THIS
PLANET EARTH: THE VISION AND MAJESTY OF
NASA’S REMOTE SENSING LEGACY
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The
TERRA Era
SCANNING SAN FRANCISCO WITH ASTER
Image 1 Animation
The
Advanced Spaceborne Thermal Emission and Reflection Radiometer,
otherwise known as ASTER, is not only capable of detecting
thermal data, but also recording scenes in stereo. This allows
visualizers to depict various locations in striking three-dimensional
representations, like this view of San Francisco.
As
the sequence starts, we see first an overhead picture of the
city. The scene shifts to show a number of different electromagnetic
bands of information reflected back to the instrument, each
providing different information to researchers. Finally, we
fly in close to the city and soar over virtual depictions
of its surrounding hills, over its fabled harbor, and finally
come in for a landing at the airport.
CIRCLING
THE CRATER: ASTER SEES MT. ST. HELENS
Image
2 Animation
This
Advanced Spaceborne Thermal Emission and Reflection Radiometer
(ASTER) instrument on the Terra satellite acts like a zoom
lens for the other four instruments on board. It is uniquely
capable among them in generating elevation data, land surface
temperatures and much more. ASTER is the Terra instrument
that most allows for mapmaking of a given region.
This
visualization is an excellent example. Taken on August 8,
2000, the sequence shows a virtual fly-around of Mt. St. Helens
volcano in Washington State. The area shown is 37 kilometers
by 51 kilometers (roughly 18.6 by 30.6 miles).
A
part of the Cascade Range of mountains, Mt. St. Helens began
to wake from more than a hundred years of slumber in the beginning
of 1980. When it ultimately erupted on May 18 of that year,
the energy released actually caused the disintegration of
the mountain’s top. Heavy ash and gas rose like an ominous
gray plume high into the atmosphere. Around 60 people died
in the eruption and its resulting after effects. The blast
also killed just about everything in an area about 180 square
kilometers (70 square miles), while residents much farther
away found themselves contending with a fine rain of volcanic
ash for days to come.
Since
the primary eruption, the total height of the summit has fallen
by more than 400 meters, and the crater opening now yawns
wide. Periodic spurts and burps of volcanic life gurgle from
the mountain.
Computer
experts enhanced the colors shown in this visualization to
reflect a more familiar look of a wooded area. ASTER data
taken in the visible and near infrared parts of the spectrum
were draped over a digital topographic model, itself created
by the 3-D stereo imaging capabilities of the instrument.
The vertical relief of the image has been exaggerated by a
factor of two to enhance the surface features.
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CASE
STUDY: EL RENO, OKLAHOMA
 Image
3 Animation
One
of the more powerful aspects of Terra’s remote sensing capabilities
is how it can collect data at not only different spatial levels
of resolution, but also can assimilate different kinds of
data into highly synergistic research issues.
In
this visualization we see an example. Starting with a close
look at El Reno, Oklahoma, we see data from the ASTER instrument
showing us a variety of local features, from vegetation to
surface temperature to evaporation rates.
As
the scene pulls back to show a different scale, we compare
a false color vegetation index with a more natural, or true
color, image.
The
scene expands again to show Oklahoma in context of the greater
United States. Starting with a true color picture of the lower
forty-eight, we dissolve to colors showing the "leaf
area index". Leaf area index is a measure of total coverage
over a given section of ground by leaves and other foliage.
This is useful for understanding not only if an area is healthy,
but how it behaves in relation to growing conditions, like
climate, weather, and human influence. In this part of the
sequence, we also see surface temperatures for the nation.
Finally,
we pull back to the ultimate perspective. We see a true color
representation of the globe first. The colors shift to show
overall biological productivity, depicted by a color scale
of light blue to dark green in terms of grams of carbon per
square meter per day.
At
different levels of resolution, with different types of data
available to researchers, Terra can show regions of the Earth
in comparative terms. By those comparisons, experts can assess
what present conditions are on the ground to a highly accurate
degree, as well as recognize how a given region is changing
over time, or changing in relation to surrounding regions.
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A
THICKNESS OF SKY
Image 4 Animation
In
these side by side images we see how MISR can help researchers
better understand how events on the ground can have significant
effects on the atmosphere. On the left we see a stripe of
ground running across the border between Idaho and Montana.
As the image scrolls down, we see tendrils of smoke drifting
up from fires burning. On the right side of the screen we
see a corresponding image showing data about the atmosphere
above the region. The data displayed is a visual representation
of what’s called "aerosol optical thickness", a
measurement of the amount of light absorbed by the smoke and
haze in the atmosphere.
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FIRE
SIGNATURES IN IDAHO
Image 5 Animation
ASTER
can not only get in close to a subject on the ground, but
can also assess its thermal characteristics. This can provide
useful information for officials on the ground in dealing
with fires, volcanoes, and other natural events. Here we see
ASTER data of fires that happened last summer in Idaho. The
scene first shows the area of the fires, zooms in for a closer
look, then shows a thermal signal indicating hot spots on
the ground. Notice the blackened area surrounding the hot
regions. These are burn scars from the fire.
