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What is the Quasi-Zenith Satellite System (QZSS)?

Why is QZSS required today?

Main area where QZSS is avalable

Main area where QZSS is avalable

Navigation functions—such as those used in car navigation systems, smartphones, and mobile phones—have come into widespread usage because they are very convenient. These convenient functions use positioning signals from satellites for satellite positioning services that make it possible to detect your current location. These services are also used in fields such as surveying, disaster prevention, etc.

However, the satellite positioning services so far had utilized GPS satellites operated by the United States. Due to reasons such as the small number of satellites in the field of vision, services were not always stable.

QZSS (Michibiki) has been in operation since November 2018 to develop a satellite positioning service that can be used stably in all locations at all times. This system is compatible with GPS satellites and can be utilized with them in an integrated fashion. In this way, the satellite positioning service environment was advanced dramatically.

QZSS can be used even in the Asia-Oceania regions with longitudes close to Japan, so its usage will be expanded to other countries in these regions as well.

Satellite deployment plan

The first Quasi-Zenith Satellite (QZS-1) was launched on September 11, 2010, and the Japan Aerospace Exploration Agency (JAXA) operated QZSS.
Afterward, the cabinet decided in September 2011 that the government would establish a four-QZS constellation and complete a seven-QZS constellation in the future. This system is also regarded as an important political measure in the Basic Plan on Space Policy of January 2013.

Due to these circumstances, the government decided to develop the additional three satellites (two with a quasi-zenith satellite orbit [QZO] and one with a geostationary orbit [GEO]), which were launched in FY2017. In this way, a four-satellite constellation has been operated since November 2018. Development and operation are conducted via a private finance initiative (PFI) project and Quasi-Zenith Satellite System Services Inc. (QSS) operates the four satellites, including QZS-1.

Increasing the number of QZS to improve positioning errors

To improve multipath errors (caused by reflection off buildings and other objects) and satellite constellation errors, it is necessary to increase the number of satellites used for positioning. However, there is not a sufficient number of GPS satellites to perform high-precision positioning with these satellites alone. And because GPS satellites are operated by the United States, the number cannot be increased. Therefore, positioning errors will be improved by increasing the number of QZS that are compatible with GPS.

It is said that more visible satellites are ideal for carrying out stable, highly precise positioning. QZSS has become a four-satellite constellation from 2018, increasing the number of visible satellites together with GPS. It enables stable positioning even in urban or mountainous areas where fields of vision are obstructed by buildings, trees, or other objects.

Positioning with multiple frequencies to resolve ionospheric errors

To resolve ionospheric errors, it is necessary to perform positioning that combines multiple frequencies (L1 with L2 or L5). The initial-stage GPS satellites—excluding military frequencies—had only one frequency, but the latest GPS satellites have multiple frequencies. All four QZS can transmit multiple frequencies, including the first satellite.

In addition to increasing the number of multi-frequency GPS, by using it in an integrated way with QZS, it will make it possible to approach precise positioning accuracy.

Number of multi-frequency satellites(consumer use) Total GPS QZS
2021 27 satellites
(20 for L5)
23 satellites
(16 for L5)
4 satellites
2027 (forecasted) 36 satellites
(29 for L5)
29 satellites
(22 for L5)
7 satellites