|Home | Aerial Photography Applications | Aerial Cameras | Spaceborne Platforms | Bibliography | Contact|
Our Remote Sensing Applications
What is Remote Sensing?It is a process of obtaining information about an object, area or phenomenon of interest by an instrument which is not in contact with the object, area or phenomenon under investigation. The instruments used for this purpose may employ any of a variety of physical energy distributions. Sonars for example work on the principle of acoustic wave distribution, optical instruments such as the photographic camera and multispectral scanner use electromagnetic energy distribution. Radars, which belong among "active sensors" use lower frequency electromagnetic energy in the microwave region of the spectrum. An active sensor, as opposed to the passive ones, provides its own illumination source and measures the radiation returned.
For the purpose of this short introduction to remote sensing, I will limit my discussion to two optical sensors; namely the photographic camera and the multispectral scanner. These can be mounted on airborne or space-borne platform. The optical instruments of our interest here, make the use of atmospheric windows in the range of 400-2500 nm, 3-5 mm, and 8-14 mm of the electromagnetic spectrum.
Electromagnetic Energy in Remote Sensing
An overview of the electromagnetic spectrum.
Some portion of solar radiation passing through the atmosphere does not reach the earth's surface, and is effectively absorbed by atmospheric constituents such as water vapor, carbon dioxide and ozone. The wavelengths of sun's radiation which are particularly transmissive are used for remote sensing and are referred to as atmospheric windows.
The fate of radiation incident upon the earth surface is well expressed by the following energy balance equation:
EI(l) = Incident Energy at wavelength (l), ER(l) = Energy Reflected, EA(l) = Energy Absorbed, ET(l) = Energy Transmitted
For most surface materials, a portion of the incident radiation will be reflected and scattered back away from Earth's surface, a portion will be absorbed and another portion may be transmitted through the material. Some absorbed radiation may be re-emitted at wavelengths longer than those absorbed (Wickland E.D., 1991). The proportions of energy reflected, absorbed, and transmitted will vary for different earth features, depending on their material type and condition. The wavelength dependency means that even within a given feature type, the proportion of reflected, absorbed, and transmitted energy will vary at different wavelengths. Therefore, two features may be indistinguishable in one spectral range and be very different in another wavelength band (Lillesand and Kiefer, 1987).
Out of the energy reflected from a particular object, only some actually reaches the sensor. The remaining energy may be scattered throughout the atmosphere. Alternatively, some scattered energy may contribute to the background noise signal the sensors receive as well. Elevated background noise/signal ratios are common with sensors located at higher altitudes, or those receiving signal passing through atmosphere with higher concentration of energy scattering particles. Large differences in scattering can be observed between heavily humid and dust ladden tropical maritime atmosphere, and that of considerably dryer and clearer continental polar atmosphere. Scattering causes the atmosphere to have a radiance of its own. The atmospheric luminance at solar altitude angle in the neighborhood of 20o to 30o reaches a maximum of about twice the value reached at solar altitude of 90o. The effect of atmospheric radiance on aerial photography (as well as on other remote sensors) is a function of many variables, such as: sensor altitude, concentration, size-distribution and nature of atmospheric aerosols, solar altitude, spectral sensitivity range of the sensor, angle of view from nadir and its azimuth with respect to the Sun, and polarization of light.
Scattering plays an important role in remote sensing, be it the effect of atmosphere on the quality of the received signal, or interference with the signal received from within the aquatic environments. Rayleigh scatter is common when radiation interacts with atmospheric molecules and other particles that are much smaller than the wavelength of the interacting radiation. The effect of Rayleigh scatter is inversely proportional to the fourth power of wavelength. This implies that there is a much stronger tendency for shorter wavelengths to be scattered than longer wavelengths. A clear atmosphere with a large component of nonselective scattering typically shows 50% more scattering in the blue spectral region than in the red. The wavelength dependence of atmospheric scattering accounts for the blue of the sky and the red of the Sun when seen through a long atmospheric path (Sunset). Another type of scatter is Mie scatter. This comes about when particle diameter essentially equal the energy wavelength being sensed. This type of scatter tends to influence longer wavelengths compared to Rayleigh scatter (Doyle F.J et al., 1983; Lillesand and Kiefer, 1987).
All radiation received by the sensor is
termed radiance. The radiance received by the sensor, and
corrected for atmospheric scattering and the angle of incident radiation
(angle of solar radiation) using mathematical models is termed
mentioned earlier, reflectance varies spectrally. It is directionally dependent
and it also varies vith the direction of the incident energy.
C 2010 Eco-Scientific Consultants.