1 00:00:10,080 --> 00:00:14,400 Hi, I'm Tom Neumann, I'm the project  scientist on the ICESat-2 mission. 2 00:00:14,400 --> 00:00:20,240 NASA's Ice, Cloud and land Elevation Satellite-2,  or ICESat-2 as we call it, recently celebrated 3 00:00:20,240 --> 00:00:26,800 its second year on orbit after launching from  Vandenberg Air Force Base on September 15, 2018. 4 00:00:26,800 --> 00:00:32,400 Hi, I'm Kaitlin Harbeck and I'm the data product  manager for NASA's Ice, Cloud and land Elevation 5 00:00:32,400 --> 00:00:38,400 Satellite-2, or ICESat-2 for short. For this  Photon Phriday example, we actually took data 6 00:00:38,400 --> 00:00:43,600 from the same orbit collected on two different  dates to capture the clearest, least cloudy data 7 00:00:43,600 --> 00:00:48,080 examples. Neumann: ICESat-2's main objective  is to measure the elevation of the Earth, and 8 00:00:48,080 --> 00:00:53,200 as the name suggests, it's optimized to measure  changes in icy areas. The way it does that is to 9 00:00:53,200 --> 00:00:59,520 transmit a very small pulse of laser light--green  laser light in our case--and precisely time how 10 00:00:59,520 --> 00:01:04,720 long that light takes to go from the spacecraft  to the ground and back up again. By combining 11 00:01:04,720 --> 00:01:09,920 that round-trip travel time information with  information on where the satellite is in orbit 12 00:01:09,920 --> 00:01:14,400 and what direction it's pointing, through ground  processing, we can determine what the elevation 13 00:01:14,400 --> 00:01:19,760 of the Earth is beneath the satellite. This is an  example of the data we collect, known as ATL03, 14 00:01:19,760 --> 00:01:25,360 the global geolocated photon product. Hi I'm  Nathan Kurtz, the deputy project scientist for 15 00:01:25,360 --> 00:01:29,920 ICESat-2. Now we're into the Arctic, this is  where I'm familiar with, especially Arctic sea 16 00:01:29,920 --> 00:01:34,400 ice. You can see it kind of looks like an ocean  return, but there's not quite as much structure. 17 00:01:34,400 --> 00:01:41,200 The surface isn't as rough as, say, an ocean that  has a lot of swell and waves. So that return isn't 18 00:01:41,200 --> 00:01:47,440 as thick. The little structure that you do see is  really from the ridges and things that are in the 19 00:01:47,440 --> 00:01:53,840 ice. So the ice kind of gets crushed together  in places, and so that causes those returns. 20 00:01:53,840 --> 00:01:57,920 So now we're over Greenland and you can see  Greenland's really high. You can see this really 21 00:01:57,920 --> 00:02:03,920 like dome-like structure, and that's interesting  because it's how the ice sheet forms. Like it gets 22 00:02:03,920 --> 00:02:09,360 really high up near the center and gravity is  just pushing this down all the time, and so the 23 00:02:09,360 --> 00:02:15,120 ice is slowly flowing out to the ocean. So you get  this dome-like structure, but obviously here we're 24 00:02:15,120 --> 00:02:20,800 seeing more topography, things like mountains and  stuff that the ice flows around. Neumann: For each 25 00:02:20,800 --> 00:02:26,960 of the photons detected by ICESat-2 on orbit,  in ground processing, we determine the latitude, 26 00:02:26,960 --> 00:02:32,080 the longitude and the elevation of each one of  those photons. Harbeck: These large collections 27 00:02:32,080 --> 00:02:37,440 of photons make up what we call the photon cloud.  The photon clouds you're seeing on the screen now 28 00:02:37,440 --> 00:02:44,080 are examples of this ATL03 data product collected  along reference ground track number 1352. 29 00:02:44,080 --> 00:02:47,680 These photon heights have been corrected  for various geophysical phenomena, 30 00:02:47,680 --> 00:02:52,720 such as atmospheric effects and tides. Neumann:  The photons colored in green indicate photons 31 00:02:52,720 --> 00:02:58,000 that ground processing has identified as most  likely reflecting off the surface of the Earth. 32 00:02:58,000 --> 00:03:02,640 Over the oceans, we can see that the surface  is very flat, much like we would expect. 33 00:03:02,640 --> 00:03:07,520 If we zoomed in on some of these areas we can  see that the ATLAS instrument aboard ICESat-2 34 00:03:07,520 --> 00:03:12,800 is actually able to detect individual waves and  ocean swells that are just slightly higher or 35 00:03:12,800 --> 00:03:18,320 slightly lower than the surrounding area. We also  notice that the width of the photon cloud changes 36 00:03:18,320 --> 00:03:23,840 around the orbit. Over oceans where the surface  is relatively flat, that photon window is fairly 37 00:03:23,840 --> 00:03:28,960 narrow because we have a very good idea ahead of  time of where the ocean surface should be. As we 38 00:03:28,960 --> 00:03:34,400 transition onto land the width of that photon  cloud gets much larger because there's a lot of 39 00:03:34,400 --> 00:03:39,520 roughness and topography over the terrestrial  parts of Earth or over the ice sheets. And so 40 00:03:39,520 --> 00:03:45,510 ICESat-2 sends down a much wider band of photons  over these areas in order to capture that surface. 41 00:03:45,510 --> 00:03:47,440 42 00:03:47,440 --> 00:03:51,840 We notice that in some areas there's relatively  few photons, and at other points around the orbit, 43 00:03:51,840 --> 00:03:57,680 the plot is almost solid with photons, and  that's mainly due to the effects of the Sun. 44 00:03:57,680 --> 00:04:02,960 ATLAS uses green laser light, and it turns out  that the Sun also emits a lot of light in the 45 00:04:02,960 --> 00:04:07,280 green part of the spectrum. So when ICESat-2  is collecting data over sunlit surfaces, 46 00:04:07,280 --> 00:04:12,560 we see a lot of background photons in addition  to the photons reflected off the Earth. 