# The First Result of Relative Positioning and Velocity Estimation Based on CAPS
## 1 Introduction  
With the highly accurate performance of Positioning, Navigation and Timing (PNT), Global Navigation Satellite Systems (GNSSs), being of all-weather and real-time operational PNT service over land, sea, and even aerospace, have been developed rapidly around the globe [1]. Currently, the main GNSSs include the American GPS (Global Positioning System), the Russian GLONASS (Global Navigation Satellite System), the European Galileo and the Chinese BDS (Beidou Satellite Navigation System) [2]. These systems are featured by specialized satellites that can broadcast signalswith navigation messages to users to estimate PNT parameters, such as location, velocity and time. However, there are also some challenges that have to be faced by the growing development of GNSS systems [3], i.e., (1) service capabilities through positioning, velocity estimation and time service without communication, (2) the weakness of the received signal, with fixed satellites/frequencies/code making the signal vulnerable to interference, and (3) the PNT accuracy of current GNSSs is significantly dependent on the performance of satellite clock which is extremely difficult to be kept stable in orbit [4]. Thus, it has for a long time been a global dream to design and establish a satellite-based system which can not only integrate the functions of communication and navigation, but also avoid the aforementioned problems. In view of this, [PERSON] et al. [3; 5] from the National Astronomical Observatories of the Chinese Academy of Sciences proposed an innovational navigation concept based on communication satellites including the GEO (geostationary orbit), DGEO (decommissioned geostationary orbit) and IGSO (inclined geosynchronous orbit) satellites in 2002. This proposed concept has been named the Chinese Area Positioning System (CAPS). In the CAPS, the ranging code, time and navigation messages are generated and uplinked directly to the satellite from the ground control station, before being downlinked and broadcasted to users via the communication satellite transponder [5]. Compared with the traditional GNSSs like GPS, BDS, GLONASS and Galileo, CAPS does not require the installation of an atomic clock on satellites in orbit and could use a higher stability atomic clock in the ground station, since the navigation signal is generated in the ground control station. Moreover, CAPS can be flexibly established by renting the transponders on the communication satellites [6]. As a result, it significantly reduces costs and saves the system construction time when compared with the traditional GNSSs. In addition, CAPS uses C-band frequencies to transmit the navigation signals and is designed as a dual-frequency system. The frequencies of downlink signals are designed on 3862.02 and 4143.15 MHz as C1 and C2 frequencies [7; 8]. Currently, only the C1 frequency is available. The special security code with a rate of 10.23 MHz is modulated on the C1 frequency. The chip length of the CAPS signal is about 30 m. Since the wavelength of the CAPS signal is about 1/3 of the GPS L2 signal (the wavelength of the CAPS C1 signal is 7.8 cm while that of the GPS L2 signal is about 24.4 cm), the precision of the carrier-phase measurement would be theoretically higher than that of the L-band. This feature is very advantageous for achieving positioning, velocity estimation and timing with a relatively high accuracy. CAPS was first demonstrated by the Chinese Academy of Sciences based on two rented communication satellites from China Satellite Communications Co., Ltd. in 2005. Based on this demonstration, some experiments had been carried out from 2006 to 2010 by integrating the communication and navigation function in vehicles and ships from different areas over land and sea. Around 2011, CAPS was selected as the essential part of the BDS experiment system and aimed to support the BDS construction and development by testing the new or on-going navigation technology.