Date of this Version
Published in Remote Sensing of Drought: Innovative Monitoring Approaches, edited by Brian D. Wardlow, Martha C. Anderson, & James P. Verdin (CRC Press/Taylor & Francis, 2012).
Satellite measurements of the biosphere have now become common place in various aspects of large-scale environmental monitoring, including drought and crop monitoring. This was not the case until the launch of the Advanced Very High Resolution Radiometer (AVHRR) instrument on June 27, 1979, on board the first National Oceanic and Atmospheric Administration (NOAA) Advanced Television Infrared Observation Satellite (TIROS-N/NOAA-6) polar-orbiting satellite. Initially, the NOAA AVHRR satellites were designed to observe the earth’s weather patterns: primarily cloud dynamics, vertical soundings of the atmosphere, and sea surface temperatures. Early studies on remote sensing of vegetation were focused on understanding seasonality. The vernal advancement and retrogradation of vegetation (e.g., spring green-up, summer abundance, and fall dry-down) was first studied over the north–south expanse of the U.S. Great Plains using data from the Earth Resources Technology Satellite (ERTS) Multi-Spectral Scanner (MSS) instrument (Rouse et al., 1974a,b). Rouse et al. (1974a) and others demonstrated that biophysical characteristics of vegetation over this rangeland and cropland region could be inferred from satellite spectral measurements despite solar zenith angle differences across a long latitudinal gradient (Deering et al., 1975). Rouse et al. (1974b) developed a difference ratio metric between the red and near-infrared (NIR) radiances over their sum to normalize the effects of the solar zenith angle. This derivation is based on the unique spectral response function of vegetated surfaces compared to other surface matter in the visible and NIR portion of the electromagnetic spectrum, as shown in Figure 2.1.