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A General Introduction to MISR
If Earth were a movie screen, there would be no need for MISR. Movie screens
are made of a material that reflects light nearly equally in all directions. So for
people sitting in the right, left, and center sections of the theater, the movie looks
equally sharp and bright.
But Earth is a more complicated place: its surface, the clouds, even tiny particles
floating in the air, each reflects light differently when viewed at different angles.
This matters to us because the light being reflected is sunlight, which carries the
energy that heats Earth. The details of how sunlight is scattered by forests,
deserts, snow- and ice-covered surfaces, cumulus, stratus, and cirrus clouds, and smoke
from forest fires, soot, and other by-products of industry -- all affect our climate.
Most satellite instruments look only straight down, or toward the edge of the planet.
To fully understand Earth's climate, and to determine how it may be changing, we need
to know the amount of sunlight that is scattered in different directions under natural
conditions. MISR is a new type of instrument, designed to address this issue; it
views Earth with cameras pointed in 9 different directions [see
illustration]. As the instrument flies overhead,
each piece of Earth's surface below is successively imaged by all 9 cameras, in each
of 4 wavelengths (blue, green, red, and near-infrared).
In addition to improving our understanding of the fate of sunlight in Earth
environment, MISR data can also distinguish different types of clouds, particles and
surfaces. Specifically, MISR monitors the monthly, seasonal, and long-term trends in:
- The amount and type of atmospheric particles (aerosols), including those formed
by natural sources and by human activities
- The amounts, types, and heights of clouds
- The distribution of land surface cover, including vegetation canopy structure
Aerosols tend to cool the surface below them, since most aerosols are bright
particles that reflect sunlight back to space, reducing the amount of solar radiation
that can be absorbed at the surface. The magnitude of this effect depends on
the size and composition of the aerosols, and on the reflecting properties of the
underlying surface. It is thought that the cooling effect due to increased amounts
of aerosols may reduce or delay some of the expected warming due to increases in the
amount of atmospheric carbon dioxide. But key details about aerosol properties needed
to calculate even their current effect on surface temperatures are not known. MISR
data makes it possible to determine global aerosol amounts with unprecedented
accuracy, and to estimate particle size and composition.
Because they are very common, clouds play a major role in controlling Earth's
climate. A cloud may warm or cool Earth, depending on its thickness and height above
the surface. Because clouds are so variable, their effect on global climate is
difficult to measure -- this is currently the leading uncertainty in climate prediction
models. Stereo images from MISR provides new data about cloud top heights.
The multi-angle observations will also yield information about the structure of
clouds, the properties of cloud particles, and the way clouds reflect
incoming solar energy.
The land surface of Earth is constantly changing. There are natural variations, such
as the progress of seasons, and changes caused by human activities, like deforestation
and desertification in overgrazed regions. There are social and political reasons
for keeping track of these changes. But again because of the importance of solar
energy in determining Earth's climate, we also care about the amount and manner in
which surfaces around the globe reflect sunlight. MISR is able to characterize in detail
the reflection properties of Earth's surface. From these observations, we will be able
to tell where the surface is changing, as well as what effect these changes are likely
to have on the climate of Earth.
Now let's look at some specific examples of this: Click on each of the following
thumbnail images to see enlarged images that illustrate typical conditions of Earth's
atmosphere and surface that need to be better understood in order to further our
knowledge of Earth's climate, and the ways that MISR can be used in studying them.
These are not the entire range of goals for MISR, but are intended
to illustrate the diversity of the MISR experiment.
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Los Angeles sunset: Sunset on a smoggy summer day in
Los Angeles during the early 1990's. You can see faintly the tallest
buildings in the downtown area poking up through the mist and smog
layer, and you should just be able to make out the Los Angeles River
wending its way across East Los Angeles in the foreground, and the
Hollywood Hills in the background. MISR will measure aerosol
amount globally, including the particles that constitute smog and mist,
and collects information about particle size, shape, and
composition.
[400 x 271 pixels. 36 kilobytes.] |
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Brush fire in central Oregon: Two plumes of particles emanate from a brush fire in central Oregon in this mid-summer scene from 1991. MISR monitors particle amounts over land and ocean. [400 x 277 pixels. 68 kilobytes.] |
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Cloud layer over the Colorado River: A thick cloud layer seen
from an airplane over the Colorado River in the western United
States, probably originating in the Gulf of California as part of the
typical summer monsoon pattern. Thunderheads appear to be growing out
of the cloud mass in this late afternoon photograph. Having 9 cameras
with different look angles, MISR will view clouds in "hyper-stereo."
This makes it possible for MISR to measure the height and texture of
cloud tops such as those seen here.
[400 x 277 pixels. 40 kilobytes.] |
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Cumulus clouds over the Midwest of the United States: A pattern of
cumulus clouds casts its shadows on the farmland below, in this autumn 1991
aerial scene. MISR is able to classify clouds distributed over the entire Earth
based on their height, shape, and texture.
[265 x 400 pixels. 76 kilobytes.] |
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The Mississippi River near St. Louis, Missouri: A variety of surface
properties is seen in this August 1990 view of the Mississippi River,
surrounded by riparian woodland, farmland, and forest. MISR classifies
different types of surfaces on Earth, and measure their brightness and
directional reflectance.
[400 x 270 pixels. 60 kilobytes.] |
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Oxbow bend in the Snake River: Dense dark vegetation and inland
water cover vast areas of Earth, especially in the Northern Hemisphere,
such as in this July 1994 scene of the Snake River in Oregon. Such areas
warrant special study because of their climatic significance. MISR
simultaneously measures properties of such surfaces and of the atmospheric
particles above them.
[400 x 278 pixels. 72 kilobytes.] |
Now let's look briefly at the MISR instrument itself: To accomplish
its scientific objectives, the MISR instrument measures Earth's
brightness in 4 spectral bands, at each of 9 look
angles spread out in the forward and aft directions along the flight
path. Spatial samples are acquired every 275 meters. Over a period of 7 minutes,
a 360 km wide swath of Earth comes into view at all 9 angles. Special attention
has been paid to providing highly accurate absolute and relative calibration,
using on-board hardware consisting of deployable solar diffuser plates and
several types of photodiodes. To complement the on-board calibration effort,
a validation program of in situ measurements are being conducted, involving
field instruments, one of which is the "PARABOLA III", which automatically
scans the sky and ground at many angles, and a
multi-angle aircraft camera (AirMISR). Global coverage
with MISR is acquired about once every 9 days at the equator;
the nominal lifetime of the mission is 6 years.
MISR was built for NASA by the Jet Propulsion Laboratory in Pasadena,
California, and is one of five instruments launched into polar orbit aboard
NASA's Terra spacecraft in August 1999. The spacecraft flys in a "sun-synchronous"
orbit, designed so that it crosses the equator every 98 minutes, always at
10:30 a.m. local time, as Earth rotates below.
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