The Quasi-Zenith Satellite System (QZSS) is a three-satellite regional time transfer system in development and the satellite-based augmentation system for the Global Positioning System that would be receivable within Japan. The first satellite "Michibiki" was launched on 11 September 2010. Full operational status was expected by 2013. In March 2013, Japan's Cabinet Office announced the expansion of the Quasi-Zenith Satellite System from three satellites to four. The $526 million contract with Mitsubishi Electric for the construction of three satellites is slated for launch before the end of 2017. The basic four-satellite system is planned to be operational in 2018.
Authorized by the Japanese government in 2002, work on a concept for a Quasi-Zenith Satellite System (QZSS), or Juntencho eisei shisutemu (?????????) in Japanese, began development by the Advanced Space Business Corporation (ASBC) team, including Mitsubishi Electric, Hitachi, and GNSS Technologies Inc. However, ASBC collapsed in 2007. The work was taken over by the Satellite Positioning Research and Application Center. SPAC is owned by four departments of the Japanese government: the Ministry of Education, Culture, Sports, Science and Technology, the Ministry of Internal Affairs and Communications, the Ministry of Economy, Trade and Industry, and the Ministry of Land, Infrastructure, Transport and Tourism.
QZSS is targeted to provide highly precise and stable positioning services in the Asia-Oceania regions, while maintaining compatibility with GPS.
The third satellite was launched into orbit on August 19, 2017. The fourth was launched on October 10, 2017.
Video Quasi-Zenith Satellite System
Orbit
QZSS uses three satellites, each 120° apart, in highly inclined, slightly elliptical, geosynchronous orbits. Because of this inclination, they are not geostationary; they do not remain in the same place in the sky. Instead, their ground traces are asymmetrical figure-8 patterns (analemmas), designed to ensure that one is almost directly overhead (elevation 60° or more) over Japan at all times.
The nominal orbital elements are:
Maps Quasi-Zenith Satellite System
Satellites
QZSS and positioning augmentation
The primary purpose of QZSS is to increase the availability of GPS in Japan's numerous urban canyons, where only satellites at very high elevation can be seen. A secondary function is performance enhancement, increasing the accuracy and reliability of GPS derived navigation solutions.
The Quasi-Zenith Satellites transmit signals compatible with the GPS L1C/A signal, as well as the modernized GPS L1C, L2C signal and L5 signals. This minimizes changes to existing GPS receivers.
Compared to standalone GPS, the combined system GPS plus QZSS delivers improved positioning performance via ranging correction data provided through the transmission of submeter-class performance enhancement signals L1-SAIF and LEX from QZSS. It also improves reliability by means of failure monitoring and system health data notifications. QZSS also provides other support data to users to improve GPS satellite acquisition.
According to its original plan, QZSS was to carry two types of space-borne atomic clocks; a hydrogen maser and a rubidium (Rb) atomic clock. The development of a passive hydrogen maser for QZSS was abandoned in 2006. The positioning signal will be generated by a Rb clock and an architecture similar to the GPS timekeeping system will be employed. QZSS will also be able to use a Two-Way Satellite Time and Frequency Transfer (TWSTFT) scheme, which will be employed to gain some fundamental knowledge of satellite atomic standard behavior in space as well as for other research purposes.
QZSS timekeeping and remote synchronization
Although the first generation QZSS timekeeping system (TKS) will be based on the Rb clock, the first QZSS satellites will carry a basic prototype of an experimental crystal clock synchronization system. During the first half of the two year in-orbit test phase, preliminary tests will investigate the feasibility of the atomic clock-less technology which might be employed in the second generation QZSS.
The mentioned QZSS TKS technology is a novel satellite timekeeping system which does not require on-board atomic clocks as used by existing navigation satellite systems such as GPS, GLONASS, NAVIC or Galileo system. This concept is differentiated by the employment of a synchronization framework combined with lightweight steerable on-board clocks which act as transponders re-broadcasting the precise time remotely provided by the time synchronization network located on the ground. This allows the system to operate optimally when satellites are in direct contact with the ground station, making it suitable for a system like the Japanese QZSS. Low satellite mass and low satellite manufacturing and launch cost are significant advantages of this system. An outline of this concept as well as two possible implementations of the time synchronization network for QZSS were studied and published in Remote Synchronization Method for the Quasi-Zenith Satellite System and Remote Synchronization Method for the Quasi-Zenith Satellite System: study of a novel satellite timekeeping system which does not require on-board atomic clocks.
See also
- Global Navigation Satellite System (GNSS)
- Multi-functional Satellite Augmentation System (MSAS)
- Inclined orbit
References
External links
- Government Of Japan QZSS site
- JAXA QZSS site(in Japanese)
- JAXA MICHIBIKI data site (in Japanese)
- JAXA MICHIBIKI data site, English subsite
- JAXA Quasi-Zenith Satellite-1 "MICHIBIKI"
- JAXA MICHIBIKI Special Site
- JAXA QZSS Twitter account (in Japanese)
- ESA Navipedia QZSS article
Source of article : Wikipedia
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