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MODIS Data Product Non-Technical Description - MOD 09

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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Curator: Brandon Maccherone
NASA Official: Shannell Frazier

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