| Tutorial: Our Earth-Sun System
Understanding the interaction between the Sun's
radiation and the Earth's reflectance
By Clyde H. Spencer As discussed in the first tutorial, presented in the November 1997 issue of EOM, passive remote sensing systems primarily depend on natural electromagnetic radiation that comes from our Sun. The choice of what portion of the electromagnetic spectrum to use is dictated by the characteristics of the Sun's radiation, the wavelength-related absorption of the earth's atmosphere, and the absorption characteristics of the target materials on the earth's surface. As the ultimate source for virtually all radiation used in passive remote sensing systems, understanding the Sun is important. The Sun
Our Sun, some 8 light-minutes distance away, is a self-contained thermonuclear reactor radiating electromagnetic energy in a manner very similar to that predicted for a perfect emitter (called a Black Body) at a temperature of about 6,000 degrees kelvin. The shape of the radiation emission curve - plotted as wavelength versus energy - is asymmetrical. The peak is at about 0.5 microns; there is a long tail into the infrared (IR). However, substantial levels of x-rays, ultraviolet (UV), and even microwaves are emitted also. Imposed on the actual emission curve are thousands of very narrow absorption features - known as Fraunhofer Lines - created through an absorption by the gasses in the Sun's outer atmosphere. These are of no concern for broad-band remote sensing systems. However, the more prominent ones may have to be reckoned with in future hyperspectral systems.
The radiation output of the Sun is remarkably constant. What is called the Solar Constant - the rate at which solar radiation is received outside the atmosphere on a surface perpendicular to the path of the radiation - typically varies by no more than 0.3% over time periods of a few weeks. However, during periods of sunspot maxima - roughly every 11 years - there is an increase in the relative proportion of X ray and ultraviolet radiation, leading to an overall increase of energy received by as much as 3%. The Solar Constant is approximately 6,500 foot-candles (1.99 calories per square centimeter per minute).
The unique combination of the spectral distribution of the Sun's output, in combination with the absorption and scattering characteristics of our atmosphere, constrain the design of passive remote sensing systems. Earth's Atmosphere
One of the unfathomable coincidences of our Earth-Sun system is that the atmosphere is quite transparent to electromagnetic radiation in the region of peak emission from the Sun. There are bands in the UV and IR regions that have strong absorptions; therefore, if the Sun were slightly hotter or cooler, the peak emission might coincide with a major band of absorption and complicate imaging systems considerably. If the Sun's peak emission were in the far-UV region, very little 'light' would make it through to the surface, stratospheric ozone layer or no.
However, the atmosphere still acts as a critically important modifier of solar radiation. On average, about 19% of the solar energy incident on the top of the atmosphere is absorbed before it reaches the ground.
There is very strong attenuation of UV and soft x-rays, resulting in virtually no radiation shorter than 0.3 microns reaching sea-level (even with the presence of the infamous Antarctic so-called "ozone hole!").
Dust, smoke, aerosols, and condensed water vapor can scatter and block visible light. However, the atmospheric gasses of nitrogen, oxygen, carbon dioxide, and water vapor alone are remarkably transparent to visible light.
Rayleigh scattering subtracts UV and blue light from directly illuminated scenes and adds UV and blue light to shadowed areas. Rayleigh scattering is a form of coherent scattering caused by particles much smaller than the wavelength of the light; the scattering increases inversely with the 4th power of the wavelength. Consequently, the shortest wavelengths are scattered most strongly.
As one moves out into the infrared region, there are numerous absorption bands. Two of the most notable ones are the result of water vapor and carbon dioxide and can be found at approximately 1.4 and 1.9 microns. The attenuation is so strong that one might call both: transmission "black holes." As a consequence, these broad absorption features effectively preclude using solar radiation near these wavelengths for the imaging or identification of surficial materials. This is an annoyance in the use of hyperspectral systems since one can not obtain continuous spectra to match against laboratory spectra. Out of necessity, one must resort to using one or more selected regions on either side of the "water holes." The Landsat Thematic Mapper band 5 is just above the 1.4 micron absorption feature and the 1.9 micron feature lies between TM bands 5 and 7. There are two other strong water-absorption bands at about 3 and 6 microns, also. TM band 6 (thermal IR) lies between the 6 micron absorption feature and a broad region of water-vapor absorption from about 20 microns to about 500 microns. This last broad absorption band effectively spans the transition region from infrared to microwaves.
