Calibration, Validation of Satellites Products, Atmospheric Corrections and Radiative Transfer for science applications
Perhaps never before has it been so critical that mankind increase his knowledge of the world in which he lives, specifically, the Earth and its atmosphere. The elements of land, water, air, and life are interconnected. Environmental problems are of great concern to many. A great number of individuals, scientists, organizations, governments, and members of academia are working independently and collaboratively, utilizing state-of-the-art technology and equipment, in a massive effort, a "Mission to Planet Earth," to acquire a greater understanding of the planet so that proper and effective solutions to problems can be sought. As with any problem or conflict, having more accurate and detailed information allows one to surmise a more legitimate solution. Organized, tested, and validated data is the key.
This is precisely what the MODIS Land Surface Reflectance Science Computing Facility group, led by Dr. Eric Vermote, seeks. Dr. Vermote is involved with the MODIS (MODerate-resolution Imaging Spectroradiometer) project which the objective is to provide "a comprehensive series of global observations of the Earth's land, oceans, and atmosphere in the visible and infrared regions of the spectrum in such a way as to view the entire surface of the Earth every two days. MODIS is the primary tool on the Earth Observing System satellites for conducting global change research.
The MODIS instrument is the sensor with which our group, the Land Surface Reflectance Science Computing Facility, is associated. MODIS is capable of viewing the entire globe daily at moderate resolutions, ranging from 250 meters square to 1 kilometer square (about 0.5 square miles) pixels. With a sweeping 2,330-km-wide viewing swath, MODIS sees every point of our world every 1-2 days in 36 discrete spectral bands. It tracks a wider array of the earth's vital signs than any other Terra sensor. The sensor measures the atmosphere, ocean, and land processes, and, thus, structurally and logically, the MODIS project is divided into three separate disciplines: atmosphere, ocean, and land, each a separate entity, yet interacting in conjunction. In a collaborative effort, two other groups, Quality Assurance and Validaion, act with the other groups to maintain data accuracy and consistency, thereby playing a vital role in the overall process. Still further, these groups are divided into more specialized disciplines, or projects. For further information visit the MODIS Website.
The MODIS Terra/Aqua Surface Reflectance (MOD09/MYD09)is a seven-band product computed from the MODIS Level 1B land bands 1, 2, 3, 4, 5, 6, and 7. The product is an estimate of the surface spectral reflectance for each band as it would have been measured at ground level as if there were no atmospheric scattering or absorption. It corrects for the effects of atmospheric gases, aerosols, and thin cirrus clouds. The Surface Reflectance product is a major input utilized in the generation of several land products: Vegetation Indices, BRDF, Land Cover, Snow Cover, Thermal Anomalies, and LAI/FPAR. The surface reflectance is inverted with the help of a 6S radiative transfer code using atmospheric inputs taken from NCEP (ozone, pressure) or directly derived the MODIS data (aerosol, water vapor).
The 6S code is a basic RT code used for calculation of lookup tables in the MODIS atmospheric correction algorithm. It enables accurate simulations of satellite and plane observation, accounting for elevated targets, use of anisotropic and lambertian surfaces and calculation of gaseous absorption. The code is based on the method of successive orders of scatterings approximations and its first vector version (6SV1), capable of accounting for radiation polarization. It was publicly released in May, 2005.
For stage II validation, the accuracy of the surface reflectance has been evaluated for 157367 cases for TERRA and 118195 cases for AQUA; the product performs well since approximately 90% of the data are within the theoretical errors bars.