Quasi-Zenith Satellites System (QZSS)

QZSS (Michibiki) has been operated as a four-satellite constellation from November 2018, and three satellites are visible at all times from locations in the Asia-Oceania region. QZSS can be used in an integrated way with GPS, ensuring a sufficient number of satellites for stable, high-precision positioning. QZS are compatible with GPS and receivers can be procured at a low cost, so it is expected that position information businesses utilizing geographical and spatial information will be developed.

Satellite-based positioning systems have been used in a broad array of fields including car navigation and land surveying, and have become indispensable to daily lives. The Quasi-Zenith Satellites System (QZSS) uses multiple satellites that have the same orbital period as geostationary satellites with some orbital inclinations (their orbits are known as “Quasi-Zenith Orbits”). These satellites are placed in multiple orbital planes, so that one satellite always appears near the zenith above the region of Japan. The system makes it possible to provide high accuracy satellite positioning service covering close to 100% of Japan, including urban canyon and mountain terrain.

Through the development and deployment of this system, Japan aimed to enhance Japan’s satellite positioning technology and contribute to building safe and secure society with enhanced satellite based positioning, navigation and timing technologies. The satellite positioning system is not just for car navigation. It is imperative for mapping, measurements for construction work, monitoring services for children and senior citizens, automatic control of agricultural machinery, detecting earthquakes and volcanic activities, weather forecasting and many other applicable fields. Therefore, an improvement in accuracy and reliability is called for from various areas. New service using more accurate positioning data may be born when positioning accuracy is further improved by the QZSS.

In some urban or mountainous areas, positioning signals from four satellites are often hampered by kyscrapers or mountains, and that has often caused significant errors. The QZSS consists of a multiple number of satellites that fly in the orbit passing through the near zenith over Japan. By sharing almost the same positioning signals for transmission with the currently operated GPS as well as the new GPS, which is under development in the U.S., the system enables us to expand the areas and time duration of the positioning service provision in mountainous and urban regions in Japan.

Furthermore, the QZSS aims at improving positioning accuracy of one meter to the centimeter level compared to the conventional GPS error of tens of meters by transmitting support signals and through other means. In order to have at least one quasi-zenith satellite always flying near Japan's zenith, at least three satellites are necessary. The first quasi-zenith satellite "MICHIBIKI" carried out technical and application verification of the satellite as the first phase, then the verification results will be evaluated for moving to the second phase in which the QZ system verification will be performed with three QZ satellites.

The orbit of the quasi-zenith satellite is unique to applications in Japan. The geostationary orbit is generally above the equator. If the geostationary orbit is inclined to move, this creates an orbit that passes just above Japan. However, that is not enough to have a satellite always on top of Japan because the orbit is inclined, thus a satellite is gradually changing its angle and moving toward the south or the north due to the Earth’s rotation. Consequently, a satellite can stay over Japan only for seven to nine hours. By flying a multiple number of satellites that take turns, there would always be one satellite above Japan all the time.

The footprint of a quasi-zenith satellite orbit looks like the figure "eight (8)" (or the satellite looks like depicting "eight (8)" as it flies) if you look at its flight path while having the Earth stationary. Therefore, it was originally nicknamed a "Figure-8 satellite." Some other orbits apart from the figure-8 orbit were also considered. If the orbit is inclined to the equator and has a larger eccentricity to have its apogee position over Japan, it is possible to maximize the time duration of the satellite staying over Japan. In such a case, the orbit becomes "asymmetry figure-8" or the "shape of a tear." The orbit that is adopted for the Quasi-Zenith Satellite System has an advantage that it can easily switch connection from satellite to satellite when the system is used for satellite broadcasting services. Its merit is that a satellite can stay a longer time over Japan compared to the symmetry figure-8 orbit.

The MICHIBIKI was launched by the H-IIA Launch Vehicle No. 18 on September 11, 2010. After being injected into the quasi-zenith orbit, the MICHIBIKI was under a three-month initial functional verification. Then, its technical and application verification will be carried out in cooperation with concerned organizations.

The usage of the QZS at high elevation angles in combination with GPS, improves availability of satellite positioning for areas that enough numbers of GPS signals could not be received such as urban canyon and mountain terrain. In order to facilitate GPS availability enhancement, the navigation signals and messages of the QZSS have complete interoperability with those of GPS. Users can receive advanced positioning service combining GPS and QZSS without being aware of the difference between the two systems. Enhanced GPS performance contributes to improving users' convenience by achieving high accuracy and reliability of positioning through the transmission of error correction data, availability, and other GPS signal information received at the reference staions for a quasi-zenith satellite.