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How GPS Knows Where You Are

頭上の30の衛星が時計を見せ合うことで、あなたの場所が分かる。

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Whenever you check your phone for directions, you are quietly receiving signals from at least four satellites about twenty thousand kilometers above your head. The system that sends them, GPS, was originally designed by the United States military. Today, it is open to anyone with a compatible device. The basic trick is timing. Each GPS satellite carries an extremely accurate clock and broadcasts its current time, every second, in all directions. Your phone receives those broadcasts and notes how long each signal took to arrive. Because radio waves travel at a known speed, even a tiny difference in arrival times — millionths of a second — translates into a position. With one satellite, your phone knows roughly how far you are from that satellite. With three, it can pinpoint a single spot on the surface of the Earth. With four, it can correct for errors in its own clock. The mathematics is older than the satellites; the precision is what is new. We rarely think about it, but billions of small location decisions every day depend on a quiet conversation happening above the clouds.

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  • Whenever you check your phone for directions文法

    スマホで道を確認するたびに

    〈whenever〉= 『〜するたびに』、複合関係詞。

  • The system that sends them, GPS, was originally designed by the United States military.文法

    それを送るシステム『GPS』は、もともとアメリカ軍によって設計された。

    関係詞 + コンマで挟む同格 (GPS)。受動態 was designed。

  • broadcasts its current time, every second, in all directions文法

    現在の時刻を毎秒、全方向に放送している

    副詞句の3並列。技術記述の典型。

  • translates into a position表現

    位置に変換される

    〈translate into〉= 『〜に変換される』、抽象動詞。

  • it can pinpoint a single spot on the surface of the Earth語彙

    地球表面の一点を絞り込める

    〈pinpoint〉= 『正確に特定する』、動詞。

  • The mathematics is older than the satellites文法

    数学は衛星より古い

    〈mathematics〉は語尾 -s だが単数扱い。

  • billions of small location decisions every day depend on a quiet conversation happening above the clouds文法

    毎日数十億の小さな位置決定が、雲の上で起きている静かな会話に依存している

    〈depend on〉+ 抽象主語。〈happening above the clouds〉= 現在分詞による後置修飾。

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