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Remote sensing, by definition, is the acquisition of information about objects
or events without making physical contact. In the broad usage, remote sensing
refers to the use of both the force fields (such as gravity and magnetism) and
electromagnetic spectrum to detect, classify, and identify the objects or events.
In the restricted usage, the term remote sensing generally refers to the use of
electromagnetic spectrum to acquire information about the earth’s surface (land
and ocean) and atmosphere using sensors onboard airborne (aircraft, balloons) or
spaceborne (satellites, space shuttles) platforms. Remote sensing in its restricted
usage is recognized as the only viable means to acquire regional- and mesoscale
information of the earth surface and atmosphere in a globally consistent and
economically feasible manner. According to the spectral response range of sensors,
earth observation by remote sensing can be roughly classified to visible and
near-infrared remote sensing (VNIR), thermal infrared (TIR) remote sensing, and
microwave remote sensing. Given the broad scope of remote sensing, this book is
limited to deal with the TIR remote sensing.
As TIR sensors onboard the spaceborne platforms only measure spectral radiances
at the top of the atmosphere, these measured radiances are influenced not only
by the surface parameters (emissivity and temperature) but also by the composition
and thermal structure of atmosphere along the path between the surface and the
sensors. Theoretically, only two surface parameters (land surface emissivity and
land surface temperature) can be directly retrieved from TIR data; other surface
parameters or variables such as soil moisture and land surface evapotranspiration
may be indirectly deduced. |