## 1 Introduction  
As one of the Global Navigation Satellite Systems (GNSSs), the BeiDou navigation satellite system (BDS) has been a relevant and valuable complement for establishing improved positioning, navigation and timing (PNT) services. The regional BDS (BDS-2) has been providing stable and continuous PNT services for customers in the Asia-Pacific region since 27 December 2012. Currently, the BDS-2 constellation consists of six geostationary orbit (GEO) satellites (C01-C05, C17), six inclined geosynchronous orbit (IGSO) satellites (C06-C10, C15/C13), and three medium earth orbit (MEO) satellites (C11, C12, C14). At present, the BDS-2 GEO satellite C17 has still not been put into use. All BDS-2 satellites are able to provide B1, B2 and B3 signals with a center frequency of 1561.098, 1207.14 and 1268.52 MHz, respectively [1]. The BDS implementation has been performed in three steps in terms of its deployment timeline: BeiDou navigation demonstration system (BDS-1) by 2000, BDS-2 by 2012 and global BDS (BDS-3) by 2020. Following the \"three-step\" strategy, the BDS-3 is under construction. The constellation deployment of BDS-3 is expected to be completed by 2020, and the enhanced PNT services will be able to be provided to the global users at that time. The full constellation of BDS-3 will be composed of five GEO satellites, three IGSO satellites and 27 MEO satellites. As of 29 March 2018, a total of eight BDS-3 satellites (C19-C22, C27-C30) have been launched. All these satellites are currently unavailable because they are still in the phase of flight tests. A BDS-3 demonstration system (BDS-3S) was established before the launch of the first BDS-3 satellite, including three MEO satellites (C33-C35) and two IGSO satellites (C31, C32). The BDS-3S satellite C35 was launched on 1 February 2016, but its antenna failed to transmit navigation signals. The BDS-3S is used to validate the design of BDS-3, including inter-satellite links, new hydrogen clock and complex ranging signals. All the BDS-3S satellites, except C31, are capable of transmitting three new navigation signals, namely, B1C centered at 1575.42 MHz, B2a at 1176.45 MHz, and B2b at 1207.14 MHz, which overlap with Galileo E1/E5a/E5b and GPS L1/L5 signals so as to enhance the compatibility and interoperability with other GNSS systems. In addition, all these satellites, including C31, can also broadcast signals on the two legacy frequencies B1 and B3 to ensure that the BDS-2 can be smoothly transited to BDS-3 [2]. With the signal transmission of BDS-2 satellite M1, a new type of systematic biases existing in code observations was identified [3]. As for other BDS-2 satellites, [PERSON] et al. [4] also observed the systematic code biases. [PERSON] and [PERSON] [5] provided more details. They concluded that the code biases with a varying range of approximately 1 m from horizon to zenith were both frequency- and elevation-dependent. They also found significant differences between code bias variations of IGSO-type and MEO-type satellites. In addition, the systematic code biases should be attributed to the transmitting satellites, because they were not dependent on the station location or receiver type. Therefore, the bias is termed satellite-induced code bias (SICB) in this study. The mechanism of SICB has now been identified clearly, and the root cause is the spacecraft internal multipath. The impedance mismatch between antenna elements and power divider network causes reflection signals. The downlink antenna retransmits the reflection signals. At the satellite end, the inner reflection signals depend on the bore-sight angles. Consequently, the elevation-dependent multipath errors can be observed at the user end [6]. Figure 1 shows the principle of multipath. The objects near the receiver will reflect the direct signals, and thus the reflected signals are generated. The reflected signals go through a longer path than the direct signals, which causes the large propagation delay. The received signals are the superposition of the direct and the reflected signals.