Page 156 - Intro to Space Sciences Spacecraft Applications

Remote Sensing 143
Radiative Transfer Equation. The amount of energy that a remote sensor
receives from a target area is very much frequency-dependent and may
come from a variety of sources as a result of the interactions described
above. Figure 6-3 demonstrates the types of energies that a remote sensor
may be looking to receive and reveals some of the interactions that energy
may have during its transmission through the atmosphere.
In the figure, IT represents the total energy received by the satellite sen-
sor which has been designed to observe phenomena within a specific fre-
quency range. In the situation depicted, the illumination of the scene is
provided by the energy of the sun which has the known spectral charac-
teristics described in Chapter 4. In other cases, the received energy may
instead be produced by the natural radiations of the earth and atmosphere,
or may be supplied by the transmitter of an active remote sensor. Each of
the radiations which make up the total received energy is also a function
of frequency and, for some observed frequencies, some of the components
shown may not contribute to the received energy at all.
As shown in the figure, the sun's radiation, denoted by F(h), illuminates
the scene from an angle 8, called the zenith angle. One source of energy
received by the remote sensor may come from some of this radiation that
is scattered off small particles within the atmosphere. This aerosol scut-
tered energy (IA) is uniformly radiated from the particles in all directions.
Another type of scattering of energy radiates in an anisotropic (not uni-
form) manner from the molecular constituents of the atmosphere. This

-"a- y
\ 'R" '0, \ I\

Figure 6-3. Remotely sensed energies. The energies received by a sensor in
space may come from a number of sources and/or interactions.