## 1 Introduction  
Due to the fact that the Global Navigation Satellite System (GNSS) has the ability to provide position, velocity and time information to all users near the Earth's surface, many countries have focused on the development of their own GNSSs for strategic considerations [1]. Currently, the United States' Global Positioning System (GPS) and the Russian Global Navigation Satellite System (GLONASS) are already providing services and are available to civilian users [2,3]. Recently, the Chinese BeiDou Navigation Satellite System (BDS) has been developing rapidly and now meets the open service requirements for civilian users [4]. The service area of the current BDS is 55 \(\lx@sectionsign\)-55 \(\lx@sectionsign\) N, 70 \(\lx@sectionsign\)-150 \(\lx@sectionsign\), as shown in Figure 1. As a user in Taiwan, it is important to know the benefit of applying the BDS because we can receive the BDS satellite signals all day. This research is based on actual BDS signal in space (SIS) data which is used to analyze the differences between the BDS and the GPS. Moreover, this research also provides suggestions to users who want to combine both systems for positioning. Compared with the current GPS constellation, which consists of 31 medium earth orbit (MEO) satellites, the BDS constellation had five Geostationary Earth Orbit (GEO) satellites, five Inclined Geosynchronous Orbit (IGSO) satellites, and four MEO satellites as of May of 2014. GPS does not have any GEO and IGSO satellites in the constellation. However, the two system's MEO satellites are not at the same orbit altitude. The orbit altitude of the BDS MEO satellites are 21,528 km, and the GPS satellites are operating at an altitude of 20,200 km. The GPS satellites appear at the same place at almost the same time because the periods for GPS satellites are about 12 h. Due to the fact that the orbit altitude of BDS MEO satellites is higher than that of the GPS satellites, the periods of the BDS  
Figure 1: BDS service area. MEO satellites are almost 13 h. As a result, the BDS satellite geometry will change for a fixed user at the same time every day. Under the premise of different constellations, Table 1 illustrates the main differences between the BDS and GPS. The BDS coordinate system is the China Geodetic Coordinate System 2000 (CGCS2000), and the GPS coordinate system is the World Geodetic System 1984 (WGS84). Both of the coordinate systems are based on the idea of Earth-centered Earth-fixed (ECEF) coordinates, but the definition of the ellipsoid parameters is different. However, the maxima difference in the latitude and longitude between the WGS84 and CGCS2000 is less than 1.1 \(\times\) 10\({}^{-3}\) m [5]. The slight difference between the two coordinate systems will be ignored in this research. The BDS uses BeiDou navigation satellite system Time (BDT) for its time system, which started calculating at 00:00:00 on 1 January 2006 in Coordinated Universal Time (UTC). The GPS time (GPST) started at UTC at 00:00:00 on January 6 th, 1980. The time difference between the BDT and GPST is about 14 s. Finally, the BDS signal's multiplexing mode is code division multiple access (CDMA), which is the same as that of the GPS [6]. Finally, this research analyzes the single frequency signal for a user for which the frequency of the BDS B1I signal is 1561.098 MHz, and the GPS L1 signal is 1575.42 MHz. The time system is based on the GPST to analyze the results for both systems (_i.e._, \(\text{GPST}=\text{BDT}+14\)). Under ideal conditions, all the differences between the different GNSS constellations could be determined after the coordinate and time conversion calculations. In reality, however, there are many other issues need to be resolved when a user wants to integrate different GNSS signals. For example, in order to integrate the different GNSSs for a centimeter accuracy positioning by the conventional double difference method, the inter-system biases should be determined first [7].