Solar Occultation
The solar occultation measurement technique is a very simple method of measuring
vertical profiles of atmospheric optical depth profiles from an Earth orbit
using the sun as a light source. As the spacecraft orbits the Earth, the instrument
points toward the sun and measures its intensity. It observes sunsets when the
spacecraft moves from the sunlit toward the dark side of the Earth. Before each
sunset starts, the line-of-sight (LOS) between the spacecraft and the sun is
unobstructed by the atmosphere so that the sun's intensity as measured by the
instrument is unattenuated. But, when the spacecraft starts to dip below the
horizon so that the LOS passes through a portion of the atmosphere, the sun's
intensity will be attenuated due to aerosols and gases in the atmosphere that
scatter and absorb sunlight. During sunrise events, when the spacecraft moves
from the dark towards the sunlit side of Earth, the sun is first viewed through
the atmosphere, and then along an unobstructed path when the spacecraft rises
above the horizon. Thus, the measurement sequence during sunrise is just the
reverse of that during sunset. In both instances the instrument acquires a measure
of attenuation caused by aerosols and gases in the atmosphere, thereby making
it possible to quantify these species as a function of altitude.
