The Quasi Zenith Satellite System (QZSS) is a regional space-based positioning system that uses a constellation of satellites placed in multiple orbital planes. The satellites have the same orbital period as a traditional equatorial geostationary orbit, however, they have a large orbital inclination and therefore move with respect to the Earth. The QZSS orbits are also elliptical and are sometimes known as “highly-inclined elliptical orbits” or HEO. The system covers regions in East Asia and Oceania centering on Japan, and is designed to ensure that users are able to receive positioning signals from a high elevation at all times.

QZSS enhances GPS services in the following ways: 1) availability enhancement (improving the availability of GPS signals); and 2) Performance enhancement (increasing the accuracy and reliability of GPS signals).

QZSS enhances standalone GPS availability for any user that has visibility to, and can track, one or more QZSS satellites. This enhancement will be the greatest for users in the region of Japan because the constellation design is optimized for that area. However, users in many other Asia-Pacific areas will also benefit from the enhanced geometric arrangement made possible by QZSS. This increases area and times at which positioning is possible in both urban and mountainous locations. The system also improves positioning accuracy by transmitting signals (L1C/A, L1C, L2C and L5) that are equivalent to modernized GPS signals.

Because the GPS availability enhancement signals transmitted from QZSS satellites are compatible with modernized GPS signals and interoperability is ensured, the system uses modernized GPS signals as a base, transmitting the L1C/A signal, L1C signal, L2C signal and L5 signal. This minimizes changes to specifications and receiver designs.

QZSS further improves standalone GPS accuracy by means of ranging correction data provided through transmission of submeter-class performance enhancement signals L1-SAIF and LEX. QZSS 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.

The QZSS project will be promoted incrementally in accord with the government's basic policy released on March 31, 2006 as follows.
First step: First QZSS satellite will be launched in 2009. Technical validation and application demonstration will be conducted.
Second Step: The 2nd and 3rd QZSS satellites will be launched several years later. Full system operation will be demonstrated.