WEBVTT FILE 1 00:00:00.050 --> 00:00:04.090 When we do a lunar calibration, it's once a month, and we do the maneuver 2 00:00:04.110 --> 00:00:08.140 only at night time - or basically, when the spacecraft is at night time - 3 00:00:08.160 --> 00:00:12.170 so we don't have to worry about the sun hitting the spacecraft someplace we don't want it to hit. 4 00:00:12.190 --> 00:00:16.200 So we do all our maneuvering in the dark. 5 00:00:16.220 --> 00:00:20.230 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 6 00:00:20.250 --> 00:00:24.250 the slew is starting, the spacecraft is maneuvering to point to the moon 7 00:00:24.270 --> 00:00:28.300 It slews out to the moon and it stands by there for just a short time 8 00:00:28.320 --> 00:00:32.320 until it starts the scans. 9 00:00:32.340 --> 00:00:36.350 It's a push-broom instrument, so it basically consists of 7000 detectors that build up an image 10 00:00:36.370 --> 00:00:40.390 of the earth, like you would push a broom along the surface. 11 00:00:40.410 --> 00:00:44.430 The actual focal plane is made up of a number of modules. There's actually 14 modules across 12 00:00:44.450 --> 00:00:48.490 the focal plane. Each one has about 500 detectors. 13 00:00:48.510 --> 00:00:52.500 And the spacecraft has to take each one and run it down 14 00:00:52.520 --> 00:00:56.550 the centerline of the moon. Our scan pattern is a lot like just how a farmer would 15 00:00:56.570 --> 00:01:00.620 plow a field. You plow down the field one way, turn around, 16 00:01:00.640 --> 00:01:04.640 and move over, and come back. It basically moves the spacecraft so that 17 00:01:04.660 --> 00:01:08.650 each one sees exactly the same field of view across the moon 18 00:01:08.670 --> 00:01:12.650 We actually do this over a period of two orbits, because we don't have enough time in one orbit to get all 19 00:01:12.670 --> 00:01:17.700 14 focal plane modules. That takes about 18 minutes for 8 scans 20 00:01:17.720 --> 00:01:23.440 And then it will return back to Earth just as we're entering daylight. 21 00:01:23.460 --> 00:01:26.760 And then the second orbit will do the rest of them. 22 00:01:26.780 --> 00:01:29.790 The main reason we look at is because the moon is a stable source. 23 00:01:29.810 --> 00:01:36.820 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. 24 00:01:36.840 --> 00:01:40.860 By looking at the moon we can see drifts in the instrument much more precisely 25 00:01:40.880 --> 00:01:44.880 than any other technique that we have. So if you're looking at something changing on the Earth 26 00:01:44.900 --> 00:01:56.029 you know that it's really the Earth changing and not the instrument.