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NINE
EYES ON MONTANA FIRES
Image 6 Animation
Montana
suffered severe woodland losses to intense fires during the
summer of 2000. Fires tormented authorities and residents
across the western United States while evidence of the disaster’s
immense scale floated across the country. Heavy smoke and
aerosols traveled as far east as the Great Lakes.
Scientifically,
there was much to be learned.
The
power of the Terra platform is its ability to look at an event
with a variety of different instruments. As we saw in the
previous section, ASTER is well suited for determining thermal
properties of places on Earth. In the case of the Montana
fires we consider a different instrument on board Terra. It’s
called MISR, the multi-angle spectroradiometer.
MISR
is a single instrument composed of nine different cameras.
By using images from those cameras either in combination,
alone, or in sequence, sophisticated information can be gleaned.
The first component of this example shows a still picture
of the fire region in Montana. We see haze over the area as
smoke drifts high into the atmosphere.
As
MISR’s nine cameras cycle through the scene, the change in
perspective allows us not only to see different angles of
the ground and the particles of clouds themselves, but also
to measure cloud height. The process is clever. Higher altitude
clouds appear to move further as the camera images cycle from
side to side. Lower altitude clouds move less. By knowing
the absolute height of the spacecraft relative to the Earth’s
surface, it’s a relatively straightforward mathematical process
to derive the height of clouds. As clouds are a major component
of climate, as well as a significant tell-tale of more regional
Earth conditions, cloud height measurements can tell an expert
a great deal about the area of the Earth he or she is studying.
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CATCHING
THE LIGHT IN THE CRACKS
Image 7 Animation
MISR’s
nine cameras are useful for surface measurements, too. On
the Pine Island glacier in Antarctica, a large crack has recently
appeared. In very little time last year, the crack spread
more than 25 kilometers (15 miles) across the glacier. Researchers
are watching it carefully; they expect it to break off and
become a huge iceberg sometime next year.
The
value of a space based perspective is the ability to take
in a wide area. With MISR, detailed analysis of the surface
feature can be studied across the entire length of the fissure.
By using images taken by the forward, nadir, and aft cameras,
we can see differences in reflectance in the crack very clearly.
This helps scientists track the crack as it grows as well
as better understand the forces that led to its formation.
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SIDEBAR:
WHAT CALIBRATION MEANS
 Image
8 Animation
Imagine a symphony orchestra taking their seats and lifting
their instruments without the concertmaster playing a tuning
note. On the first downbeat, the audience might recognize
the advertised concerto, but they certainly wouldn’t appreciate
the raucous din of all those musicians out of tune. Said in
terms of scientific hardware, the various musicians would
not have properly calibrated their instruments to each other.
Calibration
is simply a term that describes a process of conforming to
a standard. For example, all thermometers will measure the
freezing point of water, but unless properly calibrated they
cannot determine how much the temperature of a quantity of
water had changed.
In
terms of remote sensing, calibration is vital for analysis
of data gathered by different instruments. Consider the following
example: two spacecraft with different types of instrumentation
are to be sent over a particular part of the Earth’s surface.
Unless certain standards for measurement are determined by
the research teams and imposed on the instruments, the measurements
will have no absolute relevance to each other. Researchers
might be able to identify an image, but the value of that
data as compared to other relevant instrumentation would be
zero.
To
put it in terms of our musicians again, if a clarinet and
a violin each play the same note, but the musicians have not
tuned their instruments to each other in advance, their simultaneous
playing of that same note will sound dissonant. No standard
will have been determined, and thus the resulting product
will be of marginal, if any, value.
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The
Dawning Future: EO-1 Operational on Orbit
HYPERION:
A NEW VIEW OF EARTH
Image 9 Animation
It’s not so much that the Hyperion instrument will be able
to see the Earth more "close up" or have a higher
spatial resolution than previous instruments. Yet Hyperion’s
goals are nothing less than ambitious. The instrument is designed
to gather highly complex data from a given region on the Earth
by viewing the surface in terms of 220 distinct "bands"
or colors of light. Think of looking at a photograph in black
in white and then comparing the exact same frame in color.
Even though there is no greater resolution to the image, no
change in perspective, lighting, or magnification, the amount
of data presented to the viewer has greatly increased. Project
managers designed Hyperion to fill in that kind of data in
observed regions on the ground.
The
uses for an instrument than can make such fine spectral distinctions
include studies of land use, changes in land cover, mineral
resource assessment, research into coastal processes, changes
in the atmosphere and more.
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OBSERVATIONS
ON THE EDGE OF THE FUTURE
Image 10
In
this visualization we see data gathered by Hyperion over a
portion of Argentina. As the ground scrolls by in the imaging
window, we see how the instrument assesses the surface features.
Data about the surface features being observed by the instrument
appear in the graphical readout. As scientists begin to use
Hyperion data more and more, they expect to be able to quantify
surface features in ways never before possible at planetary
scales.
