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30 Principles of Remote Sensing

Types of Remote Sensing

Remote sensing can be either passive or active. ACTIVE systems have their
own source of energy (such as RADAR) whereas the PASSIVE systems depend
upon external source of illumination (such as SUN) or self-emission for remote


Radiation from the sun, when incident upon the earth’s surface, is either
reflected by the surface, transmitted into the surface or absorbed and emitted
by the surface (Fig. 3). The EMR, on interaction, experiences a number of
changes in magnitude, direction, wavelength, polarization and phase. These
changes are detected by the remote sensor and enable the interpreter to obtain
useful information about the object of interest. The remotely sensed data
contain both spatial information (size, shape and orientation) and spectral
information (tone, colour and spectral signature).

Figure 3: Interaction of Energy with the earth’s surface. ( source: Liliesand & Kiefer, 1993)

From the viewpoint of interaction mechanisms, with the object-visible and
infrared wavelengths from 0.3 µm to 16 µm can be divided into three regions.
The spectral band from 0.3 µm to 3 µm is known as the reflective region. In
this band, the radiation sensed by the sensor is that due to the sun, reflected

ER (λ) = Reflected energy

EI (λ) = ER(λ) + EA (λ) + ET (λ)

EI (λ) = Incident energy

EA(λ) = Absorbed energy ET(λ) = Transmitted energy

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38 Principles of Remote Sensing

spectrum are very useful for identifying and characterizing earth and
atmospheric features.


Campbell, J.B. 1996. Introduction to Remote Sensing. Taylor & Francis, London.

Colwell, R.N. (Ed.) 1983. Manual of Remote Sensing. Second Edition. Vol I: Theory,
Instruments and Techniques. American Society of Photogrammetry and Remote Sensing
ASPRS, Falls Church.

Curran, P.J. 1985. Principles of Remote Sensing. Longman Group Limited, London.

Elachi, C. 1987. Introduction to the Physics and Techniques of Remote Sensing. Wiley Series
in Remote Sensing, New York.

Joseph, G. 1996. Imaging Sensors. Remote Sensing Reviews, 13: 257-342.

Lillesand, T.M. and Kiefer, R.1993. Remote Sensing and Image Interpretation. Third Edition
John Villey, New York.

Manual of Remote Sensing. IIIrd Edition. American Society of Photogrammtery and Remote

Sabins, F.F. 1997. Remote Sensing and Principles and Image Interpretation. WH Freeman,
New York.

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