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90 meters. The

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distance between each laser pair. In order to measure the slope

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across the laser's path in a single pass of the satellite, ATLAS

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uses three pairs of beams spaced 90 meters apart with a 

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reference track between the beams. Then on following

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passes, even if the two beams are slightly upslope or downslope from

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where they were on the first pass, scientists can still calculate the elevation

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change of that key track.
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