QZSS: design visibility around a region

Japan’s Quasi-Zenith Satellite System, QZSS, is also known as Michibiki. It was developed to complement satellite positioning in the Asia-Oceania region, especially around Japan. The historical four-satellite service began in November 2018. The program includes both spacecraft in quasi-zenith orbits and spacecraft in geostationary orbits, so the family name does not imply that every member follows the same path. Michibiki-7 is one later spacecraft in that program.

In a city or a mountain valley, a receiver may have an obstructed view of much of the sky. A satellite at a high elevation angle — open definition can sometimes remain visible when a lower one is hidden. The engineering problem is therefore not only how many satellites exist, but where their signals appear in the user’s sky and whether those signals can be received.

A quasi-zenith orbit is a geosynchronous orbit selected so the satellite spends a long part of its day high over the intended region. Its inclined, slightly elliptical motion produces an asymmetric figure-eight ground track — open definition. That figure eight is a trace over the rotating Earth, not a figure-eight path through space.

The spacecraft moves through the sky; it does not remain motionless directly above Tokyo. Several satellites can take turns providing favorable viewing geometry. A truly geostationary satellite remains over the equator, so simply asking one to hover over a non-equatorial city does not describe the same orbital solution. QZSS demonstrates how orbit design can be matched to a regional user need.

QZSS provides positioning signals and augmentation services. For example, its Sub-meter Level Augmentation Service transmits information used to reduce certain orbit, clock and ionospheric errors. Receiving corrections is different from receiving enough suitable ranging observations. A correction message cannot manufacture a missing line of sight through a building.

In a fictional valley experiment, one receiver obtains fresh correction information but sees too few usable positioning signals for its intended solution. The appropriate conclusion is that correction availability alone is insufficient. Check the receiver’s supported service, visible signals and measurement geometry. Neither a service name nor a high-elevation satellite guarantees a particular accuracy in every local environment.