constellation of seven satellites—enough for sustainable operation and some redundancy

The four satellites will follow an orbit that, from the perspective of a person in Japan, traces an asymmetrical figure eight in the sky. While the orbit extends as far south as Australia at its widest arc, it is designed to narrow its path over Japan so that at least one satellite is always in view high in the sky—hence the name quasi-zenith. This will enable users in even the shadowed urban canyons of Tokyo to receive the system’s error-correcting signals.

“Errors can be caused, for example, by the satellite’s atomic clock, orbital shift, and by Earth’s atmosphere, especially the ionosphere, which can bend the signal, reducing its speed,” says Sato.

To correct the errors, a master control center compares the satellite’s signals received by the reference stations with the distance between the stations and the satellite’s predicted location. These corrected components are compressed from an overall 2-megabit-per-second data rate to 2 kilobits per second and transmitted to the satellite, which then broadcasts them to users’ receivers.

via Japan’s Plan for Centimeter-Resolution GPS – IEEE Spectrum.