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訳していただく時に下の語句を参照してください。
The Global Positioning System （GPS） enables anyone with a GPS receiver to determine exactly where they are on the Earth’s surface. It is a miracle for anyone who has ever been lost. It is based on 24 satellites nearly 18,000 kilometers above the Earth’s surface, each orbiting the Earth every 12 hours.
The way the system works is perhaps easiest to understand in two dimensions. If you ask someone in Hong Kong how far away from them you are and they say that you are 900 kilometers away, you draw a circle 900 kilometers in radius around Hong Kong and know that you are somewhere on that circle. If someone in Manila then tells you that you are 1,200 kilometers away from them, you draw a circle 1,200 kilometers in radius around Manila. This circle intersects the Hong Kong circle at two points, and you know that you must be at one of those points. If someone in Hanoi then says that you are 700 kilometers away, this third circle will intersect with only one of those two points. And this is where you are: in a boat about 200 kilometers off the coast of Vietnam.
The same principle works in three dimensions using four centers and four spheres. Your GPS receiver tells you how far it is from four satellites. If it knows exactly where each satellite is, it can tell you exactly where you are. The orbits of the satellites are very predictable and each GPS receiver has the details of where every satellite should be at any given time. The gravitational pull of the sun and moon can alter an orbit very slightly. These changes are monitored by the U.S. Department of Defense and this information is sent via the satellite to the receiver. The distance between the satellite and the receiver is calculated based on the time the satellite signal takes to reach the receiver. This requires the use of very accurate clocks.
By fixing a GPS receiver at one spot on the Earth’s surface and taking repeated readings, scientists can tell if that spot is actually moving. The Tsing Ma Bridge in Hong Kong, one of the world’s longest suspension bridges, is built to move with winds and traffic. But too much movement could seriously damage the bridge. Fourteen permanent GPS receivers are attached to various parts of the bridge to record the vertical and horizontal of each part 10 times a second. In Japan, a grid of more than 1,000 GPS receivers has been set up to monitor small, slow movements of the Earth that could provide early warning of an earthquake. Each receiver receives a signal every 30 seconds and can detect movements of less than one centimeter.
The GPS was developed by the U.S. military to guide its missiles and bombers to their targets （and to tell its soldiers where they are）. It is now offered free for civilian use. But the European Union is worried that because the GPS is operated by the U.S. military, there may be a conflict between its military and civilian use at some time in the future. So the EU has decided to develop its own system, to commence operations around 2007, to guarantee uninterrupted use by civilians.
【語句】
Global Positioning System （GPS） 「全地球測位システム」
radius 「半球」
Manila 「マニラ」
Hanoi 「ハノイ」
sphere 「球体」
details 「詳細」
the U.S. Department of Defense 「アメリカの国防総省」
Tsing Ma Bridge 「青馬橋」
European Union 「欧州連合 （EU）」
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