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Hi, I'm Tom Neumann, I'm the project  scientist on the ICESat-2 mission.

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NASA's Ice, Cloud and land Elevation Satellite-2,  or ICESat-2 as we call it, recently celebrated

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its second year on orbit after launching from  Vandenberg Air Force Base on September 15, 2018.

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Hi, I'm Kaitlin Harbeck and I'm the data product  manager for NASA's Ice, Cloud and land Elevation

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Satellite-2, or ICESat-2 for short. For this  Photon Phriday example, we actually took data

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from the same orbit collected on two different  dates to capture the clearest, least cloudy data

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examples. Neumann: ICESat-2's main objective  is to measure the elevation of the Earth, and

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as the name suggests, it's optimized to measure  changes in icy areas. The way it does that is to

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transmit a very small pulse of laser light--green  laser light in our case--and precisely time how

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long that light takes to go from the spacecraft  to the ground and back up again. By combining

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that round-trip travel time information with  information on where the satellite is in orbit

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and what direction it's pointing, through ground  processing, we can determine what the elevation

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of the Earth is beneath the satellite. This is an  example of the data we collect, known as ATL03,

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the global geolocated photon product. Hi I'm  Nathan Kurtz, the deputy project scientist for

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ICESat-2. Now we're into the Arctic, this is  where I'm familiar with, especially Arctic sea

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ice. You can see it kind of looks like an ocean  return, but there's not quite as much structure.

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The surface isn't as rough as, say, an ocean that  has a lot of swell and waves. So that return isn't

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as thick. The little structure that you do see is  really from the ridges and things that are in the

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ice. So the ice kind of gets crushed together  in places, and so that causes those returns.

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So now we're over Greenland and you can see  Greenland's really high. You can see this really

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like dome-like structure, and that's interesting  because it's how the ice sheet forms. Like it gets

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really high up near the center and gravity is  just pushing this down all the time, and so the

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ice is slowly flowing out to the ocean. So you get  this dome-like structure, but obviously here we're

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seeing more topography, things like mountains and  stuff that the ice flows around. Neumann: For each

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of the photons detected by ICESat-2 on orbit,  in ground processing, we determine the latitude,

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the longitude and the elevation of each one of  those photons. Harbeck: These large collections

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of photons make up what we call the photon cloud.  The photon clouds you're seeing on the screen now

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are examples of this ATL03 data product collected  along reference ground track number 1352.

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These photon heights have been corrected  for various geophysical phenomena,

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such as atmospheric effects and tides. Neumann:  The photons colored in green indicate photons

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that ground processing has identified as most  likely reflecting off the surface of the Earth.

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Over the oceans, we can see that the surface  is very flat, much like we would expect.

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If we zoomed in on some of these areas we can  see that the ATLAS instrument aboard ICESat-2

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is actually able to detect individual waves and  ocean swells that are just slightly higher or

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slightly lower than the surrounding area. We also  notice that the width of the photon cloud changes

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around the orbit. Over oceans where the surface  is relatively flat, that photon window is fairly

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narrow because we have a very good idea ahead of  time of where the ocean surface should be. As we

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transition onto land the width of that photon  cloud gets much larger because there's a lot of

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roughness and topography over the terrestrial  parts of Earth or over the ice sheets. And so

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ICESat-2 sends down a much wider band of photons  over these areas in order to capture that surface.

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We notice that in some areas there's relatively  few photons, and at other points around the orbit,

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the plot is almost solid with photons, and  that's mainly due to the effects of the Sun.

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ATLAS uses green laser light, and it turns out  that the Sun also emits a lot of light in the

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green part of the spectrum. So when ICESat-2  is collecting data over sunlit surfaces,

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we see a lot of background photons in addition  to the photons reflected off the Earth.

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In contrast, over dark surfaces at night,  we see relatively few background photons,

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and we get a really clear surface return. Kurtz:  That sharp diagonal feature in the photon return

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is from something called the transmitter echo  path, or TEP. It comes from routing of some of

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the transmit laser power through to the detectors  directly. Because we have two different TEP paths

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to know exactly how long the path delay should  take, we're able to use deviations in the TEP

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to calibrate the instrument to account for  things like thermal variations.

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Harbeck: At times ATLAS' onboard signal finding that is  used as a first filter to approximate where the

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ground surface is located is unable to find  the surface returns, owing to the reflection

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of sunlight from clouds. In those cases, the  telemetry band may or may not include the surface.

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The telemetry band can change every 200  shots, or roughly 140 meters along the

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satellite's track, which takes roughly 5  one hundredths of a second to complete.

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The discontinuous bands of telemetered data and  blocky features that you may be seeing on your

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screen are due to ATLAS detecting clouds  and not necessarily the ground surface.

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Neumann: As we transition across the center of Antarctica,  one area of interest for me anyway is a place

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called Hercules Dome. The National Science  Foundation has funded a number of deep ice

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coring efforts in Antarctica over the past several  decades that are all aimed to drill down through

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the ice sheet and collect ice from far back in  time. The ice closest to the surface of the ice

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sheet is the youngest, having fallen as snow  relatively recently. And as we drill deeper

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and deeper into the ice sheet, we collect ice  that fell as snow many thousands of years ago.

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Hercules Dome is an interesting place to drill  an ice core because it sits near the mountains,

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right at the edge between the East Antarctic  ice sheet and the West Antarctic ice sheet.

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Models and observations suggest that the  West Antarctic ice sheet is a much more

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dynamic ice sheet than the much larger and most  likely much older East Antarctic ice sheet.

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And by drilling right at Hercules Dome,  scientists hope to be able to understand

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how those two ice sheets have changed  through time relative to each other.

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Harbeck: ICESat-2 has a 91-day repeat  orbit cycle and a 92-degree inclination.

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ATL03 is one of the primary sources for  all of the photon information required

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by higher-level data products, such as land  ice height and sea ice freeboard.

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Neumann: As ICESat-2 transitions from taking data  over land to collecting data over ocean,

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at times you'll notice we have a return off the  ocean surface, but we also have a fainter second

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return. And that's due to photons penetrating  down through the water column and reflecting off

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the seabed before returning to ICESat-2. In that  way we're able to use ICESat-2 data to determine

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shallow water bathymetry around the planet.  Determining bathymetry is not one of ICESat-2's

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primary objectives, but it is something the  scientific community is very interested in.

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In very clear water, photons from ICESat-2  can penetrate down through the water column,

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reflect off the ocean floor and travel back up  to the spacecraft. By looking at our data so far,

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we note that ICESat-2 can measure up to 30  meters of water depth, or nearly 100 feet. As

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the turbidity of water increases or there's more  waves at the surface, we can only see very shallow

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water depths, perhaps just a few meters. Measuring  bathymetry with ICESat-2 is one of the many things

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that we've discovered in our data over just the  first two years of the mission. Scientists are

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just publishing initial papers, and we expect  many more discoveries over the following years.