| MISR, David Diner |
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M-I-S-R, the Multi-angle Imaging SpectroRadiometer, or MISR, is
a new type of instrument that's never flown in space before. It's performing
extremely well, and providing novel ways to physically
characterize the Earth's surface, atmosphere, and clouds, and how
they interact with sunlight, the primary energy source for Earth's
climate system. |
| Sunrise |
MISR's digital cameras are capturing exquisitely detailed color
imagery, such as this view of sunrise over Greenland and Baffin
Bay. The instrument observes reflected sunlight over a swath 400
kilometers wide, and can see objects as small as 275 meters, or
about the size of a sports stadium.
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MISR images the entire day side of every orbit as Terra flies from
pole to pole. What makes this instrument unique is that it has nine
separate cameras--looking forward, straight downward, and aftward
of the vertical--over a wide range of angles. As the spacecraft
moves along its flight path, it takes only seven minutes for any single
area to be imaged, in succession, at all nine angles. Let's visit a
few places, and explore them using this new type of vision.
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Our first destination is Hudson and James Bay in Canada. On the
left is a conventional view from the camera looking straight down,
and we see little color in this icy winter landscape. On the right, we
combine data in a single color band but from three different cameras,
encoding the forward view as blue, vertical as green, and aftward
as red. Our automated data processing software geometrically
superimposes every pixel from the different camera views. Color
serves as a proxy to illustrate how different scene elements reflect
light differently at different angles.
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| James Bay Pan |
Clouds, showing up as purple, now clearly stand out against the ice.
The glint-like reflection from smooth ice makes it appear light blue.
Rough ice appears orange because it preferentially reflects light
back toward the Sun. Variations in physical structure and texture are
manifested by different angular reflectance signatures.
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We next look at a view of the eastern United States from Lake
Ontario to Georgia, spanning the Appalachian mountains. Again we
begin with the conventional, straight downward view, and we see
what generally appears to be a clear scene.
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As we progressively increase the angle of view, we detect a pall of
haze over the Appalachians. This is similar to the effect you get
when you look at different angles out an airplane window. The slant
paths of the oblique images significantly increase our sensitivity to
atmospheric particulates.
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One application of this is for observing pollution. Here is a view of
the Himalayas, looking south from the Tibetan Plateau in the foreground
into the Ganges Basin of India in the background. In this
view, we've draped imagery from the vertical camera over the surface
topography.
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Switching to imagery from one of MISR's oblique angles clearly
emphasizes that India's low-lying areas are shrouded in haze. Such
variations in appearance enable MISR to determine haze amounts
and to distinguish different types of airborne particles. This is
important because they can impact our regional and global climate
in different ways.
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Our final example shows an assortment of clouds over Florida and
Cuba. This view is reoriented so that MISR's flight path is from left
to right, and we see Florida turned on its side.
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As we cycle through the nine cameras and change the angle of view
from forward to aftward, we see that the clouds show a displacement
from right to left. The majority of this displacement is due to a
geometric effect called parallax, and not true motion.
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Parallax is what you experience when you place a finger in front of
your nose and blink one eye and then the other, and your finger
appears to change position. This stereoscopic vision gives us our
depth perception, and this same principle applies to MISR. The
greater apparent motion of the cirrus clouds tells us that they are
higher than the low-level cumulus. This ability to localize clouds in
3-D is a necessary step to being able to associate different cloud
types with their amount of solar reflection.
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These visualizations are just a few illustrative examples of MISR's
new capability. Since every pixel in the images provides an accurate
measure of reflected sunlight, our computers can process this digital
information quantitatively. In the years ahead, this will help us paint
a more detailed portrait of our planet's changing environment and
climate, and some of the factors that affect Earth's habitability.
MISR, along with its companions on Terra, has just given us a fine
new set of brushes, and a new palette, with which to paint that portrait.
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