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HYPERION,
THINLY SLICED
Image 11 Animation
The
principal reason Hyperion offers such powerful research opportunities
for scientists is its ability to slice reflected light into
more than 220 individual wavelengths. It doesn’t see much
more of the spectrum, but it sees light in significantly more
subtle gradations. In this visualization, we see how light
is broken into bands for processing by the Landsat instrument.
Next to it we see a comparison to Hyperion’s spectral capabilities.
Hyperion slices the spectrum into thin colors, offering highly
precise measurements of surface features.
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THE
FUTURE OF EARTH IMAGING: THE ADVANCED LAND IMAGER
 Image
12 Animation
In
many ways, the Advanced Land Imager (ALI) embodies the engineering
ideal that less is more. A principle component to the EO-1
mission, ALI is an Earth observing instrument designed to
generate images of the planet based on various wavelengths
of light reflected from the surface. Project designers developed
the instrument to be comparable with or exceed the capabilities
of Landsat’s Enhanced Thematic Mapper Plus. Further, the EO-1
project team designed ALI to deliver these images at a significant
reduction in weight, technical complexity, and cost-- all
vital features to facilitating development of advanced Earth
observing satellites.
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CLEARING
THE VIEW: THE ATMOSPHERIC CORRECTOR
Image 13 Animation
Between
the Earth and any satellite on orbit lies an ocean through
which all information must pass. It’s the atmosphere, and
to the highly precise sensors of delicate orbiting systems,
it can be just like looking through a cloudy or warped window.
For researchers, this is a problem that must always be taken
into account when looking at Earth from space. But the EO-1
project will test a new device designed to compensate for
atmospheric distortion. It’s called The Atmospheric Corrector
(AC). If proven effective, such a device will likely be applicable
to other scientific or commercial remote sensing missions
where water vapor or other particles in the atmosphere might
cause measurements of the surface to degrade.
Until
now, experts have generally compensated for atmospheric distortion
by using predicted or modeled mathematical values for how
much the atmospheric layer between the Earth and their instrument
causes changes to images. But EO-1’s Atmospheric Corrector
changes that strategy. By gathering actual, real time information
about how the atmosphere distorts images from the ground,
scientists can calibrate their sensors to create significantly
clearer images of what they’re studying. The device should
provide significant improvements in generating accurate surface
reflectance measurements for land imaging missions. Further,
the algorithms developed for use with the Atmospheric Corrector
will enable more accurate measurements and classification
of land resources and better models for land management in
the future.
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A
STRING OF PEARLS: ENHANCED FORMATION FLYING
Animation
By
design, EO-1 is primarily tasked to study the surface of the
Earth. But the satellite’s reason for being is the essentially
the next logical step in fulfilling the mandate put forth
by the 1992 Land Remote Sensing Policy Act (Section 105 of
Public Law 102-555). That act calls for the development of
a sound data policy for information collected by Landsat 7.
So, if the law speaks about Landsat, how does EO-1 fit in?
If
the technologies prove their promise, the new experimental
satellite begins to light a way for future, continuing development
of the Landsat data legacy. EO-1’s Advanced Land Imager, its
Hyperion hyperspectral imager, and the new Atmospheric Corrector
all have direct application to the issue of providing next
generation Landsat type data; the two satellites share a common
ancestry.
To
that end, a novel experiment is being conducted with EO-1
and Landsat 7 working in concert. In the first satellite maneuver
of its kind, EO-1 and Landsat 7 will assume an orbital "formation",
flying approximately one minute apart on the same ground track.
In terms of distance, this will place the two spacecraft approximately
270 miles (450 kilometers) apart, plus or minus 30 miles (50
kilometers) or so.
This
affords scientists and engineers the opportunity to do some
valuable tests. By flying the same route so close together,
nearly identical images taken by each satellite can be compared
on the ground. As potentially powerful improvements to existing
technologies, use of the Advanced Land Imager and the Hyperion
instruments on EO-1 in concert with Landsat overflights refine
the calibration. Of more immediate interest is the opportunity
to try EO-1’s Atmospheric Corrector as a tool for refining
data collected by its fellow satellite Landsat, flying one
minute ahead.
There’s
a lot of information expected from EO-1. For each scene the
spacecraft generates, over 20 gigabits of scene data from
the Advanced Land Imager, Hyperion, and Atmospheric Corrector
will be collected and stored on the on-board solid state data
recorder at high rates. When the EO-1 spacecraft is in range
of a ground station, the spacecraft will automatically transmit
its recorded image to the ground station for temporary storage.
Enhanced
Formation Flying (EFF) also tests highly sophisticated software
systems, including so-called "fuzzy logic" algorithms
to resolve navigational and operational conflicts that inevitably
occur in flight. Some of the benefits of flying satellites
in formation come in the area of risk management. By using
small fleets of less expensive, less complex satellites in
place of singularly large, highly sophisticated platforms,
a catastrophic failure does not necessarily cause irreparable
harm to an overall mission. Further, by flying a suite of
sensors in formation, researchers can essentially create one
enormous "virtual" satellite by integrating the
data collected individually by each smaller instrument.
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