Sensors carried aboard many satellites record the amount of visible and infrared light reflected by the land surface (and atmosphere). Some sensors are able to gather observations on a daily basis, while others do so weekly or twice a month. Daily images can be used directly, or composited into 8-day and bi-weekly observations. Creating composites allows for the removal of pixels representing cloud and atmospheric contaminants. This enables scientists to work with the best data available for the composited period.
Once the data have been pre-processed computers are used to convert them into various measurements or indices. One commonly used index is the Normalized Difference Vegetation Index (NDVI) which can be used to describe vegetation greenness or the amount of green leaf material in the forest. In the Spring, NDVI values will start to rise as more healthy green vegetation begins to emerge. In the Fall, NDVI measurements will decrease due to the changing colors of the leaves and the impending dormancy of the trees. When NDVI is examined over time scientist may see patterns in these increases and decreases. They indicate the vegetation health of a region and areas that have experienced changes due to deforestation, wild fires, disease, or alteration of climate patterns. All of these observations help scientist understand the landscape phenology and ecology of our the study region.