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