Photon Phriday: One Phull Orbit



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