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There’s a reason that sunrises and sunsets always look
more brilliant in the country than they do from urban centers.
Tiny particles in the air, such as aerosols and gasses diffuse,
diffract, and scatter light passing by them, causing the brilliant
colors of the rising or setting sun to become more muted in
particle-dense city air than in the cleaner, more particle-free
country air.
The principle that the tiny elements suspended in air can
affect the light passing by (and thus change the way you perceive
whatever you’re looking at) holds especially true when
you’re looking at something from a great distance (like
the planet’s surface) as opposed to something relatively
close up (like a sunrise).
The MODIS instrument views the Earth at a distance of 705
km above the planet’s surface; the rough equivalent
of driving from New York City to Washington, DC, twice. MODIS
gathers data by measuring and recording light that bounces
off of Earth’s various surfaces and into MODIS’
“eye.” Because different objects reflect light
in different patterns, MODIS scientists are able to create
products that look at very specific features, such as the
density of worldwide vegetation, how productive the world’s
oceans and large bodies of water are, and how many clouds
are detected over a given area in a certain period of time.
In a perfect world (from the satellite's point of view),
the light heading back out to space after reflecting off of
some surface on the planet would not be affected by anything
else it encountered. But in the real world, this doesn’t
happen, because the gasses, aerosols, and thin cirrus clouds
throughout the atmosphere all affect the light passing by
or through them, and end up changing the data. Though atmospheric
gases, aerosols, and thin cirrus clouds are not visible to
the naked human eye, they do affect the light that passes
through them by causing it to pick up some of their own characteristics.
This atmospheric "interference" means that the observations
collected by the instrument are a product not just of the
surface, but also of the atmosphere.
For scientists studying land surface features like vegetation
distribution or fires, it’s important to remove, or
correct for the atmospheric effects from the data. This correction
takes place in the Surface Reflectance product, which is a
measure of what the true surface reflectance would have been
at a given point in place and time if there were no atmosphere
to get in the way. Accurate land surface reflectance is the
foundation for a variety of MODIS products: vegetation indices,
BRDF (Bidirectional Reflectance Distribution Function, thermal
anomalies (fires), and FPAR/LAI (Fraction of Photosynthetically
Active Radiation/Leaf Area Index). The surface reflectance
product will be routinely used in land cover characterizations,
global and regional climate models, and surface energy balance
modeling.
Just as we want to see the most brilliant, and hence uncontaminated,
sunrises and sunsets, MODIS scientists want to observe land
and ocean surfaces as they truly are, not through the interference
of the atmosphere. The MODIS Surface Reflectance product is
the filter that is the helps them achieve that goal.
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