1 00:00:00,000 --> 00:00:10,240 Explain how scientists use LIDAR to help them measure the aerosols in the atmosphere. 2 00:00:10,240 --> 00:00:15,440 Here in our lab at NASA Langley, we use a technique called active remote sensing. 3 00:00:15,440 --> 00:00:17,920 Now that means that we carry our own light source. 4 00:00:17,920 --> 00:00:20,840 We don't wait around for the sun to shine on the object. 5 00:00:20,840 --> 00:00:26,440 And we use, what we do is we use short pulses of laser light to probe the atmosphere. 6 00:00:26,440 --> 00:00:28,600 This technique is called LIDAR. 7 00:00:28,600 --> 00:00:31,520 LIDAR stands for Light Detection and Ranging. 8 00:00:31,520 --> 00:00:37,040 A LIDAR uses short pulses of laser light to detect aerosols in the atmosphere. 9 00:00:37,040 --> 00:00:43,360 NASA Langley is involved in active remote sensing from the ground and in the air. 10 00:00:43,360 --> 00:00:50,040 At NASA Ames in California, LIDAR is flown in a high altitude ER-2 aircraft to record 11 00:00:50,040 --> 00:00:52,000 atmospheric data. 12 00:00:52,000 --> 00:00:57,840 And at NASA Dryden, also in California, a high altitude solar powered unpiloted airplane 13 00:00:57,840 --> 00:01:03,560 is being developed that can stay aloft for weeks, even months, at a time to make atmospheric 14 00:01:03,560 --> 00:01:04,560 measurements. 15 00:01:04,560 --> 00:01:07,320 So, how does LIDAR work? 16 00:01:07,320 --> 00:01:15,840 Well, first of all, we open this trap door and then align our LIDAR under the open sky. 17 00:01:15,840 --> 00:01:20,120 Then we shoot a pulsed laser beam into the atmosphere. 18 00:01:20,120 --> 00:01:25,840 Some of that laser beam scatters off the tiny aerosol particles and scatters light into 19 00:01:25,840 --> 00:01:27,280 this telescope. 20 00:01:27,280 --> 00:01:30,320 The light is then captured by this detector. 21 00:01:30,320 --> 00:01:35,480 By precisely timing the laser pulse going out into the atmosphere and the reflected 22 00:01:35,480 --> 00:01:41,160 light coming back to the telescope, scientists can accurately measure the location and number 23 00:01:41,160 --> 00:01:42,160 of aerosols. 24 00:01:42,160 --> 00:01:47,360 Now, remember, this is active remote sensing, much like that flash on the camera. 25 00:01:47,360 --> 00:01:53,720 Okay, I understand passive and active remote sensing and how LIDAR works, but how do you 26 00:01:53,720 --> 00:01:57,640 measure the distance from the ground to the aerosols in the atmosphere? 27 00:01:57,640 --> 00:01:59,120 You can't use a meter stick. 28 00:01:59,120 --> 00:02:06,920 Okay, let's say you wanted to measure something far away, say like aerosols in the sky. 29 00:02:06,920 --> 00:02:11,360 You're right, you wouldn't use a meter stick. 30 00:02:11,360 --> 00:02:14,720 Scientists at NASA Langley use mathematics. 31 00:02:14,720 --> 00:02:21,200 A pulse of laser light is shot from point A. The beam travels from point A to the aerosols 32 00:02:21,200 --> 00:02:22,200 at point B. 33 00:02:22,680 --> 00:02:28,360 Then, the light reflects off the aerosols and bounces back to point A. If you know how 34 00:02:28,360 --> 00:02:33,680 fast it takes for a pulse of laser light to travel, and you know a little math, then you 35 00:02:33,680 --> 00:02:37,480 can calculate how far away the aerosols are in the atmosphere. 36 00:02:37,480 --> 00:02:38,480 Check it out. 37 00:02:38,480 --> 00:02:45,600 It takes a pulse of light one nanosecond to travel one third of a meter. 38 00:02:45,600 --> 00:02:47,120 What is a nanosecond? 39 00:02:47,120 --> 00:02:50,800 A nanosecond is one billionth of a second. 40 00:02:50,800 --> 00:03:00,960 Basically, it's a really small amount of time. 41 00:03:00,960 --> 00:03:12,160 Anyway, if a scientist shoots a pulse of laser light at the sky, and that beam reflects back, 42 00:03:12,160 --> 00:03:24,680 say in 6,000 nanoseconds, the aerosols in the sky are really 3,000 nanoseconds away. 43 00:03:24,680 --> 00:03:25,680 Why? 44 00:03:25,680 --> 00:03:32,400 Because you have to divide the total time by two in order to find the time one way. 45 00:03:32,400 --> 00:03:33,400 Got it? 46 00:03:33,400 --> 00:03:39,440 So, if you multiply the time one way, 3,000 nanoseconds, by the number of meters in a 47 00:03:39,440 --> 00:03:44,440 nanosecond, one third, you get 1,000 meters. 48 00:03:44,440 --> 00:03:48,480 If you know how to convert meters to kilometers, you can calculate that the aerosols in the 49 00:03:48,480 --> 00:03:51,880 sky are one kilometer away. 50 00:03:51,880 --> 00:03:55,600 Well, that should answer your question, Van. 51 00:03:55,600 --> 00:03:59,560 But you know, in order to get the whole picture, we need to measure aerosols from space. 52 00:03:59,560 --> 00:04:03,280 Let me call a few colleagues of mine over at Hampton University who are working with 53 00:04:03,280 --> 00:04:07,880 NASA Langley Ball Aerospace and the French Space Agency to get them to explain to you 54 00:04:07,880 --> 00:04:10,280 how we can measure aerosols from space.