Page 146 - Intro to Space Sciences Spacecraft Applications

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Remote Sensing
mental monitoring satellites, there are a host of fine spatial resolution
satellites that owe their heritage to the pioneering work done by NASA’s
Landsat and Seasat satellites. These satellites employ electronic cameras
and special radars to observe the earth and its oceans in great detail for
military, scientific, and commercial purposes.
Remote sensing from space can be categorized in a variety of different
ways, such as the type of sensor employed, the application purpose,
and/or the sponsorship of the mission. These themes will be expanded in
the sections that follow but are first outlined in this introduction.

Sensors

Sensors can first be categorized as being passive or active. A passive
sensor receives information from the earth and the atmosphere. A camera
is a good example of a passive sensor relying on solar illumination of the
object(s) in view to provide input to the camera. By contrast, an active
sensor provides the source of energy used to view the area of interest.
Radars and lasers are examples of active sensors.

Both passive and active sensors operate over a wide portion of the elec-
tromagnetic spectrum, The precise frequencies or wavelengths chosen are a
function of first, the application, and second, the influence of the propaga-
tion path between the satellite and the earth. The signatures provided by the
earth and its atmosphere result from highly complex interactions of induced,
in the case of an active sensor, or natural, in the case of a passive sensor,
electromagnetic radiation. These interaction mechanisms include
reflectance, emission, absorption, and scattering, and the proper blend of
sensor wavelengths can reveal information about the types and condition of
soil, vegetation, water, ice, and the atmosphere. One of the most popular
regions of the electromagnetic spectrum employed by remote sensors are
the visible bands that provide picture-like observations of the earth’s surface
and the clouds that envelope the earth. However, many other portions of the
electromagnetic spectrum are used to provide additional information about
the earth and its resources. Infrared and microwave sensors provide infor-
mation on atmospheric, surface, and even subsurface conditions.
Microwave sensors have the added benefit of being able to observe the
earth’s surface through clouds and the capability of operating day or night.