# _remote sensing_
## 1 Introduction  
The BeiDou Navigation Satellite System (BDS) is one of the four global navigation satellite systems (GNSSs). The first-generation BeiDou system (the BeiDou Navigation Satellite Experimental System, BDS-1) was first used in 2000 and offered limited coverage and navigation services mainly for users in China and neighboring regions. The second generation of the system, officially called the BeiDou Regional Navigation Satellite System (BDS-2), was operational in December 2011. Since December 2012, it has provided services to customers in the Asia-Pacific region. In 2015, the construction of the third-generation BeiDou system (BDS-3) began. On 27 December 2018, BDS-3 started to provide global services. The 35 th and final BDS-3 satellite was launched into orbit on 23 June 2020.  
Compared with BDS-2, BDS-3 has been upgraded from being a regional service to providing navigation and positioning services for global users, and its service performance is better [1; 2]. With respect to its designed signal, BDS-3 provides navigation signals of multiple frequencies and can provide high-precision services via combined multi-frequency signals. BDS-3 is backward compatible with the B1I and B3I signals of BDS-2 and adds B1C and B2a signals. Two new navigation messages, B-CNAV1 and B-CNAV2, were modulated on the two new signals, and they adopted new orbital parameters (18 parameters) with higher orbit description accuracy [3; 4; 5]. Compared to the rubidium atomic clock of BDS-2 [6; 7; 8], BDS-3 uses a new type of rubidium atomic clock with a daily stability of \(10^{-14}\) and a new type of hydrogen atomic clock with a daily stability of \(10^{-15}\), which improves the performance and service life of satellites [9; 10; 11]. In addition, BDS-3 satellites are equipped with an inter-satellite link (ISL) payload. Since monitoring stations are only deployed in China, ISLs play an important role in broadcast message generation and update [9; 12; 13].  
A signal-in-space range error (SISRE) mainly includes satellite orbit errors and clock errors, and its accuracy has a direct impact on satellite navigation and positioning [14; 15; 16]. Changes in the accuracy of SISRE are an important factor affecting the performance of GNSS services [13; 17; 18]. Therefore, it is of great significance to study and analyze the orbit and clock errors of a satellite broadcast ephemeris for evaluating the basic service performance of GNSSs. Currently, there are many studies evaluating the SISRE, orbit, and clock errors of GNSSs [19; 20; 21; 22; 23; 24], and some studies have arrived at significative conclusions with respect to BDS [10; 14; 15; 16; 17; 21]. [PERSON] evaluated BDS-2's navigation messages [21]. [PERSON] assessed the long-term SISRE accuracy of BDS-2 from 2013 to 2016 [25]. An average root mean square (RMS) of 1 to 2 m was computed for SISRE [14; 15]. For BDS-3, the SISRE accuracy shows a noticeable improvement compared to BDS-2 [1; 2]. [PERSON] assessed the technical characteristics and service performance of BDS-3, and [PERSON] carried out an initial assessment of BDS-3 SISRE [10; 17]. They arrived at similar conclusions: the 3D RMS of the broadcast orbit error was less than 0.6 m, and the RMS of the clock error was about 0.5 m. Since the time span of the data used was less than 2 months, there is a lack of analysis with respect to the long-term annual evolution of BDS-3 SISRE. Furthermore, the accuracy analysis of B-CNAV navigation messages has not attracted sufficient attention, and the accuracy of hydrogen and rubidium atomic clocks should be counted separately to grasp the service performance of new atomic clocks.  
In this paper, the orbit, clock error, and the SISRE evolution of the 24 medium Earth orbit (MEO) satellites of BDS-3 were analyzed and compared using data for the entire year of 2022. In order to evaluate the accuracy of the D1 and B-CNAV navigation messages of the 24 MEO satellites of BDS-3, this article used the precision products of the Center for Orbit Determination in Europe (COD) as reference values.