| TUTORIAL: The Electromagnetic Spectrum and Remote Sensing
By Clyde Spencer Introduction
Remote sensing is very mysterious to many people. It is commonly viewed as a black art practiced by the military and government agencies with three-letter names. While there is some truth to that perception, the principles upon which remote sensing is based are actually rather simple.
Remote sensing is the technology of using sensors such as conventional cameras, or electronic scanners sensitive to electromagnetic radiations outside the visible region, to obtain imagery for analysis. Active radar, band-limited passive microwave, infrared, ultraviolet, and multispectral or hyperspectral instrumentation have been used successfully.
After more than two decades of use, satellite-based remote sensing has become commonplace and has successfully penetrated many civilian applications. To appreciate how remote sensing works and is applied, we will examine the essential foundations of the technology. Electromagnetic Radiation
Phenomena associated with a periodically varying electric and magnetic field, which are traveling at the speed of light, are called electromagnetic radiation. Electromagnetic radiation is characterized by measurable features of amplitude,wavelength, frequency, and phase. The range of potential wavelengths is open-ended, hence there is a continuum of wavelengths that are referred to as the electromagnetic spectrum.
Frequency and wavelength are inversely related. That is, as frequency increases, wavelength decreases. For most regions of the electromagnetic spectrum used for remote sensing, wavelength measurement is more useful than frequency. Therefore, the radiations are commonly characterized in terms of the wavelength.
Radiation from different regions of the electromagnetic spectrum interact with matter in different ways. Therefore, these regions have been given different names. However, there is usually a transition between regions that makes it difficult to place a precise boundary.
The commonly recognized regions are as follows, starting with the longest wavelength (lowest frequency) phenomena:
¥ Radio waves
¥ Microwaves
¥ Infrared
¥ Visible light
¥ Ultraviolet
¥ X rays
¥ Gamma rays Radio Waves
Radio waves effectively exist from about the region of 1 kilohertz (1 thousand cycles per second) to about 1 gigahertz (1 billion cycles per second). That is equivalent to wavelengths between 300,000 meters to 0.3 meters.
Radio waves are best known as a medium of communication. However, the characteristic behavior of their ability to pass through non-conductors (dielectrics) and absorption by conductors, allows some utility in remote sensing. Unfortunately, the very long wavelength limits the size of objects that can be resolved to tens of meters. Radio waves have found some use in geophysical exploration where rather poorly outlined conductive anomalies (typically sulfide-rich ore bodies) have been detected. Radio waves do not lend themselves to creating images as we have come to know the term in remote sensing. Microwaves
Microwaves - perhaps best known for making popcorn - are also useful for communication and are key to the success of RADAR (RAdio Detection And Ranging). They occupy the region from about 0.3 meters to about 0.0003 meters in wavelength. Like radio waves, microwaves pass through materials with low dielectric constants, but are reflected strongly by materials such as soils and rocks containing water and metals. Some ranges of frequencies are attenuated strongly by vegetation and atmospheric water, while others have considerable power to penetrate even clouds and thick jungle canopies.
Both passive and active microwave imaging systems have been built and experimented with. Passive systems use reflected microwaves originating with our sun.
The application of RADAR technology to active imaging systems has resulted in what is called SAR (Synthetic Aperture Radar). SAR has found utility in mapping terrains as diverse as the Amazon Basin and the sulfuric acid cloud-covered surface of Venus. It does an excellent job of detailing the surface topography, irrespective of cloud cover and vegetation. It has also been found that SAR can penetrate very dry desert sands to reveal buried topography. Interpretation of the image is not as straightforward as with visual/infrared multispectral scanners and there are numerous post-processing problems with correcting the data. However, enough work has been done to allow identification and even differentiation of crop types with multifrequency systems. It is less useful for identifying rock types because the primary factors that affect the return signals are: water content and orientation and size of the reflecting surfaces (essentially surface texture).
SAR has been experimented with as a remote sensing tool for several years; early SAR systems were flown on aircraft. The first experiments in spaceborne SAR imagery were the U.S. Space Shuttle Imaging Radar, SIR-A and SIR-B. The Space Shuttle experiments used rather simple approaches of imaging based on the amplitude of a fixed-frequency return signal. Recently, a SIR-C experiment was also flown.
The U.S. and Russian intelligence communities have flown SAR satellites to gain all-weather ship movement information. The European Space Agency and Japan have experimental SAR systems in orbit and Canada recently launched their RADARSAT polar-orbiting platform to provide worldwide, all-weather coverage for commercial interests.