47 00:04:12,560 --> 00:04:16,640 In contrast, over dark surfaces at night,  we see relatively few background photons, 48 00:04:16,640 --> 00:04:22,630 and we get a really clear surface return. Kurtz:  That sharp diagonal feature in the photon return 49 00:04:22,630 --> 00:04:30,160 50 00:04:30,160 --> 00:04:34,720 is from something called the transmitter echo  path, or TEP. It comes from routing of some of 51 00:04:34,720 --> 00:04:39,760 the transmit laser power through to the detectors  directly. Because we have two different TEP paths 52 00:04:39,760 --> 00:04:44,720 to know exactly how long the path delay should  take, we're able to use deviations in the TEP 53 00:04:44,720 --> 00:04:50,960 to calibrate the instrument to account for  things like thermal variations. 54 00:04:50,960 --> 00:04:56,240 Harbeck: At times ATLAS' onboard signal finding that is  used as a first filter to approximate where the 55 00:04:56,240 --> 00:05:00,800 ground surface is located is unable to find  the surface returns, owing to the reflection 56 00:05:00,800 --> 00:05:06,800 of sunlight from clouds. In those cases, the  telemetry band may or may not include the surface. 57 00:05:06,800 --> 00:05:11,840 The telemetry band can change every 200  shots, or roughly 140 meters along the 58 00:05:11,840 --> 00:05:16,240 satellite's track, which takes roughly 5  one hundredths of a second to complete. 59 00:05:16,240 --> 00:05:20,560 The discontinuous bands of telemetered data and  blocky features that you may be seeing on your 60 00:05:20,560 --> 00:05:26,550 screen are due to ATLAS detecting clouds  and not necessarily the ground surface. 61 00:05:26,550 --> 00:05:34,080 62 00:05:34,080 --> 00:05:38,880 Neumann: As we transition across the center of Antarctica,  one area of interest for me anyway is a place 63 00:05:38,880 --> 00:05:43,920 called Hercules Dome. The National Science  Foundation has funded a number of deep ice 64 00:05:43,920 --> 00:05:49,520 coring efforts in Antarctica over the past several  decades that are all aimed to drill down through 65 00:05:49,520 --> 00:05:55,600 the ice sheet and collect ice from far back in  time. The ice closest to the surface of the ice 66 00:05:55,600 --> 00:06:00,080 sheet is the youngest, having fallen as snow  relatively recently. And as we drill deeper 67 00:06:00,080 --> 00:06:06,640 and deeper into the ice sheet, we collect ice  that fell as snow many thousands of years ago. 68 00:06:06,640 --> 00:06:10,960 Hercules Dome is an interesting place to drill  an ice core because it sits near the mountains, 69 00:06:10,960 --> 00:06:15,440 right at the edge between the East Antarctic  ice sheet and the West Antarctic ice sheet. 70 00:06:15,440 --> 00:06:19,040 Models and observations suggest that the  West Antarctic ice sheet is a much more 71 00:06:19,040 --> 00:06:25,120 dynamic ice sheet than the much larger and most  likely much older East Antarctic ice sheet. 72 00:06:25,120 --> 00:06:28,800 And by drilling right at Hercules Dome,  scientists hope to be able to understand 73 00:06:28,800 --> 00:06:35,240 how those two ice sheets have changed  through time relative to each other. 74 00:06:35,240 --> 00:06:42,080 Harbeck: ICESat-2 has a 91-day repeat  orbit cycle and a 92-degree inclination. 75 00:06:42,080 --> 00:06:46,240 ATL03 is one of the primary sources for  all of the photon information required 76 00:06:46,240 --> 00:06:51,440 by higher-level data products, such as land  ice height and sea ice freeboard. 77 00:06:51,440 --> 00:06:56,560 Neumann: As ICESat-2 transitions from taking data  over land to collecting data over ocean, 78 00:06:56,560 --> 00:07:01,360 at times you'll notice we have a return off the  ocean surface, but we also have a fainter second 79 00:07:01,360 --> 00:07:06,480 return. And that's due to photons penetrating  down through the water column and reflecting off 80 00:07:06,480 --> 00:07:13,200 the seabed before returning to ICESat-2. In that  way we're able to use ICESat-2 data to determine 81 00:07:13,200 --> 00:07:18,160 shallow water bathymetry around the planet.  Determining bathymetry is not one of ICESat-2's 82 00:07:18,160 --> 00:07:22,400 primary objectives, but it is something the  scientific community is very interested in. 83 00:07:22,400 --> 00:07:27,360 In very clear water, photons from ICESat-2  can penetrate down through the water column, 84 00:07:27,360 --> 00:07:33,040 reflect off the ocean floor and travel back up  to the spacecraft. By looking at our data so far, 85 00:07:33,040 --> 00:07:38,560 we note that ICESat-2 can measure up to 30  meters of water depth, or nearly 100 feet. As 86 00:07:38,560 --> 00:07:43,680 the turbidity of water increases or there's more  waves at the surface, we can only see very shallow 87 00:07:43,680 --> 00:07:49,760 water depths, perhaps just a few meters. Measuring  bathymetry with ICESat-2 is one of the many things 88 00:07:49,760 --> 00:07:54,320 that we've discovered in our data over just the  first two years of the mission. Scientists are 89 00:07:54,320 --> 00:08:02,517 just publishing initial papers, and we expect  many more discoveries over the following years.