问答题 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.《问题》：【M5】