十万个为什么·科学启蒙30篇（1）
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How Do GPS Devices Know Your Exact Location?
GPS设备如何知道你的精确位置？
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GPS devices receive signals from at least four satellites orbiting Earth at high speed.
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Each signal includes the satellite’s location and the exact time the message was sent.
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The device calculates distance by measuring how long each signal took to arrive.
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Using geometry called trilateration, it finds where three spheres intersect in space.
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A fourth satellite helps correct timing errors and improves accuracy to within five meters.
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GPS works day or night, in rain or clouds, because radio waves pass through them easily.
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Your phone combines GPS with Wi-Fi and cell tower data to get even faster fixes indoors.
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Maps and apps use this location data to give directions or suggest nearby places.
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Military systems first developed GPS, but now civilians use it worldwide every day.
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Engineers keep improving satellites so future GPS may guide self-driving cars safely.