## 3 Signal Performance
### Phase and Pseudorange Code Evaluated by Precise Orbit Determination  
The residuals of phase and pseudorange code have also been analyzed in precise orbit determination procedure. As the first order of the ionosphere delay can be eliminated by dual-frequency combination observation data, thus, a carrier phase and pseudorange code Ionosphere-free (IF) combination data are used in POD. The IF combination can be expressed as  
\[ \begin{array}{l}IF=aR_{i}+bR_{j}\\ \left\{\begin{array}{l}a=\frac{f_{i}^{2}}{f_{i}^{2}-f_{j}^{2}}\\ b=-\frac{f_{j}^{2}}{f_{i}^{2}-f_{j}^{2}}\end{array}\right.\end{array}  \tag{2}\]  
where \(IF\) represents the combined data of the IF carrier phase and pseudorange code, \(a\) and \(b\) are the combination coefficients, \(f\) and its subscripts are the same meanings as formula (1), and \(R\) is the range observation of phase or pseudorange. Positioning And Navigation Data Analyst software (PANDA) [24], developed by Wuhan University, Wuhan, China, is used for the POD in the study, and the undifference algorithm is adopted in the software. Four open service signals are used and four kinds of IF combinations, (B1I, B2a), (B1I, B3I), (B1C, B2a), (B1C, B3I), are set up herein. The information about these combinations is listed in Table 2. The IF combination coefficients of the selected four situations are given in the table, and the noise  
Figure 4: The signal to noise (S/N) performance of the observation data recorded by the HMNS station, where the S/N values of BD3 B1I/B1C/B2a/B3I signal are shown in the left, Galileo E1/E5a/E5b and GPS L1/L2/L5 signal performance are shown in the middle and right panel, respectively. level of the four kinds of IF combinations are calculated using the error spread rule. \(\sigma_{B1I}\), \(\sigma_{B1C}\),\(\sigma_{B2a}\) and \(\sigma_{B3I}\) in Table 2 are the standard deviation (STD) values of the B1I, B1C, B2a and B3I phase or pseudorange observation. Considering that the iGMAS stations can track all the open service signals of BD3 satellites at present, only the iGMAS stations are used for the BD3 precise orbit determination in the paper. Observation data from DOY 030 to 059 2019 is utilized in the study and a 72-h arc length is adopted. Table 3 summarizes the observation models and force models used in our POD processing. In addition, the receiver and satellite clock offsets are also estimated together with the orbit determination process, and the white-noise constraint is used between adjacent epochs. Note that the coordinates of iGMAS stations are estimated with a tight constraint of about 5 cm, and the Earth Orientation Parameters (EOP) are fixed to the IERS EOP 14 C04 series. Besides, the above four types of IF combinations are used for the BD3 satellites POD. The satellites antenna phase center offset (PCO) parameters supplied by the BeiDou Operational Control Center are used in the study. It should be noted that the space vehicle number (SVN) is absent in the BD3 FOC satellites and it is assigned and used within the IGS-MGEX project. The residuals of phase and pseudorange are presented in Figure 5, where the four types of IF combinations are displayed by different colors. According to the results, the B1I-B2a combination performs best in the IF pseudorange residuals, while B1C-B3I is the worst. The corresponding statistical results are listed in Table 4. In addition, the combination signal seems to be contaminated by B1C signal, which is consistent with the results acquired from the multipath combination. However, when it comes to the IF phase combination, the residual of B1C-B2a is the minimum and all four combinations are in the same magnitude, which is consistent with the noise level derived in Table 2.