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  3. When early explorers sailed round the world, they use the stars to show【M1

When early explorers sailed round the world, they use the stars to show【M1

游客2023-10-16  32
问题       When early explorers sailed round the world, they use the stars to show【M1】________
them their position.Now drivers can use satellite navigation systems,【M2】________
satnavs for short, to tell them their route in exact the same way.The【M3】________
only difference is that satnavs use artificial stars-satellites.
      There are more then 20 satellites in the Global Positioning System【M4】________
（GPS） .The GPS satellites are about 20, 000 kilometers on the Earth.【M5】________
They have atomic clocks which tell the time to an accurate of one second【M6】________
in 300,000 years.
      The satnav compares the positions three or four satellites and works【M7】________
out exactly where it is.It does this several times by a second and is usually【M8】________
accurate to 20 meters anywhere in the world.But the satnav user need to【M9】________
give it accurate informations.Drivers sometimes get lost and end up in the【M10】________
wrong street, the wrong town, or even the wrong country. [br] 【M8】
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