Chapter 4: All the Easy Missions Are Done
Narration: Lauren Ward
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The shape of what we
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build live, work, study, operate--on whether it be on the Earth,
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the Moon, Mars, wherever we're going--matters.
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Knowing that at a scale where we can understand what's going to happen,
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what has happened and predict what could happen, is really important.
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Laser altimetry, as developed here at Goddard, went from an idea
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to try to capture that into something we can actually do.
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NASA's launched two next-gen lidar missions
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specifically to look closely at our changing planet.
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But if over three decades of lidar has taught us anything,
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it's that laser altimetry at Goddard is an evolution of technology,
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propelled by scientific curiosity in the face of almost certain setbacks.
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Take the story of the tree-measuring lidar: the Global Ecosystem
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Dynamics Investigation, or GEDI.
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GEDI had its genesis really in all
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the innovative work that had been done with lidar at Goddard.
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Some of those innovators, this was Jack Bufton, Bryan Blair at Goddard.
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Bryan had an instrument called SLICER that was flying around,
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taking these cool lidar transects.
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And then they put an instrument up in space, the Shuttle Laser Altimeter.
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And I saw some of that data and I thought, Wow, this is really cool.
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We've never been able to look at canopies of three dimensions like this.
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There's certainly got to be some applications to this.
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The Shuttle Laser Altimeter was the first real test for lidar
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and provided the momentum for MOLA to take on Mars.
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But it also gave us a glimpse at what lidar could measure on our own planet.
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And so the push for the Vegetation Canopy Lidar, or VCL, began.
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It was a really innovative mission.
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We were trying to do something
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that hadn't been done before, but we were optimizing it for vegetation.
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Vegetation is very different than if you're looking at ice,
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or if you're looking at Mars or if you're looking at the Moon
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because you have to have enough laser power to get through the canopy
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and get a strong return underneath the ground.
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The VCL team could build lasers strong enough,
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but they couldn't get them to last very long.
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And that proved too risky for very cautious NASA in the nineties.
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After that happened, we focused on the airborne lidar program,
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and this is again with Bryan Blair using that really innovative instrument
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he had called LVIS, the Land, Vegetation and Ice Sensor, I believe it's called.
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But if you really want to get down to really high resolution
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and looking at the sort of landscape-scales
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changes over the Earth, you need a swath mapping system.
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So sort of in the mid nineties we started working on LVIS, and you know,
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we really worked on that in large part because people said it couldn't be done
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and you know, it's really kind of drove us to
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to see how much we could get out of that system.
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missions were successful at keeping that momentum going,
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especially in the long periods between satellite launches.
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Sort of a core of us kept going year after year,
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going from one instrument opportunity to another
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and sort of making opportunities if we didn't have any.
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The thread that kept us all going was the airborne system.
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Airborne lidar really plays a role helping us
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understand how things work in real world settings.
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Yeah that's definitely definitely the best way to go
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was to build the hardware, get some data over real terrain and
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and actually, you know,
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show that it meets the requirements, that you can scale it to the space.
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But along the way also as we were flying, as we were collecting those data sets,
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we were releasing those publicly and letting people experiment with them
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and get comfortable with them.
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Dubayah, Blair and others leveraged the success of LVIS to propose
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a new satellite mission, DESDynI, a combined radar and lidar mission
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that could see through clouds down to tree canopies.
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However, NASA's budget cuts sidelined a couple of Earth science
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missions, and DESDynI was grounded indefinitely.
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And that was a devastating blow because we now been trying from 1995
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and now it's 2010, we've been trying to get a lidar
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that was meant just for vegetation structure into space
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using the best people in the world were at NASA's Goddard to do this.
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And at that point, I've been doing this 15 years.
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Maybe I'll just quit.
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But of course we really didn't quit.
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So we said, Well, let's look for another opportunity.
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That opportunity was on board the International Space Station with
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the GEDI instrument.
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GEDI wasn't just another successful lidar.
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It was the end of a very long road, hard fought by scientists and engineers,
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dedicated to pushing the limits of what lidar could do.
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We've been really pretty happy about the success of GEDI thus far.
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GEDI again is the first lidar that's been in space
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that was optimized to to measure vegetation structure.
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And it has it's created an enormous amount of data.
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We've conservatively done about ten billion estimates,
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about getting those tree heights and getting that canopy structure.
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But ultimately, we really wanted to get at the carbon content.
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What role do forests play in the carbon cycle?
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GEDI has been steadily gathering data,
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chipping away at the global question of just how much carbon dioxide trees
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take out of the atmosphere, a big piece of the climate puzzle.
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lidar missions are all about building on the past.
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Things that we in fact have only just begun
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to think about from pictures, now we have the third dimension.
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ICESat will add the third dimension.
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ICESat-2 will add the third dimension, the elevation.
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Pushing the technology to get at deeper science questions.
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And so came the next generation of ice-focused laser
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altimeters, aptly named ICESat-2.
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Its single instrument, ATLAS, was designed to precisely
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small changes in the shrinking, icy poles of Earth.
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To get down to that level of accuracy from space,
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everything had to be much better.
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It's this story of these incremental improvements through time,
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and with each mission, you're leveraging the lessons of the last mission.
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It wasn't a short process for ICESat-2, even though we knew a lot
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and had learned a lot over the last 20 or 30 years.
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Each mission has its own challenges.
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All of the easy missions are done, as they say.
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The first iteration of the
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instrument was going to be very similar to GLAS.
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As it turned out, the group that wanted the more complicated instrument won.
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So then they came back and said, okay, instead of digitizing,
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you know, 40 hertz or 50 hertz laser or whatever, we're going to fire
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this beam to the ground.
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And then individually by time tag each photon that comes back.
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There were so many requirements, there were so many constraints.
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We had constraints on the software capability.
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We had constraints on the storage space.
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We had constraints on the memory.
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By photon tagging, I mean they'd built a detector system and detector
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electronics, they were just--it was like
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a firehose of data coming in to us.
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ATLAS has six beams and it records elevations
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for each of those six beam 10,000 times a second, as long as there is reasonably
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clear skies that the laser light can go from the spacecraft to the ground
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and back again.
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The Earth is much more complicated to work with because of the clouds.
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The algorithm could easily be confused
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and start following, you know, the cloud surface.
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I was the lead for the receiver algorithms team.
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The responsibility of making this work fell on my shoulders.
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I had sleepless nights.
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I have to tell you,
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thinking that I wasn't going to be able
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to make this work.
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In order to maximize the return from these data,
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a key component was determining the location on Earth of the laser
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bounce point, a process called geolocation.
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So what we do and geolocation, we get the position of the satellite
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We get the pointing of the laser beam very accurately, and
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then we have the range from the altimeter, and we add all those together
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to give us where that bounce point came from.
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Without the geolocation, you have lots and lots of error and you wouldn't be able
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to measure the change in the height of the ice sheets.
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We overcame what I
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think was fairly insurmountable problems,
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but everybody took their own piece of the puzzle and everybody worked it.
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ICESat-2 launched in 2018 and months later began gathering data
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that shed new light on how fast the ice sheets are changing,
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how thick the sea ice cover was in the Arctic, and even measured
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beneath the surface of the water up to 30 meters, a kind of bonus science result
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for a team that worked tirelessly to push the limits
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of the ATLAS instrument.
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So you cannot just build just one lidar.
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You need to sustained team who's been building
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lidar for some time.