Some Remote Sensing Systems for Monitoring Drought

Landsat

Early use of satellite data was pioneered by the Landsat series originally known as the Earth Resource Technology Satellite (ERTS; http://landsat7. usgs.gov/index.php). Landsat was the first satellite specifically designed for broad-scale observation of the earth's land surface. A series of Landsat satellites (Landsats 1-5 and 7; 6 failed at launch) have provided visible and near-infrared data since 1972, with additional bands in the shortwave infrared and thermal regions for Landsat 4, 5, and 7. The Landsat satellites orbit the earth in sun-synchronous mode with a repeat cycle over any given location of 16 days. The earlier Landsat satellites provided imagery with a multispectral scanning instrument (MSS) at a resolution or pixel size of 80 m. Landsats 4 and 5 carried an additional sensor known as the thematic mapper (TM) that provided image data at a 30-m resolution over the visible, near-, and shortwave-infrared wavelengths with a 120-m thermal band. Currently, Landsat 7 carries an enhanced thematic mapper (ETM+) sensor that provides improved radiometric calibration, a panchromatic band at 15 m, visible through shortwave infrared bands at 30 m, and a thermal band at 60 m spatial resolution.

SPOT

The Systeme Probatoire pour l'Observation de la Terre (SPOT) satellite sensor series (http://www.spotimage.fr/home/), developed by Center National d'Etudes Spatiales (CNES) in France, was first launched in 1986 and provides data at a spatial resolution of 10 m (panchromatic mode) and 20 m (multispectral mode) using what is known as the high-resolution visible

(HRV) sensor. The HRV instruments have been in service on SPOT 1, 2, and 3. Although the SPOT HRV repeat cycle of 26 days is longer and has only 3 spectral bands as opposed to 7 on Landsat TM and ETM+, the spatial resolution is an improvement, and the SPOT sensors can also acquire stereoscopic pairs of images for a given location, which is useful in topographic mapping. SPOT 4 was launched in May 1998 with an additional sensor named VEGETATION (VGT), designed specifically to monitor land surface parameters on a global basis at 1 km spatial resolution. VGT monitors vegetation dynamics on a daily basis and globally in four spectral bands, from visible to shortwave infrared.

NOAA-AVHRR

The advanced very high resolution radiometer (AVHRR) instrument on board the NOAA series of satellites (http://www.nesdis.noaa.gov/) has provided daily data on the global environment for the last 20 years. Although the NOAA series of satellites was designed for weather and climate observations, its broad bands in the visible through thermal-infrared portions of the spectrum have been used effectively for large-scale monitoring of vegetation dynamics, especially in arid and semiarid areas (Tucker et al., 1983; Justice, 1986). Since 1981, the compilation of vegetation measurements from NOAA-7, -9, -11, -14, and currently NOAA-16 has provided a continuous stream of high temporal resolution data. Although the nominal spatial resolution for NOAA data is 1 km, satellite storage limitations enforced degrading of 1-km data to 4-km data to achieve global coverage on a daily basis. Data at about 1 km resolution can only be gathered if there is a local receiving station when the satellite passes overhead.

The Indian Space Research Organization (ISRO; http://www.isro.org/ programmes.htm) currently has several satellites under the Indian Remote Sensing Satellite (IRS) system for natural resource monitoring and management. These data are distributed by ANTRIX Corporation Ltd., the commercial arm of the ISRO, and also by Space Imaging Corporation in the United States. The IRS-1C and IRS-1D satellites together provide continuous global coverage with many advanced capabilities. The IRS-P5 and IRS-P6 offer a very high-resolution panchromatic camera for cartographic applications as well as specific capabilities for agricultural applications.

MODIS

One of the most advanced remote sensing systems for land surface studies is NASA's Moderate Resolution Imaging Spectroradiometer (MODIS; http://modis.gsfc.nasa.gov/) sensor on board the Earth Observing System

(EOS) Terra and Aqua (http://aqua.gsfc.nasa.gov/) platforms, launched in December 1999 and May 2002, respectively. The MODIS instruments provide global coverage in 36 spectral bands with a spatial resolution ranging from 250 m to 1 km. A number of spectral bands and band combinations on this instrument are invaluable to drought, vegetation, and climate studies. Bands 1 and 2 provide 250-m resolution imagery in the red and near-infrared (NIR) regions, and bands 3-7 provide 500-m resolution imagery in the visible, NIR, and SWIR regions. This includes bands 5 (1230-1250 nm) and 6 (1628-1652 nm). In these bands, leaf water content influences the canopy reflectance response. The remaining bands are at 1 km spatial resolution and include measurements in the thermal infrared as well as optical data useful for ocean, atmosphere, and cryosphere applications. This sensor therefore fills the spatial resolution data gap between the Landsat and SPOT satellite series and the coarser NOAA-AVHRR data. Already a number of valuable land products have been assembled and are publicly available for use through various distribution centers (http://edcdaac.usgs.gov/modis/dataprod.html). The products include the traditional normalized difference vegetation index (NDVI) data, as well as the enhanced vegetation index (EVI) data, which offer improvements over the NDVI by minimizing saturation problems at high biomass conditions and by reducing atmosphere and soil background noise (Huete et al., 2002; Justice et al., 2002).

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