## 5 Experiment Setup and Result Discussion  
We use two self-developed GPS/BDS receivers as the base and the rover respectively. The receiver is developed based-on a DSP + FPGA architecture as shown in the right part of Figure 2. It can receive GPS L1/L2 and BDS B1/B2 signals via 96 parallel channels and can process L1 and B1 signals at this moment. We use a commercial survey antenna (left picture of Figure 2) and place it on the rooftop of Weiqing Building, Tsinghua University, Beijing, China. Three static experiments with a zero baseline, a 5.9 km baseline and a 9 km baseline for each are conducted. The zero baseline experiment is designed to verify the feasibility of the proposed model, and the other two short baseline tests are to evaluate its performance. In the zero baseline test, a radio frequency splitter was connected to the rooftop base antenna so as to split signals for the base and the rover receivers respectively. In the other two experiments, another antenna with the same performance as the base antenna was setup at an open-sky parking lot at the South gate of the North park of the Olympic Forest Park and the rooftop of the office building of China Academy of Civil Aviation Science and Technology respectively. The base and rover locations are drawn in Figure 3. The  
Figure 2: Experiment devices (**a**) antenna (**b**) receiver circuit board. approximate locations calculated from pseudorange measurements for the base and the rovers in the tests as well as other experiment settings are listed in Table 1. The broadcast ephemeris and raw carrier phase and pseudorange measurements generated by the receivers are recorded during the tests. We implemented the proposed GPS/BDS combined RTK model on Matlab and post-processed the recorded measurements. Figure 4 is the number of available satellites during the testing period. Figure 5 shows the East, North and Up (ENU) positioning results of the zero baseline experiment. For comparison, the results of \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Item**} & **Zero Baseline** & **5.9 km Baseline** & **9 km Baseline** \\ \hline \multirow{8}{*}{Base} & Time (UTC) & 11:48–12:55 & 03:12–04:19 & 12:41\(\sim\)14:21 \\  & & 13 May 2014 & 21 May 2014 & 31 July 2014 \\ \multirow{8}{*}{Rover} & Frequency & GPS L1, BDS B1 & GPS L1, BDS B1 & GPS L1, BDS B1 \\  & Latitude\({}^{\circ}\) & 40.0015 & 40.0015 & 40.0015 \\  & Longitude\({}^{\circ}\) & 116.3302 & 116.3302 & 116.3302 \\ \multirow{8}{*}{Rover} & Height/m & 98.9 & 98.9 & 98.9 \\  & Latitude\({}^{\circ}\) & Same as Base & 40.0228 & 39.9689 \\ \multirow{2}{*}{Rover} & Longitude\({}^{\circ}\) & Same as Base & 116.3934 & 116.4303 \\  & Height/m & Same as Base & 46.4 & 78.3 \\ \multirow{2}{*}{Data length/s} & 4000 & 4000 & 6000 \\  & Epoch interval/s & 1 & 1 & 1 \\ \multirow{2}{*}{Elevation angle threshold/ \({}^{\circ}\)} & 20 & 20 & 20 \\ \multirow{2}{*}{Ratio test threshold} & 3 & 3 & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Experiment settings. Figure 3: Base and rover locations (from Google Earth). processing the single GPS and the single BDS with the same algorithm is included in the figure. Other statistics of the three methods are listed in Table 2. The result validates the feasibility of the proposed GPS/BDS RTK model and shows that the baseline precision of combined GPS and BDS is on the same level as that of handling either single constellation. The proportion of fixed solutions are 100% for all the three methods, _i.e._, the ratios for all epochs reach above the threshold of 3. There is no difference in the first time to fix between the three approaches and they all enter fixed solution within one epoch. The reason for that should be the zero length of baseline. Figure 4: Number of available satellites _vs._ time. Figure 5: ENU baseline result of zero baseline experiment (all epochs). The results from the 5.9 km short baseline test as shown in Figures 6-8 and Table 3 illustrate some differences from the zero baseline experiment.