AVHRR/NDVI
Advanced Very High Resolution Radiometer (AVHRR) is a sensor aboard the NOAA series of polar orbiting earth satellites that have been in operation for more than three decades. Currently, NOAA-14, NOAA-16 and NOAA-17 satellites are in orbit. The main purpose of these satellites is to forecast weather and monitor regional climatic conditions. However, their potential for monitoring crop growth, assessing crop yield and monitoring forest cover has been realized only during the past decade. AVHRR is a broadband scanner, sensing in the visible (Channel 1), near-infrared (Channel 2) and thermal infrared portions (Channel 3, Channel 4 and Channel 5) of the electromagnetic spectrum. The spectral ranges of different channels are given in Table 1.
| Channel | Wavelength (μm) |
|---|---|
| 1 | 0.58 - 0.68 |
| 2 | 0.73 - 1.10 |
| 3 | 3.55 - 3.93 |
| 4 | 10.3 - 11.3 |
| 5 | 11.5 - 12.5 |
The satellite receiving system, located at the Blackland Research Center(BRC) in Temple, TX, acquires daily raw data from the three active AVHRR satellites. An automatic data processing system has been developed for radiometric, geometric, and atmospheric corrections. Besides this standard processing, algorithms developed by various researchers have been refined in computing Normalized Difference Vegetation Index(NDVI) and Land Surface Temperature (LST).
NDVI is a measure of vegetation condition and health. It is computed from the reflectance measured in visible and infrared channels (Channel 1 and Channel 2) of the AVHRR satellite using a simple formula:
| NDVI = | ch1 - ch2 |
| ch2 + ch1 |
NDVI and LST maps for Texas will be available soon through the Real-Time Vegetation Monitoring System maintained by the BRC in Temple, TX.