ABSTRACT
The information collected by Earth observation cameras is meant to identify and map various Earth surface objects. This activity is known as remote sensing and the instruments used to collect the data are called remote sensors. Remote sensors can be broadly classified as passive sensors and active sensors. Sensors that sense natural radiation, either emitted or reflected from the Earth, are called passive sensors. It is also possible to produce electromagnetic radiation of a specific wavelength or band of wavelengths as a part of the sensor system and the interaction of this radiation with the target could then be studied by sensing the scattered radiation from the targets. Such sensors, which produce their own electromagnetic radiation, are called active sensors. The technology involved in developing sensors is not the same throughout the electromagnetic spectrum. The technology for developing microwave sensors is quite different from that of optical-infrared (OIR) sensors. Therefore, from the standpoint of understanding the design and realization of the sensors, it is convenient to classify the sensors (both passive and active) as those operating in OIR region and those operating in the microwave region. The OIR and microwave sensors can be either imaging or nonimaging sensors. The imaging sensors give a two-dimensional spatial distribution of the emitted or reflected intensity of the electromagnetic radiation (as in a photographic camera), while the nonimaging sensors measure the intensity of radiation, within the field of view (FOV) of the instrument, and in some cases, as a function of distance along the line of sight of the instrument (e.g., vertical temperature profiling radiometer [VTPR]). Figure 4.1 gives a possible classification of remote sensors. In this book, we deal with passive imaging sensors operating in the OIR region.
