When we do a lunar calibration, it's once a month, and we do the maneuver
only at night time - or basically, when the spacecraft is at night time -
so we don't have to worry about the sun hitting the spacecraft someplace we don't want it to hit.
So we do all our maneuvering in the dark.
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
the slew is starting, the spacecraft is maneuvering to point to the moon
It slews out to the moon and it stands by there for just a short time
until it starts the scans.
It's a push-broom instrument, so it basically consists of 7000 detectors that build up an image
of the earth, like you would push a broom along the surface.
The actual focal plane is made up of a number of modules. There's actually 14 modules across
the focal plane. Each one has about 500 detectors.
And the spacecraft has to take each one and run it down
the centerline of the moon. Our scan pattern is a lot like just how a farmer would
plow a field. You plow down the field one way, turn around,
and move over, and come back. It basically moves the spacecraft so that
each one sees exactly the same field of view across the moon
We actually do this over a period of two orbits, because we don't have enough time in one orbit to get all
14 focal plane modules. That takes about 18 minutes for 8 scans
And then it will return back to Earth just as we're entering daylight.
And then the second orbit will do the rest of them.
The main reason we look at is because the moon is a stable source.
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.
By looking at the moon we can see drifts in the instrument much more precisely
than any other technique that we have. So if you're looking at something changing on the Earth
you know that it's really the Earth changing and not the instrument.