Measuring the reflectance of a target from different directions is really useful because geophysical media (the atmosphere, including the clouds and aerosols, the ocean, and terrestrial surfaces such as soil, vegetation and snow or ice) reflect solar light differently in different directions. In fact, the variations between the reflectances acquired from a variety of observation angles can be interpreted (with appropriate models) to document the properties of the target, just as the more familiar spectral differences are exploited to document its chemical composition.
Most imaging space-borne instruments acquire measurements for each location on Earth from a single direction at a time, usually within the limited range of (across-track) observation zenith angles allowed by the scanner or push-broom design of the sensor. The accumulation of multiangular observations with such instruments necessitates revisiting the site over rather long periods of time, from multiple days to a few weeks or more. By incorporating nine separate cameras oriented at various angles along the track of the platform, the MISR instrument is capable of acquiring multiple observations of the same site from a wide variety of zenith angles in a matter of a few minutes. This greatly facilitates the interpretation of the measurements and significantly improves the accuracy of the retrieved information.
Specifically, using its multi-angular and multi-spectral capability, the MISR instrument
- better distinguishes between objects and surfaces than would be possible on the basis of spectral variations alone;
- exhibits an enhanced sensitivity to aerosols and thin cirrus clouds, both of which are ubiquitous and usually hard to detect, especially at large observation zenith angles and/or over bright backgrounds;
- provides a three-dimensional (stereoscopic) view of clouds that allows users to estimate their height;
- makes it possible to use clouds as tracers of winds aloft due to the time lapse (about 7 minutes) between the most forward and the most backward views;
- yields at once measurements that accentuate or minimize the effect of sun glint over the ocean and other water surfaces, thereby enabling observations even when traditional sensors are hampered by the very high reflectance of these surfaces, and
- permits users to much more accurately estimate the hemispherical albedo of the target, which is thus calculated on the basis of nine different values instead of only one.
More detailed information is provided on the MISR web site Science Goals and Objectives page.