QZSS
Encyclopedia
The Quasi-Zenith Satellite System (QZSS), is a proposed three-satellite regional time transfer
Time transfer
Time transfer is a scheme where multiple sites share a precise reference time. Time transfer solves problems such as astronomical observatories correlating observed flashes or other phenomenon with each other, as well as cell phone towers coordinating handoffs as a phone moves from one cell to...

 system and Satellite Based Augmentation System for the Global Positioning System
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...

, that would be receivable within Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

. The first satellite 'Michibiki' was launched on 11 September 2010. Full operational status is expected by 2013.

Authorized by the Japanese government in 2002, work on a concept for a Quasi-Zenith Satellite System (QZSS), or Juntencho (準天頂) in Japanese
Japanese language
is a language spoken by over 130 million people in Japan and in Japanese emigrant communities. It is a member of the Japonic language family, which has a number of proposed relationships with other languages, none of which has gained wide acceptance among historical linguists .Japanese is an...

, began development by the Advanced Space Business Corporation (ASBC) team, including Mitsubishi Electric Corp., Hitachi Ltd., 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; of Internal Affairs and Communications,; of Economy, Trade and Industry; and the Ministry of Land, Infrastructure and Transport.

QZSS is targeted at mobile applications, to provide communications-based services (video, audio, and data) and positioning information. With regards to its positioning service, QZSS can only provide limited accuracy on its own and is not currently required in its specifications to work in a stand-alone mode. As such, it is viewed as a GNSS Augmentation
GNSS Augmentation
Augmentation of a global navigation satellite system is a method of improving the navigation system's attributes, such as accuracy, reliability, and availability, through the integration of external information into the calculation process. There are many such systems in place and they are...

 service. Its positioning service could also collaborate with the geostationary satellites in Japan's Multi-Functional Transport Satellite
Multi-Functional Transport Satellite
Multifunctional Transport Satellites are a series of weather and aviation control satellites. They are geostationary satellites owned and operated by the Japanese Ministry of Land, Infrastructure and Transport and the Japan Meteorological Agency , and provide coverage for the hemisphere centred on...

 (MTSAT), currently under development, which itself is a Satellite Based Augmentation System similar to the U.S. Federal Aviation Administration
Federal Aviation Administration
The Federal Aviation Administration is the national aviation authority of the United States. An agency of the United States Department of Transportation, it has authority to regulate and oversee all aspects of civil aviation in the U.S...

's Wide Area Augmentation System
Wide Area Augmentation System
The Wide Area Augmentation System is an air navigation aid developed by the Federal Aviation Administration to augment the Global Positioning System , with the goal of improving its accuracy, integrity, and availability...

 (WAAS).

The satellites would be placed in a periodic Highly Elliptical Orbit
Highly Elliptical Orbit
A highly elliptical orbit is an elliptic orbit with a low-altitude perigee and a high-altitude apogee. It is a type of high Earth orbit....

 (HEO). These orbits allow the satellite to dwell for more than 12 hours a day with an elevation above 70° (meaning they appear almost overhead most of the time) and give rise to the term "quasi-zenith" for which the system is named. Similar orbits are used by the Sirius Satellite Radio system (Tundra orbit
Tundra orbit
Tundra orbit is a type of highly elliptical geosynchronous orbit with a high inclination and an orbital period of one sidereal day . A satellite placed in this orbit spends most of its time over a chosen area of the Earth, a phenomenon known as apogee dwell...

). As of June 2003, the proposed orbits ranged from 45° inclination
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...

 with little eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

, to 53° with significant eccentricity.

QZSS and positioning augmentation

QZSS can enhance GPS services in two ways: first, availability enhancement, whereby the availability of GPS signals is improved, second, performance enhancement whereby the accuracy and reliability of GPS derived navigation solutions is increased.

Because the GPS availability enhancement signals transmitted from Quasi-Zenith Satellites are compatible with modernized GPS signals, and hence interoperability is ensured, the QZSs will transmit the L1C/A signal, L1C signal, L2C signal and L5 signal. This minimizes changes to specifications and receiver designs.

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 QZS. 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 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 QZS, 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 or the planned 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 novel system. An outline of this concept as well as two possible implementations of the time synchronization network for QZSS were studied and published in Fabrizio Tappero's PhD work.

See also

  • Global Navigation Satellite System
    Global Navigation Satellite System
    A satellite navigation or SAT NAV system is a system of satellites that provide autonomous geo-spatial positioning with global coverage. It allows small electronic receivers to determine their location to within a few metres using time signals transmitted along a line-of-sight by radio from...

     (GNSS)
  • Multi-functional Satellite Augmentation System
    Multi-functional Satellite Augmentation System
    Multi-functional Satellite Augmentation System is a Japanese SBAS , i.e. a satellite navigation system which supports differential GPS designed to supplement the GPS system by reporting on the reliability and accuracy of those signals...

    (MSAS)

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK