## 3 Signal Performance
### Phase and Pseudorange Code Evaluated by Precise Orbit Determination
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Signal**} & **Frequency** & **Combination** & \multicolumn{2}{c}{**Combination Coefficient**} & \multirow{2}{*}{**Noise Level**} \\ \cline{3-3} \cline{5-6}  & **(MHZ)** & **Type** & **a** & **b** & \\ \hline B1C & 1,575,420 & B1I-B3I & 2.9437 & \(-\)1.9437 & \(\sqrt{8.67\sigma_{B1I}^{2}+3.78\sigma_{B3I}^{2}}\) \\ B1I & 1,561,098 & B1C-B3I & 2.8436 & \(-\)1.8436 & \(\sqrt{8.09\sigma_{B1C}^{2}+3.40\sigma_{B3I}^{2}}\) \\ B2a & 1,176,450 & B1I-B2a & 2.3144 & \(-\)1.3144 & \(\sqrt{5.36\sigma_{B1I}^{2}+1.73\sigma_{B2a}^{2}}\) \\ B3I & 1,268,520 & B1C-B2a & 2.2606 & \(-\)1.2606 & \(\sqrt{5.11\sigma_{B1C}^{2}+1.59\sigma_{B2a}^{2}}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The information about the open service signals and four types of ionosphere-free (IF) combinations. \begin{table}
\begin{tabular}{l l} \hline \hline \multicolumn{1}{c}{**Item**} & \multicolumn{1}{c}{**Model**} \\ \hline Observation combination & Undifferenced IF of code and phase, B1I-B3I, B1C-B3I, B1I-B2a, B1C-B2a \\ Sampling interval & 300 s \\ Cutoff elevation & 7 deg \\ Arc length & 72 h \\ Satellites antenna phase center & supplied by BeiDou Operational Control Center \\ Geopotential & EGM 2008, \(12\times 12\) degree \\ Attitude model & Continuous yaw-steering model [25]. \\ N-body gravitation & Sun, Moon, Mercury, Venus, Mars, Jupiter Saturn, Uranus, Neptune and Pluto, \\  & PPL DE405 ephemeris are used [26] \\ Earth Orientation Parameters & Fixed to IERS EOP 14 C04 dataset \\  & The Saastamoinen model for the initial dry and wet zenith delay [27], projection \\ Tropospheric delay & function GMF is used [28], and the wet delay was estimated as constant \\  & every 2 h. \\ Solar radiation & ECOM five-parameters model with a constant acceleration bias in the \\  & along-track direction [29]. \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The observation models and force models used in precise orbit determination (POD). In addition, a two-day overlap difference between two adjacent arcs was used to evaluate the orbit precision. RMS values of the BD3 satellites overlap difference are shown in Figure 6. Within the process of POD, only the BD3 satellites are calculated using the four types of IF combinations. The statistical RMS values of the overlap in the along-track, cross-track, radial and three dimensions (3D) are presented in the figure. Besides, the mean observation counts of the four IF combinations in each arc are also demonstrated on the dual-Y axis. As Figure 6 shows, the IF combination of B1I-B3I performs best and its RMS values in the along-track, cross-track, radial and 3D directions are 14.4 cm, 8.7 cm, 3.9 cm and 20.0 cm, respectively, followed by B1C-B2a and B1I-B2a, whilst B1C-B3I shows the worst performance. Interestingly, these results are in the reverse order of the mean observation counts. The count of the observation used in the four types of IF combinations may be related to the capability of receivers. As the observation accuracy of phase is much higher than that of the pseudorange, the weights of the phase and pseudorange observation are generally set as 100:1. According to the residual data listed in Table 4, we suggest the IF combination of the B1C-B2a should be adopted first, which may enhance the \begin{table}
\begin{tabular}{c c c c c} \hline  & **B1I-B3I** & **B1C-B3I** & **B1I-B2a** & **B1C-B2a** \\ \hline LC & 5.9 & 5.2 & 4.9 & 4.8 \\ PC & 631 & 820 & 469 & 720 \\ \hline \end{tabular}
\end{table} 
Table 4: The root mean square (RMS) of B1I-B3I, B1C-B3I, B1I-B2a, B1C-B2a IF combination residuals in phase and pseudorange (units: mm). Figure 5: The phase and pseudorange ionosphere-free (IF) combination residuals as a function of epoch. precision of the parameters in the same situation. The results and mean observation count of the four IF combinations' overlap are both listed in Table 5.