## 1 Introduction
Since the BDS MEO satellites are at a higher orbit altitude than that of GPS satellites, the period of BDS MEO satellites is 7/13 sidereal day, approximately equal to 12 h and 55 min [11]. As a result, the BDS satellite geometry repeats every seven sidereal days for a fixed user. The system differences between BDS and GPS are summarized in Table 1. This research uses the GPS time (GPST) as the timing system to evaluate the results of BDS, GPS, and their integration (_i.e._, BDT = GPST \(-\) 14) [11]. The coordinate systems for these two systems are based on the Earth-centered Earth-fixed (ECEF) coordinates, and China Geodetic Coordinate System 2000 (CGCS 2000) is used for BDS and GPS uses the World Geodetic System 1984 (WGS84). According to [PERSON] [12]) the difference between the two coordinate systems is caused by that their definitions of the geocentric gravitational constant (\(\mu\)) and the rate of Earth rotation (\(\Omega_{\ell}\)) are different, and the maximum latitude and longitude differences for these two coordinates are less than 1.1 \(\times\) 10\({}^{-3}\) m, so the coordinate conversion effect is thus ignored in this paper. Under ideal conditions, the coordinate and timing conversions between different GNSSs could be calculated, and the simulation results of [12] shows that integration of two or more GNSSs offers better positioning performance than a single GNSS. However, there are many issues that need to be resolved if one would like to integrate actual signals from two or more GNSS constellations. For most of the multi-GNSS user positioning algorithms, the minimum requirement is to conduct an inter-system bias calibration [13; 14]. For example, when a user attempts to combine the measurements from several GNSS constellations to achieve precise positioning, the inter-system biases between different GNSSs would affect the resolution of integer ambiguity [15]. Because the BDS constellation includes MEO, IGSO, and GEO satellites, a BDS receiver has to consider the inter-satellite-type biases between different constellations [16]. Besides these inter-system and inter-satellite-type biases, it is of practical interest to evaluate the differences between BDS and GPS based on actual satellite data, signals and measurements. Moreover, suggestions are provided for combining the two systems for positioning. This research mainly focuses on finding all the system differences that cause bad positioning results when using combined GPS and BDS data. By analyzing real data, signals, and measurements, the actual performance of a given GNSS can be revealed. Comparisons of GPS and BDS on satellite data, signal and measurements give us a good understanding about their differences. For a complete \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **GPS** & \multicolumn{3}{c}{**BDS**} \\ \hline
**Orbit** & MEO & GEO & IGSO & MEO \\ \hline
**Orbit Radius (km)** & 20,200 & 35,786 & 35,786 & 21,528 \\
**Infination** & 55\({}^{\circ}\) & \(<\)2\({}^{\circ}\) & 55\({}^{\circ}\) & 55\({}^{\circ}\) \\
**Sat. Number (Until 2014)** & 31 & 5 & 5 & 4 \\
**Planned Sat. Number** & 24 & 5 & 3 & 27 \\
**PRN Number** & 1-32 & 1-5 & 6-10 & 11-14 \\ \(\mu\) (m\({}^{3}\)/s\({}^{2}\)) - 10\({}^{\ddagger}\) & 3.986005 & 3.986004418 & \\ \(\Omega_{\ell}\) (rad/s) - 10\({}^{\circ}\) & 7.2921151467 & 7.2921150 & \\
**Coordinates** & VGSS4 * & CGCS2000 * \\
**Time** & GPST ** & BDT ** \\
**Time Start** & 6 January 1980 & 1 January 2006 \\
**1 st Carrier** & 1575.42 MHz & 1561.098 MHz \\
**2 nd Carrier** & 1227.6 MHz & 1207.14 MHz \\ \hline \end{tabular}  
* WGS84: World Geodetic System 1984; CGCS2000: China Geodetic Coordinate System 2000; ** GPST: Global Positioning System Time; BDT: Bellou Navigation Satellite System Time.  
\end{table} 
Table 1: System comparison between GPS and BDS [2; 11]. survey of the performance of BDS and GPS, this research divides the analysis methods into data quality analysis (DQA), signal quality analysis (SQA), and measurement quality analysis (MQA).