Status of QZSS Navigation System in Japan

S. Kogure, M. Sawabe, M. Kishimoto

Abstract:	Quasi-Zenith Satellite System (QZSS) is a satellite navigation system in Japan which is the constellation consisted of several satellites orbiting different high inclined orbital planes with geosynchronous period. Each satellite is allocated on the orbit so as to pass over the same ground track at constant intervals. Eccentricity and inclination are selected so that minimum elevation angle through 24 hours in service area is to be larger than about 70 degrees. As a baseline, we are now planning to launch first QZS whose orbit parameters are eccentricity as about 0.1 and inclination as about 45 degrees, respectively. From the viewpoint of users, they can receive the signal from at least one of the satellite of QZSS where it is around zenith direction, with high elevation angle, every time. This is the origin of the name, “Quasi-Zenith Satellite System”. “More stars” is a primary essential requirement for GNSS applications from geodesy to navigation use of handheld equipment. The constellation design of the QZSS leads the improvement of availability effectively as less as number of satellite. Especially, the satellite with high elevation angle is highly important for land mobile users in the urban canyon and mountainous terrain. In Japan, there are many mountainous and urban areas with dense buildings and narrow roads. Under these conditions, it is expected that the augmentation of GPS by QZSS will improve the performance of GPS in such areas where we cannot see a sufficient number of GPS satellites due to some obstacles. Moreover, the effect of acceleration to Initial Operational Capability (IOC) of modernized civil signals in Asia Oceania region is considered in the paper as well as fundamental benefits such as availability and GDOP improvement. Since a couple of QZS can be observed in the region, we can obtain higher availability for new modernized signals earlier than other area in the world. The result of simulation, which it could be one or two years earlier to reach 100% availability of new modernized signals, is shown in this paper. Japan and US have been discussed how to implement full interoperability and compatibility between GPS and QZSS since GPS-QZSS Technical Working Group (TWG) was established in the 2nd US-Japan GPS plenary meeting in 2002. Both parties have agreed that GPS and QZSS have complete compatibility and interoperability in the 5th TWG meeting on January 26, 2006. QZSS will transmit navigation signals, which have same center frequency, spectrum, bandwidth, PRN code family and message structure as civil signals of current and modernized future GPS on L1, L2 and L5 band. These signals will be emitted to the whole visible earth surface from the satellite including not only Japanese neighbors but also East Asia and Oceania region. There are small differences on the QZSS message definitions from GPS legacy NAV message and CNAV message. Although they come from the difference of the orbit and configuration of payload, users can receive QZSS signals and calculate their position combining GPS signals with tiny modification of existing GPS receiver. The most important feature of GPS-QZSS collaboration is joint study for L1C signal. We agreed to establish joint Interface Specification document in order to maximize our interoperability between GPS and QZSS. QZSS is to be the first satellite to transmit this brand new L1C signal in the world. Adding to above mentioned navigation signals, the signal, called L1-Sub meter class Augmentation with Integrity Function (SAIF) and LEX signal will be planed on L1 and E6 band, respectively. SAIF is aiming to provide WDGPS correction message based on SBAS message format to enhance GPS performance. LEX signal, an experimental signal, is planed to be transmitted for the experimental purpose using faster bit rate characteristics. The 2 Kbps message stream is expected to utilize several experiments and validation for new error correction methods, then commercial use after experimental phase. The government of Japan decided to promote the QZSS program as a satellite navigation system after the industry’s announcement of difficulty with their commercial business. One of the main changes was to change from a multi mission role to a single mission role of navigation only for the 1st satellite. Another change was to develop the program in a step by step manner. The first step is to launch the first satellite by 2009 (objective) and to accomplish technical validation and application demonstration. The second step is to launch the additional satellites several years later and to demonstrate system operability. The satellite bus system and satellite tracking & control stations for the 1st satellite has to be developed by JAXA (Japan Aerospace Exploration Agency). JAXA have been taking charge of R&D for positioning payload and ground facilities for positioning, integration of total experiment system and conducting system validation (experimental operation) in collaboration with other Japanese institutes [1]. Preliminary Design Review (PDR) for the navigation system was finished in March, 2006 and started manufacturing of navigation payload. Besides development of onboard equipment, we performed several simulations and analyses for user positioning accuracy and QZS orbit and clock estimation as well. From the results of our analyses, the accuracy of SIS (Signal in Space, i.e., orbit + clock) URE (User Range Error) is expected approximately 30 cm (1-sigma) and the User Positioning Error (UPE) with GPS-QZSS combination is to be almost equivalent or slightly better than UPE by only GPS signals. EM of QZSS navigation payload will be finalized by the end of March, 2007 and CDR will be held in March of 2007. The QZSS system development is on going toward first satellite launch successfully. The current status and future development plan are described in this paper.
Published in:	Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006) September 26 - 29, 2006 Fort Worth Convention Center Fort Worth, TX
Pages:	2092 - 2102
Cite this article:	Kogure, S., Sawabe, M., Kishimoto, M., "Status of QZSS Navigation System in Japan," Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006), Fort Worth, TX, September 2006, pp. 2092-2102.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In