Japan's 1st navigation satellite good for GPS

Toshiaki Sato / Yomiuri Shimbun Staff Writer

In a nation crowded with mountains and skyscrapers, signals from navigation satellites sent to Global Positioning System devices on the ground, such as those in car navigation systems, are sometimes blocked by obstacles that make it difficult for the devices to accurately determine their locations.

However, Saturday's planned launch of Japan's first navigation satellite is expected to solve the problem, at least partly, as the so-called quasi-zenith satellite would spend significant time above Japan.

The Japan Aerospace Exploration Agency has been preparing to launch Michibiki (a name that means "guidance" in Japanese) to cover the blind spots left by the 30 U.S. navigation satellites that are the foundation of the existing GPS system. This would enable people using GPS locator-equipped cell phones and car navigation systems in this country to receive more accurate information.

It is now common to see people searching for the shortest route to their destination on car navigation systems or cell phones equipped with GPS locators, which calculate their location after receiving signals from multiple navigation satellites.

However, there are places where locators cannot calculate their locations accurately at certain times as signals sent from navigation satellites are sometimes interrupted by mountains and buildings.

From an altitude of about 40,000 kilometers, 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 kilometers 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.

Currently, 30 U.S. navigation satellites are covering the entire Earth. 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.

Currently, in some urban areas, about 10 percent of the time it is difficult for locators to calculate their locations accurately, but Michibiki would eliminate such gaps and enable locators to work accurately around-the-clock.

Michibiki also would help locators to correct errors in calculating their location.

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 the future, Michibiki is also expected to be used for many other purposes, including controlling airplanes, detecting tsunami in their early stages, helping drivers avoid traffic jams and directing large automated agricultural machines as they move through fields.

The GPS system was developed by the United States and was initially used only for military purposes. China, India, Russia and the European Union are developing and improving their own global navigation satellite systems.

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.

Koji Terada, manager of the Michibiki project at JAXA, said: "Michibiki's activities will determine whether the government decides to launch two more satellites. What we must do now is make sure Michibiki's technology tests will succeed."