What is QZSS (Quasi-Zenith Satellite System)?

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- everything RF

Aug 20, 2022

The Quasi-Zenith Satellite System (QZSS) is a regional navigation satellite system commissioned by the Japanese Government. QZSS is also referred to as the "Japanese GPS". It was authorized by the Japanese government in 2002 and the first satellite, was launched in 2010. It has four satellites and was developed by the Japanese government to enhance the United States-operated Global Positioning System (GPS) in the Asia-Oceania regions, with a focus on Japan. QZSS uses its compatibility with GPS to provide highly precise and stable positioning services in the Asia-Oceania region.

The four satellites of the QZSS are called “Michibiki” with serial numbers 1-4 respectively and comprise of one geostationary satellite and three satellites in the QZO orbit. These QZO orbits are Tundra-type highly inclined, slightly elliptical, geosynchronous orbits with each orbit 120° apart from the other two. 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 primary purpose of QZSS is to increase the availability of GPS in Japan's numerous urban canyons, where only satellites at very high elevations can be seen. QZSS operates at 1176.45 and 1575.42 MHz frequencies. Its compatibility with GPS ensures a sufficient number of satellites for stable, high-precision positioning in the Asia-Oceania region. Another function of QZSS is performance enhancement i.e. increasing the accuracy and reliability of GPS-derived navigation solutions. QZSS 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 by 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.

QZSS uses a timekeeping system (TKS) based on the positioning signal generated by a Rubidium clock. This eliminates the requirement of onboard atomic clocks. TKS technology is also used in GPS, GLONASS and Galileo systems and allows optimal operation when satellites are in direct contact with the ground station. QZSS can also use a Two-Way Satellite Time and Frequency Transfer (TWSTFT) scheme to gain some fundamental knowledge of satellite atomic standard behavior in space as well as for other research purposes.

Services Provided:

The QZSS provides three classes of public service:

  • The PNT (Positioning, Navigation and Timing) service complements the signals used by the GPS system, effectively acting as extra GPS satellites. The QZSS satellites sync their clocks with GPS satellites and also broadcast at bands L1C/A, L1C, L2C, and L5C, the same as GPS.
  • The SLAS (Sub-meter Level Augmentation) service provides a form of GNSS augmentation for GPS interoperable with other GPS-SBAS systems. It works on the same principle as of a Wide Area Augmentation System (WAAS). SLAS services are provided with transmissions on the L1 band.
  • The CLAS (Centimeter Level Augmentation) service provides high-precision positioning compatible with the higher-precision E6 service of Europe’s Galileo satellite system. The band is referred to as L6 or LEX.

The QZSS service will work in the East Asia and Oceania Region - With its primary region of operation being Japan.

Click here to see the frequency bands used by QZSS.