“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.
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