Page 155 - Intro to Space Sciences Spacecraft Applications

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Introduction to Space Sciences and Spacecraft Applications
interactions can take place in a plane or two-dimensional surface (su~uce
phenomena) or in three dimensions, resulting in interactions called vol-
ume phenomena. These surface and volume interactions can produce a
number of changes in the incident electromagnetic radiation, such as
changes in magnitude, direction, wavelength, polarization, and/or phase.
A remote sensor measures and records these changes, and the resulting
data are interpreted to identify the characteristics of the target or scene
that produced the changes in the electromagnetic radiation.
During the interaction between electromagnetic radiation and matter,
both mass and energy are conserved according to basic physical princi-
ples. The following complex interactions may occur:

Radiation may be transmitted, that is, passed through. The velocity of
the electromagnetic wave may change as it is transmitted from a vac-
uum into other mediums. This velocity change is characterized by the
medium’s index ofrefraction (n) which relates the velocity of electro-
magnetic propagation in the medium (c,) to that in a vacuum (c) as:

n = CJC, (6-6)

Radiation may be absorbed and give up its energy, largely due to heat-
ing the substance or medium.
Radiation may be emitted by elements in a scene as a function of ele-
ment structure and temperature. As we saw in Chapter 4, all matter at
temperatures above absolute zero emits energy.
Radiation may be scattered. The scattering process may be preferential,
tending to propagate in some specific directions, or it could scatter in all
directions and be lost ultimately to absorption or further scattering out-
side of the sensor FOV. If the illumination source is the sensor itself, the
energy returned to the sensor by this process is called backscatter
Radiation may be reflected, unchanged, from the surface of a substance
with the angle of reflection equal and opposite to the angle of incidence.

These interactions with any particular target or form of matter are selec-
tive with regard to the wavelength of electromagnetic radiation and are spe-
cific for that form of matter, depending primarily upon its optical orientation
with the sensor boresight, its wavelength scale-scattering surface or volume
properties, and its atomic and molecular structure. These interactions
between matter and energy provide the spectral and radiometric basis for
remote sensing.