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.GPS设备接收至少四颗高速绕地球运行的卫星发来的信号。
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Each signal includes the satellite’s location and the exact time the message was sent.每条信号都包含卫星的位置及消息发出的确切时间。
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The device calculates distance by measuring how long each signal took to arrive.设备通过测量各信号到达所需时间来计算距离。
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Using geometry called trilateration, it finds where three spheres intersect in space.利用一种名为‘三边测量’的几何方法，确定三个球体在空间中的交点。
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A fourth satellite helps correct timing errors and improves accuracy to within five meters.第四颗卫星有助于校正计时误差，将定位精度提升至五米以内。
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GPS works day or night, in rain or clouds, because radio waves pass through them easily.GPS可全天候工作，雨天或阴天也不受影响，因为无线电波能轻松穿透雨云。
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Your phone combines GPS with Wi-Fi and cell tower data to get even faster fixes indoors.手机将GPS与Wi-Fi及蜂窝基站数据结合，从而在室内更快获取定位。
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Maps and apps use this location data to give directions or suggest nearby places.地图和应用程序利用该位置信息提供导航路线或推荐附近地点。
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Military systems first developed GPS, but now civilians use it worldwide every day.GPS最初由军用系统研发，如今全球民众每日都在使用。
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Engineers keep improving satellites so future GPS may guide self-driving cars safely.工程师持续升级卫星，未来GPS或将安全引导自动驾驶汽车。