Data collected by the Advanced Very High Resolution Radiometer (AVHRR) sensor onboard the NOAA Polar Orbiter satellites can be used to examine the response of vegetation to changing climatic conditions throughout the year. The primary means to monitor vegetation health involves the calculation of a vegetation index using a ratio of the visible red and near infrared bands of the AVHRR sensor. Healthy green vegetation has low response in the red band and a high response in the near-infrared band. By comparison sand or concrete will have a high repsonse in both bands while dark soils and water will have low responses in both bands. Thus subtracting the red band from the near infrared band should produce higher values for vegetated areas than for areas dominated by concrete, soil or water. This difference can then be divided by the sum of the band responses to normalize the value so that it may be less sensitive to particular lighting conditions. So, the equation for the Normalized Differenced Vegetation Index(NDVI) is:
NDVI=(NIR-R)/(NIR+R)
This composite NDVI scene was generated by selecting the maximum NDVI values from scenes from multiple days. In this way the gaps caused by cloud cover can be filled in to produce a more complete image. Clouds which escape the cloud screening algorithm tend to lowever the calculated NDVI value. We there for assume the maximum NDVI values over a particular target will better represent the true vegetation health.
Spotlight 
