Looking down at Earth from space, one sees light
scattered by the atmosphere, as well as light that has been reflected by the
surface below. The ideal way to observe atmospheric aerosols from space would
be to image them over a surface that is completely dark. Some natural surfaces
come closer to this ideal than others.
The ocean surface is quite dark in the MISR red and near-infrared bands
(bands 3 and 4.) If the wind blowing at the ocean surface is known, then
the amount of light that is reflected in all directions from the ocean
surface can be calculated.
Thick forests tend to be dark in the blue and red (MISR Bands 1 and 3), but
not in the green. Surface reflection can be calculated if you know what
fraction of incoming light is sent in different directions (the so-called
"BRDF" of the surface). This is generally less well known than
the reflection properties of ocean surfaces.
There may be neither dark water nor dense, dark vegetation in a MISR scene.
But if there are features with different brightnesses, we attempt a
"Heterogeneous Land" retrieval. This is a more complicated
situation, involving an assumption that although the surface brightness
changes, the surface BRDF does not change very much.
In all cases, the MISR measurements are compared with simulations at natural
atmosphere-ocean situations (the Simulated MISR Atmospheric Radiative Transfer,
or "SMART" data.) The best-fitting situations are reported in the
MISR aerosols product.
