# Performance of BDS-3: Measurement Quality Analysis, Precise Orbit and Clock Determination
## 1 Introduction  
Since the late 20 th century, China has been working on the development of its own satellite navigation system known as BeiDou Navigation Satellite System (BDS). The development has three phases: the demonstration system (BDS-1), the regional system (BDS-2), and the global system (BDS-3) [1]. BDS-1 was declared to be operational at the mid of 2003 with two geostationary satellites and a backup satellite [2]. As a multi-function system, BDS-1 provided users in China with both positioning and short message communication services. BDS-2 has been providing official positioning, navigation and timing (PNT) services over the Asia-Pacific area since 27 December 2012 [3]. The constellation of BDS-2 consists of 14 satellites, five geostationary orbit (GEO), five inclined geostationary orbit (IGSO), and four medium Earth orbit (MEO), transmitting triple-frequency signals centered at B1 (1561.098 MHz), B2 (1207.14 MHz), and B3 (1268.52 MHz). The BDS-3 is expected to complete the constellation deployment with the launching of 35 satellites (five GEOs, three IGSOs, and 27 MEOs) by 2020, providing greatly improved services to global users [1; 2; 3; 4]. With the advent of BDS-2, many studies have been carried out to investigate various aspects of BDS-2, including signal characteristics [5], multipath effects [6], precise orbit determination (POD) [7; 8; 9], performance of satellite clocks [10; 11; 12], relative positioning [13], and precise point positioning (PPP) [14], etc. [PERSON], et al. [7] conducted initial assessment of the BDS-2 basedon observations of six reference stations, and the 3D root mean square (RMS) values of orbit overlap comparison were 1-10 m. The BDS-2 satellite orbits determined by [PERSON], et al. [15] have a radial orbit precision of about 10 cm. [PERSON], et al. [16] also reported radial orbit overlap RMS values of about 1 dm. [PERSON], et al. [8] carried out POD of BDS-2 using three day arcs. The 3D overlap precision reaches about 1.8 m for GEO and 0.3 m for IGSO and MEO, and radial overlap precision is better than 0.1 m for all satellites. In addition, the performance of the indigenous clocks of the BDS-2 has been evaluated [10; 11], showing Allan deviations roughly 2-3 times larger than the GPS IIF rubidium atomic frequency standard (RAFS) and the Galileo passive hydrogen maser (PHM). When it comes to the BDS signal characteristics, some studies based on the analysis of multipath combination revealed that the systematic code biases, which are absent for other global navigation satellite system (GNSS), are commonly found in BDS-2 IGSO and MEO satellites [17; 18; 19]. These biases, called satellite-induced code biases [19; 20], are elevation-dependent and can vary more than 1 m from horizon to zenith, which would affect BDS precise applications with code measurements, e.g., ambiguity fixing [20] and single-frequency PPP [19], based on the ionosphere-free code-carrier combination. On 30 March 2015, China launched an experimental BDS-3 satellite into space, named M1S, for its original global navigation and positioning network using a Long March-3C carrier rocket at the Xichang Satellite Launch Center [21]. The launch marked the beginning of expanding the regional system to global coverage. Up to February 2016, five experimental BDS-3 satellites, including two IGSOs (C31, C32) and three MEOs (C33, C34, C35), have completed deployment (Table 1). Currently, four satellites (C31-C34) are in operation and C35 is in a test orbit. For brevity, the BDS-3 henceforth refers to these experimental BDS-3 satellites in this paper. The task of these satellites is to test the new types of the navigation signals and inter-satellite links, as well as provide services as part of the existing BDS-2 constellation. In addition, all BDS-3 satellites are equipped with new RAFSs and PHMs, which build exclusively on Chinese technology [21].