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How Do GPS Satellites Know Where You Are?
GPS卫星如何知道你的位置？
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The Global Positioning System uses at least 24 satellites orbiting Earth at about 20,200 km, each broadcasting precise time signals.全球定位系统（GPS）至少使用24颗卫星，环绕地球运行，轨道高度约20200公里，每颗卫星持续广播精确的时间信号。
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Your phone or GPS device calculates its distance from each satellite by measuring how long the signal takes to arrive.你的手机或GPS设备通过测量信号到达所需时间，计算出与每颗卫星的距离。
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Because radio waves travel at the speed of light, even a one-microsecond timing error would cause a 300-meter location mistake.由于无线电波以光速传播，即使1微秒的时间误差也会导致300米的定位偏差。
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Satellites carry ultra-accurate atomic clocks, and ground stations constantly correct small drifts caused by relativity effects.卫星搭载超高精度原子钟，地面站则持续校正相对论效应引起的微小漂移。
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To pinpoint your location in three dimensions, your device needs signals from at least four satellites simultaneously.要在三维空间中精确定位，你的设备需同时接收至少四颗卫星的信号。
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Each signal defines a sphere around that satellite, and your position lies where all four spheres intersect.每颗卫星的信号定义了一个以该卫星为中心的球面，你的位置即为这四个球面的交点。
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Buildings, trees, or mountains can block or bounce signals, causing multipath errors that reduce accuracy to 5–10 meters.建筑物、树木或山脉可能遮挡或反射信号，引发多径误差，使定位精度降至5–10米。
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Newer GPS chips combine data from multiple systems—including Europe’s Galileo and China’s BeiDou—for faster locks and better reliability.新型GPS芯片融合多个导航系统数据——包括欧洲“伽利略”和中国“北斗”，实现更快定位与更高可靠性。
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Emergency services use assisted GPS (A-GPS), which downloads satellite positions from cell towers to speed up initial fixes.应急服务采用辅助GPS（A-GPS），通过蜂窝基站下载卫星星历，加快首次定位速度。
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Your GPS doesn’t send your location to satellites—it only receives signals and computes position privately on your device.你的GPS设备不会向卫星发送位置信息，仅接收信号并在本地设备上独立计算位置。