By contrast, active sensor provides their own source of energy. The most familiar form of this is flash photography (Eastman, 1995). However, in environmental and mapping applications, the best example is radar. Radar systems emit energy in the microwave region of the electromagnetic spectrum in figure 1. The reflection of that energy surface material is the measured to produce an image of the area sensed.

Detector used in remote sensing measure mixture of electromagnetic energy reflected from several objects. Reflectance from air particulate along a distance between measuring objects and detectors can significantly increase magnitudes of energy the detectors are measuring. Moreover, a long distance make each measurement (one reading of the incident energy from a given instantaneous field of view) cover a large area on the earth surface. A reading of the detector measures reflectance from several objects except an object is larger that the resolution of the detector. A simple mathematical representation of the detector. A simple mathematical representation of the measured energy is

Where I_r is the energy measured by a detector

I_a is the incidence energy contributed by the atmospheric particulate
R_i is the reflectance/irradiance object of i
A_i is the proportion of area cover by object i in the area seen by the detector
N is the number of objects in the area instantaneously seen by the detector
D is the energy-dissipated factor dependent on the distance Between objects and the detector

Details of radiate transfer and measurements of electromagnetic energy irradiated from earth’s objects can be found in remote sensing text books (e.g. see Lillesand and Kiefer, 1994)

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