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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.
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Your phone or GPS device calculates its distance from each satellite by measuring how long the signal takes to arrive.
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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.
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Satellites carry ultra-accurate atomic clocks, and ground stations constantly correct small drifts caused by relativity effects.
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To pinpoint your location in three dimensions, your device needs signals from at least four satellites simultaneously.
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Each signal defines a sphere around that satellite, and your position lies where all four spheres intersect.
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Buildings, trees, or mountains can block or bounce signals, causing multipath errors that reduce accuracy to 5–10 meters.
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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.
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Emergency services use assisted GPS (A-GPS), which downloads satellite positions from cell towers to speed up initial fixes.
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Your GPS doesn’t send your location to satellites—it only receives signals and computes position privately on your device.