Japan’s Plan for Centimeter-Resolution GPS

As reported by IEEE Spectrum: A stranger to Tokyo could easily get lost in its urban canyons. And GPS navigation, stymied by low resolution and a blocked view of the sky, might not be much help. But that won’t be the case after 2018. Engineers at Tokyo-based Mitsubishi Electric Corp. report that they’re on track to start up the first commercial, nationwide, centimeter-scale satellite positioning technology. As well as spot-on navigation, the technology will also usher in a variety of innovative new applications, its proponents say.

Named Quazi-Zenith Satellite System (QZSS), it is designed to augment Japan’s use of the U.S.-operated Global Positioning System (GPS) satellite service. By precisely correcting GPS signal errors, QZSS can provide more accurate and reliable positioning, navigation, and timing services.

Today’s GPS receivers track the distance to four or more GPS satellites to calculate the receiver’s position. But because of the various errors inherent in the GPS system, location can be off by several meters. In using the data from QZSS to correct the measured distance from each satellite, the accuracy of the calculated position is narrowed down to the centimeter scale.

“GPS positioning can be off by as much as 10 meters due to various kinds of errors,” says Yuki Sato, a research engineer in Mitsubishi Electric’s Advanced Technology R&D Center, the prime contractor for the space portion of the project. “And in Japan, with all its mountains and skyscrapers blocking out GPS signals, positioning is not possible in some city and country locations,” he adds.

The Japan Aerospace Exploration Agency (JAXA) got the project under way with the launch of QZS-1 in September 2010. Three additional satellites are slated to be in place by the end of 2017, with a further three launches expected sometime later to form a constellation of seven satellites—enough for sustainable operation and some redundancy. The government has budgeted about US $500 million for the three new satellites, which are to be supplied by Mitsubishi. It also apportioned an additional $1.2 billion for the ground component of the project, which is made up of 1200 precisely surveyed reference stations. That part’s being developed and operated by Quazi-Zenith Satellite System Services, a private company established for this purpose.

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.

“This is all done in real time, so compression is really important,” says Ryoichiro Yasumitsu, a deputy chief manager in Mitsubishi’s Space Systems Division. “It would take too long to transmit the original data.” 

Compression also means a practical-size antenna can be employed in the user’s receiver. In QZS-1 trial tests, Yasumitsu notes that the average accuracy is about 1.3 centimeters horizontally and 2.9 cm vertically.

This centimeter-scale precision promises to usher in a number of creative, or at least greatly improved, applications beyond car and personal navigation. Besides pointing out obvious uses like mapping and land surveying, Sam Pullen, a senior research engineer in the department of aeronautics and astronautics at Stanford, says precision farming and autonomous tractor operations will be big applications. “Unmanned aerial vehicles and autonomous vehicles in general,” he adds, “will also find centimeter-level positioning valuable in maintaining and assuring separation from other vehicles and fixed obstacles.”

In addition, the Japanese government plans to use the service to broadcast short warning messages in times of disaster, when ground-based communication systems may be damaged. As instructed by the government, the control center will transmit a brief warning message to the QZSS satellite, which will then broadcast it to users on the same frequency.

Given the range of promised applications and relatively low cost of the Japanese system compared with the €5 billion ($6.9 billion) budgeted for the EU’s Galileo, for instance, other nations will be watching and waiting to see if QZSS achieves its goals.