The Physics Behind Remote Sensing


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Remote Sensing is the science of acquiring, processing and interpreting images that record the interaction between the electromagnetic energy and matter (Sabins, 1997). Remote sensing offers extensive applications in almost every area of science from monitoring forest fires to geologic mapping . Although many aspects of remote sensing are complex and difficult to understand the basic theory behind remote sensing is simple physics.

There are four major stages of remote sensing. The first stage is the source of energy, usually the sun, which sends energy to a target. The second stage is the interactions that the energy will go through as it passes through a vacuum and the earth’s atmosphere. There is then interactions that occurs with the body on earth’s surface. The energy again, goes through the atmosphere and finally the sensor is able to detect and record the electromagnetic energy.

Electromagnetic Energy

Electromagnetic energy is a dynamic form of energy that is caused by the acceleration or oscillation of a charged particle. All substances above absolute zero (0 Kelvin) emit a range of electromagnetic energy. The sun emits electromagnetic energy and practically all of the natural electromagnetic energy injected into the earth is produced by the sun (Prakush, 2002).

Electromagnetic Radiation

Electromagnetic Radiation is the streams of mass-less particles, traveling in a harmonic, sinusoidal fusion at the velocity of light each possessing a specific amount of energy. The distance from one peak to the next is the wavelength and the number of peaks passing through a fixed point per unit time is the wave frequency (Lillesand and Kiefer, 1994). Electromagnetic radiation is Electromagnetic energy in motion and can be described by the basic wave theory.

Electromagnetic Spectrum

Electromagnetic waves are characterized by their wavelength location within the electromagnetic spectrum which is most commonly measured in micrometers. Names are often assigned to regions of the electromagnetic spectrum, but there is no clear cut dividing lines from one region to the next (Lillesand and Kiefer, 1994).

Stefan - Boltzmann Law

How much energy any objects radiates is a function of its surface temperature.

The Stefan - Boltzmann equations tells one that as a temperature of a body increase the total radiance of the body will also increase. (Sabins, 1997).

Interactions

The way electromagnetic radation interacts with matter can be detected with different sensors. How the radation interacts depends upon the properties of the medium, the wavelength of the incedent radation and the incident angle. There are four major types of interactions that occur: transmission, reflection, scattering and absorption (Lillesand and Kiefer, 1994).

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The atmosphere also has interactions with electromagnetic radiation as it passes through the atmosphere. Atmospheric scattering occurs when electromagnetic radiation entering the atmosphere interacts with particles in the atmosphere and the direction of the energy change is not predictable (Prakash, 2002). There are three major types of atmospheric scattering which are dependent on the wavelength of radiation and the size of the atmospheric particles.
Rayleigh scattering is the most common type of scattering and occurs when the electromagnetic radiation is much larger than the particles in the atmosphere. Rayleigh scattering occurs in the blue section of the electromagnetic spectrum and is the reason that the sky appears blue (Prakash, 2002).
Mie Scattering occurs when the wavelength of the incoming electromagnetic radiation is comparable in size to the atmospheric particle. Mie scattering occurs in the red and orange parts of the spectrum and explains why the sky appears orange and red at dusk and dawn. The water vapor and fumes in the atmosphere cause the electromagnetic radiation to scatter(Prakash, 2002).
Non- Selective Scattering occurs when the wavelength is smaller than the particles in the atmosphere. Non-selective scattering affects all of the parts of the spectrum equally and explains why clouds appear white. White clouds are a result of all of the visible wavelengths being scattered equally (Prakash, 2002).

Detection of Electromagnetic Radiation

The radiation that is emitted reflected or scattered from a body generates a radiant flux density in its surrounding space which contains information about the bodies properties. To measure the properties of this radiation a collector and detector are used. The collector is a collecting aperature which intercepts the radiant field and focuses the energy into a detector. The collector for viisual, infrared and ultraviolet remote sensing is usually a lens or a reflecting surface. The detector then transforms the electromagnetic radiation into another form of energy such as heat, electric current or state charge (Elachi, 1987). These forms of energy are measured and converted into digital numbers that can be transformed into images that can be processed.

Plank's Law

Plank's Law determines the emission pattern spectra of electromagnetic radation of a blackbody as a function of it's wavelength and temperature.

Planks law shows that as the temperature of a body rises the peak of the emitted radiation shifts toward shorter wavelengths (figure 1)The peak of the sun's radiation is in the visible part of the electromagnetic spectrum and the earth's peak of radiation is in the thermal part of the electromagnetic spectrum.

Satellite and Airborne Sensors

There are many different satellite sensors that are used today. Different sensors have different spatial and spectral resolutions therefore the information they obtain have different uses.

Applications

Geologic Mapping

ASTER images are often used for geologic mapping.
Not only does this image show different strata in different
colors, but it also show the topography of the region

Forest Fires

One of MODIS's main uses is to help monitor forest fires.
This MODIS image shows forest fires in Idaho during the summer
of 2002.The red area on the image show are that were burining
during image aquisition while the gray area depict burn scares.

Volcano Monitoring

An ASTER imagae of a volcanic eruption over Mt. Etna Italy.
Not only can ASTER detect the thermal properites of lava flows it
also has the capability of detecting gas emmisions from the volcano.

Sea Ice & Glaciers

A Landsat 7 image of the largest glacier in Alaska.
Malaspina Glacier is located west of Yalcutat Bay and is
3,880 km squared. Movement of glaciers and sea ice is often
studied using remote sensing.

Urban Growth

A Landsat 7 image of Detroit, Michigan.
Remote Sensing is often used to study changes
to urban area along with the effects of that growth.

Vegetation Mapping

This SPOT image is mapping diifernt type of agricultural area.
Remote sensing is often used to detect changes in vegetaion
especially due to deforestation

References Cited

Lillesand and Kiefer, 1994, Remote Sensing and Image Intrpretaion, 3rd Edition, John Wiley and Sons Inc., New York.

Elachi, C. 1987 Introduction to the Physics and Techniques of Remote Sensing, Wiley – Interscience Publications, New York.


Sabins, F, 1997, Remote Sensing Principles and Interpretations, 3rd Edition, W. H. Freeman and Company, New York.

ASTER Web Page: http://asterweb.jpl.nasa.gov/

Landsat Web Page: http://landsat7.usgs.gov/index.php

MODIS Web Page : http://modis.gsfc.nasa.gov/

Prakush, Anupma, 2002 Web Page and Class notes: http://www.gi.alaska.edu/~prakash/teaching/undergraduate/material/index.html



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