There are specific absorption bands from oxygen and water vapor in the millimeter and sub-millimeter microwave regions. Microwaves shorter than about 3 centimeters (cm) are attenuated by rain and fog. Consequently, one has to select bands to minimize absorption and avoid conditions of rain and fog if millimeter or shorter microwaves are chosen as the region for imaging.
To avoid the above problems, one must move into the X, S, L, or P band RADAR regions (3 cm to 1 meter). However, the longer the wavelength, the weaker the emissions from the Sun and the possibility exists that passive imaging systems will get 'painted' accidentally by an active RADAR source or the receiver will be swamped with man-made communication signals. Active SAR systems in this region (most use the X band) achieve excellent penetration through clouds, fog, and even tree canopies. Passive systems have many liabilities in the microwave region. Reflections on the Earth's Surface
The double entendre inherent in this points out the importance of knowing how the target materials one desires to image, reflect light. Taking the time to understand the characteristics of the target, and selecting an appropriate system will pay dividends.
Green vegetation achieves its characteristic appearance in the visible portion of the spectrum by absorbing light in the blue and red regions and absorbing less strongly in the green. However, what we can not see with our eyes is that there is very strong reflectance in the near IR region. When measured on the same scale, the green reflectance is swapped out by the much stronger IR reflectance. While there are some exceptions, most vegetation is remarkably similar in its reflectance patterns. Researchers are studying small differences such as the transition between the red absorption band and the IR reflectance - called the "red edge." Unfortunately, leaves can change their orientation, become desiccated, and be stressed by disease. This makes it difficult to differentiate species. This is an area deserving of - and receiving - more research.
Clear, clean water typically has reflectance in blue and green wavelengths and virtually no red or IR reflectance. The presence of abundant sediment modifies the reflectance curve to look like that of the dominant mineral species present in the sediment.
Minerals and rocks (which are homogeneous mixtures of minerals) have much more variable spectra than vegetation. The colors in the visible region for minerals are rather restricted. There are very few blue minerals; none of them are significant rock-forming minerals. Most rocks and minerals are colored by the presence of ferric or ferrous iron, resulting in greenish or reddish colors, respectively, in the visible portion of the spectrum. However, nearly all minerals have absorption features in the IR region, even minerals that are colorless or white in the visible portion of the spectrum. The exception to this is most black minerals, which typically have a low, flat reflection spectrum. Hyperspectral sensors are most efficient at differentiating mineral species.
Rocks are more difficult to identify since destructive interference between the different mineral spectra can, and often do, cancel the diagnostic absorption features and give a relatively flat spectra.
Soils are usually a mixture of mineral grains, clay particles, and organic materials such as humus, tannin, and cellulose. Which of the three primary constituents dominates will determine the gross shape of the reflectance spectra. However, the typical spectra increases in reflectance from the blue to the infrared.
Clouds and snow are uniformly bright across the visible and IR region, however there are characteristic IR absorptions that can be used to differentiate them.
If one is planning a remote sensing project, it is advisable to obtain a library of reflection spectra and study the shapes and features of the target(s) of interest before deciding on an imaging system to use. If, out of necessity, one must rely on a multispectral system such as the Thematic Mapper, the study of the spectra will provide the information required for choosing appropriate bands for discriminating the target(s). This is preferable to just using the standard false-color IR bands or a trial-and-error combination of many bands and transforms. About the Author:
Clyde H. Spencer operates Bio-Geo Recon, located in Sonora, California, a consulting firm specializing in technical and market aspects of remote sensing and GIS. Prior to forming this consultancy, he served as the remote sensing team leader for the city of Scottsdale, Arizona. He may be reached at 209-532-5197. Back |
