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532 nanometers. That's the

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wavelength of a bright green color, the color of the ATLAS laser

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measuring the elevation of the Earth below. When ATLAS receives the photons back

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from Earth, it has to filter out the sunlight that naturally reflects off the Earth,

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which would overwhelm the detectors. So ATLAS only lets in

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light that is exactly 532 nanometers. Sunlight in that

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wavelength range will also set off ATLAS detectors, and so computer

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programs are needed to filter out those sunlight photons in the data. 

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