application terminals (see [[http://www.BeiDou.gov.cn](http://www.BeiDou.gov.cn)]([http://www.BeiDou.gov.cn](http://www.BeiDou.gov.cn))). Since M01, the first satellite of BeiDou-2 system, was successfully launched, there have been various research projects studying the signal structure, the precise orbit determination of BeiDou satellites and the precise positioning performances. The signals of M01 have been tracked and the structure of the pseudo-random noise (PRN) code in the E2 (B1), E5b (B2), and E6 (B3) frequency bands has been analyzed [1,2]. The signal power of BeiDou M01 satellite has been found larger than that of GPS satellites and the GIOVE-B satellite of Galileo. Based on the \"BETS\" and the PANDA software, the precise orbit determination of the BeiDou satellites and PPP by using the precise orbit and satellite clock bias products have been investigated. The RMS values of the overlap differences in the radial component reached 10 cm and the daily static PPP BeiDou solutions showed an accuracy of 2 cm in horizontal components and 7 cm in the vertical component [3]. Satellite Laser Ranging (SLR) tracking data have been used to determine the orbit of BeiDou M01 with 7 days arc length. The RMS values of the 3-D overlap error varied from 11 to 50 cm in the selected periods [4]. The performances of the static and kinematic precise relative positioning have been also analyzed. The precision of static precise relative positioning solutions reached millimeter level and the kinematic solutions reached centimeter level [5]. It should be mentioned that the positioning results above were acquired when there were just six BeiDou satellites. With the increasing number of satellites, more research is being conducted on the BeiDou system. As a result of the fewer tracking stations and the lower precision of BeiDou orbit and clock biases product, the biases in the horizontal and vertical components of static PPP reached 12 cm [6]. The German Research Center for Geosciences (GFZ)'s EPOS-RT software package obtained similar results to those of the PANDA software [7]. A BeiDou PPP based on the products from a small network of six stations also provided an accuracy of several centimeters compared to the GPS-only results [8]. To improve the satellite orbits of the BeiDou regional system, the impact of the tracking geometry, the involvement of MEOs and the effect of integer ambiguity resolution were analyzed as well [9]. Meanwhile, the capacity of the BeiDou system for tropospheric remote sensing was also assessed. The experiment showed that the derived Zenith Tropospheric Delay (ZTD) was close to GPS ZTD estimated using different software packages [10].