Page 158 - Intro to Space Sciences Spacecraft Applications

Remote Sensing
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shows a portion of the infrared spectral band and not the entire electro-
magnetic spectrum. Frequency ranges with poor transmissivity are known
as absorption bands, while regions where radiations pass through rela-
tively unaffected are called windows. As an example, we can see well dur-
ing the day because the atmosphere is practically transparent to the visi-
ble frequency range; but a couple of hours at the beach doesn’t burn us too
badly because most of the ultraviolet radiations of the sun are absorbed
high in the atmosphere. A particular remote sensor’s purpose may be to
observe and measure some phenomenon at the earth’s surface, in which
case it must negotiate frequencies with significant atmospheric absorption
and use the window frequencies. Many sensors, sometimes called
sounders, wish to measure atmospheric or ionospheric conditions and will
intentionally select frequencies of observation that interact with and mea-
sure high-altitude phenomena.
The component of the total energy received by the remote sensor from
just the su$ace is given by:

where

which shows the dependency of the received energy on the spectral output
of the sun F,) and the effects of the transmissivity z(k) on the energy prop-
agation. Similar equations exist for the other components of the total
received energy which are also dependent on these factors. The total ener-
gy received by the remote sensor is the sum of these radiations which, when
expressed in equation form, constitute the radiative transfer equation:

where the total Rayleigh scattered input is given by IqtOt,, and I* is the
received aerosol scattered component.
The reduction of this raw received information is quite complicated and
requires some prior knowledge of the behavior of the atmosphere. Over
many years, algorithms have been developed which are used to reduce the
unrefined sensor source or raw data and identify the physical processes
which contribute to the total energy received by the remote sensor. Algo-