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When we do a lunar calibration, it's once a month, and we do the maneuver

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only at night time - or basically, when the spacecraft is at night time - 

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so we don't have to worry about the sun hitting the spacecraft someplace we don't want it to hit.

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So we do all our maneuvering in the dark.

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We'll take the instrument and point it away from the Earth and we'll point it up at the moon. Around the time you enter eclipse

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the slew is starting, the spacecraft is maneuvering to point to the moon

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It slews out to the moon and it stands by there for just a short time

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until it starts the scans.

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It's a push-broom instrument, so it basically consists of 7000 detectors that build up an image

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of the earth, like you would push a broom along the surface.

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The actual focal plane is made up of a number of modules. There's actually 14 modules across

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the focal plane. Each one has about 500 detectors.

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And the spacecraft has to take each one and run it down

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the centerline of the moon. Our scan pattern is a lot like just how a farmer would

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plow a field. You plow down the field one way, turn around,

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and move over, and come back. It basically moves the spacecraft so that

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each one sees exactly the same field of view across the moon

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We actually do this over a period of two orbits, because we don't have enough time in one orbit to get all

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14 focal plane modules. That takes about 18 minutes for 8 scans

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And then it will return back to Earth just as we're entering daylight.

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And then the second orbit will do the rest of them.

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The main reason we look at is because the moon is a stable source.

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It doesn't have any seasons, or any rain to change the soil color or anything like that. It's as stable a thing as we can find.

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By looking at the moon we can see drifts in the instrument much more precisely 

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than any other technique that we have. So if you're looking at something changing on the Earth

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you know that it's really the Earth changing and not the instrument.