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These Advanced Very High Resolution Radiometer (AVHRR) images are collected by the National Oceanic and Atmospheric Agency's polar-orbiting Television Infrared Observation Satellites (NOAA-11 and NOAA-12). The images have been received, processed, and distributed by the U.S. Geological Survey since 1987. The spatial or ground resolution of the AVHRR data is 1.1 kilometers. The AVHRR sensor acquires data in a wide swath (approximately 2,400 kilometers), enabling it to create an image of the entire surface of the Earth each day. The NOAA-12 satellite acquires data in early morning and the NOAA-11 satellite in early afternoon.

Because the AVHRR sensor observes all of the Earth's surface each day, the likelihood that one or more cloud-free observation of the surface (over a period of days) is increased. When all of the observations (images) for a month are combined, using a selection criteria that selects clear sky observations, nearly cloud free images can be created.

A vegetation index (the Normalized-Difference Vegetation Index) is calculated from to these images, which represents the quantity and vigor (photosynthesis activity) of the vegetation. The equation for the vegetation index makes it relatively easy to distinguish green vegetation from non-vegetated areas such as water, barren land, ice, snow, and clouds.

With this ability, changes in surface processes over short periods, such as growing seasons, can be monitored.

• Advanced Very High Resolution Radiometer (AVHRR)
• National Oceanic and Atmospheric Agency - Satellites
• U.S. Geological Survey - National Center for Earth Resources Observation and Science (EROS)