## 1 Introduction  
The BeiDou Navigation Satellite System (BDS, earlier referred to as COMPASS) has been providing position, navigation, and timing (PNT) services since 27 December 2012, covering the whole Asia-Pacific region. The system currently consists of 15 satellites, including five geostationary Earth orbit (GEO) satellites, five inclined geosynchronous orbit (IGSO) satellites, and five medium Earth orbit (MEO) satellites. It will eventually provide PNT services for worldwide users by the end of 2020, comprising five GEO, three IGSO, and twenty-seven MEO satellites [1].  
Like the other Global Navigation Satellite Systems (GNSS), the BDS has a significant potential in many applications and scientific research community. To make a better use of the BDS system, the BDS performance and signal characteristics must be fully evaluated and understood. In earlier studies, it has been shown that Precise Point Positioning (PPP) result using BDS observations alone is comparable with that using Global Positioning System (GPS) only, and that PPP convergence speed and positioning accuracy could be improved after integrating BDS with GPS observations [2; 3]. With triple-frequency observables available, characteristics of combined measurements [4] and ambiguity resolution with triple-frequency carrier phase [5; 6; 7] were also investigated. It has been commonly agreed that severe multipath effects slow down the convergence of PPP and degrade the reliability of PPP ambiguity resolution [5; 6; 8; 9]. Methods for mitigating multipath effects on both code [10; 11] and carrier phase measurements [12] have been developed.  
Previous studies showed that BDS observables of higher elevations have higher carrier-to-noise ratio (CNR) and smaller variation in multipath combinations [4; 10; 11; 13]. With the combination of GPS and BDS, high cut-off elevation angles, _i.e._, up to 30\({}^{\circ}\) for the single-frequency cases or 40\({}^{\circ}\) for the dual-frequency, could still guarantee high success-rate (nearly 100%) of ambiguity resolution in RTK positioning [14]. In addition, it was shown that multipath combinations of BDS observables display systematic variations with elevations, which is significantly different from that of GPS, GLONASS, or Galileo [10; 11]. This multipath variation should be taken into account since it affects the convergence and reliability of ambiguity resolution of BDS data.  
Our analysis finds that multipath combination of BDS observables contains not only real multipath effects but also some other multipath-like biases. Understanding and proper handling of these biases and their characteristics, as well as their difference from GPS, will help correct BDS signal errors and improve the positioning accuracy of using BDS or multi-GNSS data.  
This paper is organized as below. The mathematical models for GNSS code multipath combination and geometry-free ionosphere-free (GFIF) combination are given in Section 2. In Section 3, the BDS satellite CNR and multipath combination are investigated and compared with those of GPS and Galileo satellites. Cross-correlation analyses are conducted between multipath of different frequencies, between multipath and elevation, as well as between multipath and azimuth. To further examine the elevation-dependent bias in the multipath of BDS IGSO and MEO satellites, single-differencing between two stations is conducted. The GFIF combination of BDS satellites is also investigated. The conclusions are given in Section 4.