This chapter provides an introduction to the basic principles of hyperspectral remote sensing. The main objective is to explain how information about the earth's surface is conveyed to a remote hyperspectral imaging sensor, which are the key factors determining the nature and quality of the acquired data, and how the data should be processed to extract meaningful information for practical applications. By definition, hyperspectral imaging systems collect co-aligned images in many relatively narrow bands throughout the ultraviolet, visible, and infrared regions of the electromagnetic spectrum.
The term “remote sensing” has several valid definitions. In the broadest sense, according to Webster's dictionary, remote sensing is “the acquisition of information about a distant object without coming into physical contact with it.” For our purposes, remote sensing deals with the acquisition, processing, and interpretation of images, and related data, obtained from aircraft and satellites that record the interaction between matter and electromagnetic radiation.
The detection of electromagnetic radiation via remote sensing has four broad components: a source of radiation, interaction with the atmosphere, interaction with the earth's surface, and a sensor (see Figure 1.1). The link between the components of the system is electromagnetic energy transferred by means of radiation.
Source The source of electromagnetic radiation may be natural, like the sun's reflected light or the earth's emitted heat, or man-made, like microwave radar. This leads to a classification of remote sensing systems into active and passive types. Active systems emit radiation and analyze the returned signal. Passive systems detect naturally occurring radiation either emitted by the sun or thermal radiation emitted by all objects with temperatures above absolute zero. With active systems, like microwave radar, it is possible to determine the distance of a target from the sensor (range); passive systems cannot provide range information.
Atmospheric interaction The characteristics of the electromagnetic radiation propagating through the atmosphere are modified by various processes, including absorption and scattering. This distortion is undesirable and requires correction if we wish to study the earth's surface, or desirable if we wish to study the atmosphere itself.