MODIS is an EOS facility instrument designed to measure biological and physical properties on a global basis every one to two days. Slated for both the Terra and Aqua satellites, the instrument will provide long-term observations from which to derive an enhanced knowledge of global dynamics and processes occurring on the surface of the Earth and in the lower atmosphere. This multidisciplinary instrument will yield simultaneous, congruent observations of high-priority atmospheric (cloud cover and associated properties), oceanic (sea-surface temperature and chlorophyll), and land-surface features (land-cover changes, land-surface temperature, and vegetation properties). The instrument is expected to make major contributions to the understanding of the global Earth system, including interactions between land, ocean, and atmospheric processes.
The MODIS instrument employs a conventional imaging-radiometer concept, consisting of a cross-track scan mirror and collecting optics, and a set of linear detector arrays with spectral interference filters located in four focal planes. The optical arrangement will provide imagery in 36 discrete bands from 0.4 to 14.5 µm, selected for diagnostic significance in Earth science. The spectral bands will have spatial resolutions of 250 m, 500 m, or 1 km at nadir; signal-to-noise ratios of greater than 500 at 1-km resolution (at a solar zenith angle of 70°); and absolute irradiance accuracies of ±5 percent from 0.4 to 3 µm (2 percent relative to the Sun) and 1 percent or better in the thermal infrared (3 to 14.5 µm). MODIS instruments will provide daylight reflection and day/night emission spectral imaging of any point on the Earth at least every two days, operating continuously.
Many MODIS products are made over time intervals ranging from about one week to a month or a season. These products will be better suited for use by investigators interested in seasonal phenomena. In some cases these products are made to reduce data volume; in others they are made to provide cloud-free coverage within a defined time period. Because the ground track of the EOS spacecraft follows a 16- day repeat cycle, and because there may be biases in the multiday data sets based on viewing geometry at specific locations, the MODIS "week" has been defined as an 8-day period. The land and ocean data products are based upon 8-day weeks beginning on January 1 of each year. The atmosphere products are also based on 8-day weeks, but these begin with the first 8-day period of the Aqua MODIS sensor that is concurrent with the Terra MODIS schedule and run in an unbroken sequence thereafter. Thus, the land and ocean weeks will not, in general, coincide with the atmosphere weeks.
SOURCE: EOS Data Products Handbook, Volume 2, 2000, p. 28
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