On Saturday (September 11, 2010), Japan Aerospace Exploration Agency (JAXA) will launch its first navigation satellite, a quasi-zenith satellite, Michibiki. It aims to cover the blind spots left by the 30 US navigation satellites that are the foundation of the existing GPS system. This would enable people using GPS -equipped cell phones and car navigation systems in this country to receive more accurate information.
From an altitude of about 40,000 kilometres, Michibiki will send signals interchangeable with those of the GPS satellites, to locators. A navigation satellite transmits a stream of data on its location and the time, which is measured by an atomic clock. The receiver notes the difference between the time the data was sent by the satellite and the time it was received. Using this information, it can calculate its own distance from the satellite, and then determine its own location.
Radio waves travel at 300,000 kilometres per second, so an error of 0.000001 second in time results in an error of 300 meters in distance. To accurately determine its location, a locator needs to receive signals from at least four navigation satellites simultaneously.
Amongst 30 US navigation satellites, sometimes eight to 11 navigation satellites fly near Japan, but in mountainous areas or cities crowded with buildings, there are still times when it is difficult for GPS locators to receive signals from four satellites at the same time. Michibiki would be helpful in such cases because signals from quasi-zenith satellites are unlikely to be interrupted by obstacles.
The speed of radio waves is affected by atmospheric conditions, but the quasi-zenith satellite will send signals that order locators to adjust their calculations according to changes in the atmosphere.
Current cell phones and car navigation systems will not be able to directly receive signals from Michibiki. It will be necessary to develop receivers or software compatible with the signals the new satellite sends out. In future, Michibiki is also expected to be used for various other purposes, including controlling airplanes, detecting tsunami in its early stages, helping drivers avoid traffic jams and directing large automated agricultural machines as they move through fields.
About 10 years from now, space above Asia is expected to be crowded with navigation satellites. There will always be about 20 satellites flying over the continent, JAXA said. Countries have begun discussing how to share their navigation satellites at the United Nations.
Michibiki’s orbit takes it above Japan for about eight hours each day, so two more quasi-zenith satellites are needed to cover Japan for 24 hours.
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