## 2 Development of the BeiDou System  
The BeiDou system follows the principles of step-by-step construction according to a three-step master plan. The BeiDou test system started development in 1994, and then two Geostationary Earth Orbit (GEO) satellites were launched successfully, forming the first generation of the BDS (BDS-1) in 2000 [19]. The active positioning system was adopted to provide positioning, time service, wide area difference, and short message communication services for Chinese users. China started to develop BDS-2 in 2004 and successfully completed the deployment of 14 satellite networks by the end of 2012, which cover the Asia-Pacific region and includes a passive location system [20]. On 5 November, 2017, China officially began to build the BeiDou global satellite navigation system (BDS-3). The BDS-3 system inherits both the active service and passive service, which can provide basic navigation (positioning, speed measurement, time service), global short message communication, and international search and rescue services for global users [21]. Figure 1 shows the ground tracks of BDS-2 and BDS-3 satellites in normal operation. The existing satellites have full navigation service capability in global area. BDS-3 is downward compatible with B1I and B3I signals as well as the addition of new B1C and B2a signals. The frequencies, code rates, and modulation scheme of the BDS-3 signals are summarized in Table 1[22; 23]. In addition to the public service signals in Table 1, there are B1A in Binary Offset Carrier (BOC) (1,1) modulation, B3C Quadrature Phase Shift Keying (QPSK) (10) modulation and B3A in BOC (15,2.5) modulation authorized service signals [22], which are not be described in detail here. The B2 signals are divided into B2a and B2b signals similar to the E5a and E5b bands of Galileo. The B2a signals, with the center frequency at 1176.45 MHz, are transmitted in QPSK (10) modulation. It is worth noting that the B2a of BDS-3 replaces the B2I of BDS-2 with different frequency and modulation modes, while B2b and B2I share the same frequency but have different modulation modes. The B1C signal is divided into two components, B1C_data and B1C_pilot, which are respectively transmitted by the Quadrature Multiplexed Binary Offset Carrier (QMBOC) (6,1) and BOC (1,1) modulation with a center frequency of 1575.42 MHz.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Bands**} & **Frequency** & **Modulation** & **Code Rate** & \multirow{2}{*}{**Compatible with Interoperable Signals**} \\  & **(MHz)** & **Scheme** & **(Mcps)** & \\ \hline \multirow{3}{*}{**B1I**} & \multirow{3}{*}{1561.098} & BPSK\({}^{1}\)(2) & & \\  & & QPSK\({}^{2}\)(10) & 2.046 & \\  & & BPSK (10) & & \\ \hline B2b & 1207.140 & & 10.23 & Galileo E5b \\ B3I & 1268.520 & & 10.23 & \\ \multirow{3}{*}{B1C} & \multirow{3}{*}{1575.420} & B1C\_pilot & QMBOC\({}^{3}\)(6, 1) & 1.023 & \\  & & B1C\_data & BOC\({}^{4}\)(1, 1) & & GPS L1/Galileo E1 \\ \multirow{3}{*}{B2a} & \multirow{3}{*}{1176.450} & QPSK (10) & 10.23 & GPS L5/ \\ \cline{1-1}  & & & & Galileo E5a \\ \hline \hline \end{tabular} Abbreviations: \({}^{\text{i}}\)BPSK, binary phase shift keying: \({}^{\text{2}}\)QPSK, quadrature phase shift keying: \({}^{\text{3}}\)QMBOC, quadrature multiplexed binary offset carrier; \({}^{\text{4}}\)BOC, binary offset carrier  
\end{table}
Table 1: Available open service signals transmitted by the new-generation BDS-3 satellites.