## 1 Introduction  
An increasing number of countries have focused on the development of their own Global Navigation Satellite System (GNSS), which provides convenient real-time positioning, velocity, and time services [1]. Currently, the Global Positioning System (GPS) operated by the United States and Global Navigation Satellite System (GLONASS) operated by Russia provide global positioning services to users [2, 3]. In addition, the European Galileo and Chinese BeiDou Navigation Satellite System (BDS), both under development, aim to provide global positioning service by 2020. The Japanese Quasi-Zenith Satellite System (QZSS) and the Indian Regional Navigation Satellite System (IRNSS) are regional navigation satellite systems (RNSS). Different GNSSs have been combined to provide users with more complete and diverse satellite navigation services [4, 5]. A regional GNSS is designed to provide unique benefits for a certain area. For users in the Asia Pacific region, GPS, GLONASS, and BDS provide stand-alone positioning services at all times. Galileo is being built up and can thus currently provide only restricted positioning services in this region. For users in urban areas, the integration of different GNSS offers higher positioning availability. Figure 1 shows in-view satellite numbers for various GNSSs generated from the signal in space (SIS) data collected by a user in Taiwan. The data length is one day and the data were collected on the 19 th of October 2014. BDS has the highest average number of available satellites. Figure 2 shows the three GNSS satellite distributions at various elevation angles in increments of 10\({}^{\circ}\) generated from the data used for Figure 1. More than 50% of the BDS satellites have an elevation angle above 50\({}^{\circ}\). For GLONASS, more than 50% of satellites have elevation angles below 30\({}^{\circ}\). Satellite signal and measurement quality are positively correlated with the satellite elevation angle, and thus BDS provides better measurements for positioning for users in this region. Integration of several GNSS constellations could enhance the navigation services with better accuracy, continuity and availability than that of one GNSS constellation [6; 7]. Especially for environments where some portions of the sky are blocked, for instance, urban canyons and dense foliage environments, the user position cannot be calculated due to the insufficient number of satellite in view. On the other hand, the combined use of GNSSs could provide sufficient signals and therefore provide uninterrupted positioning service in the same environment. Moreover, more visible satellites from multiple GNSSs offer better satellite geometry for the positioning than single GNSS [8]. However, the different GNSS architectures may cause problems during system integration [9; 10], so before integrating BDS with GPS, the differences between these systems must be studied. During the experimental period presented in this paper (10 th of July to 13 th of August 2014), the BDS service area is 55\({}^{\circ}\)S-55\({}^{\circ}\)N, 70\({}^{\circ}\)E-150\({}^{\circ}\)E [11]. The BDS constellation includes five geostationary  
Figure 1: Number of available GPS, GLONASS, and BDS satellites for users in Taiwan. Figure 2: GPS, GLONASS, and BDS satellite distributions for various elevation angles for users in Taiwan. Earth orbit (GEO) satellites, five inclined geosynchronous orbit (IGSO) satellites, and four medium Earth orbit (MEO) satellites. The different BDS satellite types (_i.e._, GEO, IGSO, and MEO) are operated in different corresponding orbit altitudes. On the other hand, GPS has 31 MEO satellites and all GPS satellites are operated in similar orbit altitudes. The orbit altitude of GPS satellites is approximately 20,200 km and that of BDS MEO satellites is 21,528 km. The period of GPS satellites is 1/2 sidereal day, approximately equal to 11 h and 58 min. After two periods (23 h and 56 min), the GPS satellites appear at almost the same place.