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300 trillion. That's about how

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many photons in each laser pulse fired by the ATLAS instrument.

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It sounds like a lot, but as the photons sail through the

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atmosphere and bounce off the Earth, they reflect back, just like any

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light in all directions. ATLAS receives only the photons that reflect straight

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back to the instrument, just about a dozen photons. That window of

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photons received must be very small in order to keep the timing error of

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those photons very small, which allows scientists to calculate elevation

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very precisely.
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