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VO: The ATLAS instrument on ICESat-2 will measure heights

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on Earth by transmitting a pattern of six laser beams,

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receiving the laser photons that Earth reflects back up to the

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satellite, and recording the travel time of those photons.

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First up: Transmitting the laser.

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Tyler: Hey there, I'm Tyler Evans, an optical mechanical engineer working on the ATLAS

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project for the ICESat-2 mission. So what we have here is the optical bench, which

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has a lot of the transmitter components on it, which is where the laser starts before

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it bounces off a bunch of mirrors and goes out towards Earth to measure ice.

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So the laser comes out of the box off of the fold mirror, and the fold mirror folds it

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90 degrees. So it comes out towards the rest of the bench. Then it encounters this

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optic, which is the polarizing beam combiner, or the PBC,

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which sends the light in two different directions. So it picks off a small

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percentage of light to go through the periscope and into the laser sampling assembly.

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So one of the channels in the LSA is the start pulse detector,

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which is basically the stopwatch of starting and stopping that timer for how

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long it's going to take the photons to get back down to measure the ice and back. And that's

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really how the scientists are able to calculate what the elevation of ice is, is by knowing that

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time that each of the photons travels. The rest of the beam comes out through the PBC

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out toward the beam expander. It hits the small mirror first 

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and it's ten millimeters at this point. But when it hits this

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small mirror, the mirror has curvature to it, so what it does is it takes that ten millimeter beam

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and it starts to expand it, all the way out to 44 millimeters by

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the time it hits the second mirror. Once it hits the second mirror, it has curvature in it,

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which recolumnates it. So now, instead of expanding, it's now staying at a constant

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size of 44 millimeters all the way out to the rest of the bench.

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It then goes through the beam steering mirror, which is here. ATLAS has

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a mechanism to be able to actually steer the beam and compensate for changes

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that might happen on the bench. So as the spacecraft goes in and out of the Sun, thermal

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changes will happen to actually make the bench bend, and to compensate for those bends,

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you don't want to have the light bend as well, so we have a motorized mirror

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to be able to steer that beam so it's always pointing in the same direction no

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matter what happens to the bench. So the beam goes through the beam steering mirror,

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and then it actually goes through the DOE, which is the diffractive optical element,

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which sits right here under the bench. And it's a circular piece of glass, and it has

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an etched pattern in it, and that etched pattern basically takes that

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single beam and splits it into six different beams. So those six different beams

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then go out to Earth to be able to do the science.

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