Modern SARs offer multifrequency, dual- and quad-polarization, and variable resolution/swath width. Multispectral SAR, using two or more frequencies simultaneously, can provide additional information about the size of ground materials and their dielectric constants. Infrared
Microwaves transition into thermal infrared (TIR). The IR portion of the electromagnetic spectrum spans the wavelength region from about 300 microns (1 u = 0.000001 m) to 0.7 microns. IR transitions into visible light at the red end, hence its name. Most people think of infrared and thermal imaging as being synonymous. However, it is only the long wave-length IR (Far and Intermediate) that is useful for detecting or imaging objects at the relatively cool temperatures usually found on the surface of the Earth (see Figure 3). A body at 20¼ Celsius emits most of its radiant energy in the wavelength region from about 4 to 90 microns, with the peak at 13 microns. The so-called thermal band of the Thematic Mapper (TM) sensor on the Landsat 4 and 5 satellites covers the range of 10.4 to 12.5 microns. The TM thermal band has much coarser spatial resolution than the other bands, however.
Near-infrared radiation is only emitted in abundance by very hot bodies, such as our sun. Consequently, most near-infraredsystems are designed to passively detect reflected IR coming from the sun. The three non-thermal Landsat TM IR bands cover the region from 0.76 to 2.35 microns, discontinuously. The near IR region is useful for detection and identification of surficial materials because there are diagnostic absorption and reflectance features found in the near IR region. Hyperspectral sensor systems typically cover the visible and near IR regions continuously out to at least 2.5 microns. Visible Light
That region of the electromagnetic spectrum to which the typical human eye is sensitive is called visible (VIS) light. It is a rather narrow, but important region; it encompasses a wavelength region from about 0.4 to 0.7 microns. The human eye has its peak sensitivity to bright light at approximately 0.55 microns, which is approximately the peak of the emission curve for the sun. Surprisingly, this corresponds to a green color even though we think of the sun as having an orangish-yellow color. Because of dust and aerosol-induced Rayleigh scattering, less blue light directly reaches the surface of the Earth than arrives at the top of the atmosphere, changing the perceived color of the sun.
Until fairly recently, photographic processes using photosensitive substances - such as silver halides - were the only choice for producing permanent images of scenes. An outgrowth of television and the digital computer industry is that there are now many more options for recording images. These images also may be manipulated and analyzed in ways that were impossible a little more than a generation ago. Ultraviolet
Just beyond the blue (violet) band of the visible-light region is a more energetic electromagnetic radiation called ultraviolet (UV). It covers the region from about 0.4 to 0.01 microns. It is capable of inducing photochemical reactions and is generally detrimental to organic materials. While some animals such as bees have evolved the ability the see near UV, UV is generally more harmful than useful to life forms. Since far UV is strongly absorbed by ozone, and to a lesser degree by oxygen, it is always strongly attenuated by passage through an air column. It is also stongly attenuated by normal glass lenses, requiring special materials to make UV imaging systems.
UV light has been used to induce fluorescence in hydrocarbons, such as thin films on water resulting from tanker spills. UV has also been used to prospect for minerals such as scheelite, a calcium tungstate that has a characteristic blue-white fluorescence. X Rays
X rays cover the range from about 0.01 microns to about 0.00001 microns. They are even more energetic than UV and consequently are even more dangerous to organisms. They, however, find applications in creating contrast shadowgraphs - the typical so-called 'X Ray' - and CAT (Computer Assisted Tomography) scans.
X rays have not found practical use in classical Earth remote sensing because of the difficulty in focussing them, strong atmospheric absorption, and the inherent difficulties of handling ionizing radiation sources. Gamma Rays
Electromagnetic radiation with wavelengths shorter than about 0.00001 microns are called gamma rays. However, these very short-wavelength, high-energy phenomena begin to be dominated by interactions with matter that are more particle-like than wave-like. Indeed, gamma rays are usually characterized in terms of their photon energy as measured in millions of electron volts.
Gamma rays are the most energetic form of electromagnetic radiation. They are emitted by celestial bodies as a by-product of thermonuclear reactions and other more esoteric astrophysical processes. Consequently, the Earth is bombarded with them continuously as a component of cosmic radiation. They are highly penetrating and therefore almost impossible to focus. Gamma rays are also emitted by naturally radioactive elements during the process of spontaneous disintegration. Using air-borne gamma-ray spectrometers, images of the ground have been created that show the relative abundance of certain elements - typically uranium, thorium, and potassium - that emit gamma rays of characteristic energy. To date, most renditions of terrestrial gamma-ray intensities resemble a map more than what we think of as an image. About the Author:
Clyde Spencer is the remote sensing team leader for the city of Scottsdale's Advanced Technology Program. Prior to that, he was a consultant specializing in technical and market aspects of remote sensing and GIS. He also taught GIS courses part-time for the University of California, Berkeley, and was formerly a senior industry analyst for Dataquest's GIS market intelligence service. He may be reached at 602-994-7953 or [email protected] Images were obtained from the NASA Mission to Earth, Planetary Data System CD-ROM, Geologic Remote Sensing Field Experiment. Back |
