Remote sensing is defined as the science of obtaining and interpreting information from a distance, using sensors that are not in physical contact with the object being observed. Animals (including people) use remote sensing via a variety of body components to obtain information about their environment. The eyes detect electromagnetic energy in the form of visible light. The ears detect acoustic (sound) energy, and the nose contains sensitive chemical receptors that respond to minute amounts of airborne chemicals given off by the materials in our surroundings. Some research suggests migrating birds can sense variations in the earth's magnetic field, which helps explain their remarkable navigational ability.
At its broadest, the science of remote sensing includes aerial, satellite, and spacecraft observations of the surfaces and atmospheres of the planets in our solar system, although the earth is obviously the most frequent target of study. The term remote sensing is customarily restricted to methods that detect and measure electromagnetic energy, including visible light, which has interacted with surface materials and the atmosphere.
Remote sensing of the earth is used for many purposes, including the production and updating of planimetric maps, weather forecasting, and gathering military intelligence. The focus in this chapter is on remote sensing of agriculture and the associated environment and resources of the earth's surface. It explores the physical concepts that underlie the acquisition and interpretation of remotely sensed images, the characteristics of images from different types of sensors, and common methods of processing images to enhance their information content. For additional information on remote sensing refer to <http://www.microimages.com> for a useful tutorial on remote sensing of the environment.
The science of remote sensing began with aerial photography, using visible light from the sun as the energy source. Visible light, however, makes up only a small part of the electromagnetic spectrum, a continuum that ranges from high-energy, short-wavelength gamma rays to lower energy, long-wavelength radio waves. The portion of the electromagnetic spectrum that is useful in remote sensing of the earth's surface is illustrated in Figure 7.1. (See color section at end of chapter.) The earth is naturally illuminated by electromagnetic radiation from the sun. The peak solar energy is in the wavelength range of visible light (between 0.4 and 0.7 |j,m), and the visual systems of most animals are sensitive to these wavelengths. Although visi-
ble light includes the entire range of colors seen in a rainbow, a cruder subdivision into blue, green, and red wavelength regions is sufficient in many remote-sensing studies. Other substantial fractions of incoming solar energy are in the form of invisible ultraviolet and infrared radiation. Only tiny amounts of solar radiation extend into the microwave region of the spectrum. Imaging radar systems that are used in remote sensing generate and broadcast microwaves, then measure the portion of the signal that has returned to the sensor from the earth's surface. The nature and laws of the electromagnetic spectrum are discussed in Chapter 2.
The sensors in remote-sensing systems measure electromagnetic radiation (EMR) that has interacted with the earth's surface. Interactions with matter can change the direction, intensity, wavelength content, and polarization of EMR. The nature of these changes is dependent on the chemical makeup and physical structure of the material exposed to the EMR. Changes in EMR resulting from its interactions with the earth's surface therefore provide major clues to the characteristics of the surface materials. EMR that is transmitted passes through a material (or through the boundary between two materials). Materials can also absorb EMR. Usually absorption is wavelength specific: that is, more energy is absorbed at some wavelengths than at others. EMR that is absorbed is transformed into heat energy, which raises the material's temperature. Some of that heat energy may then be emitted as EMR at a wavelength dependent on the material's temperature. The lower the temperature, the longer the wavelength of the emitted radiation. As a result of solar heating, the earth's surface emits energy in the form of longer-wavelength infrared radiation. For this reason, the portion of the infrared spectrum with wavelengths greater than 3 (am is commonly called the thermal infrared region. EMR encountering a boundary such as the earth's surface can also be reflected. If the surface is smooth at a scale comparable to the wavelength of the incident energy, specular reflection occurs in which most of the energy is reflected in a single direction, at an angle equal to the angle of incidence. Rougher surfaces cause scattering, or diffuse reflection, in all directions (Figure 7.2).
To understand how different interaction processes impact the acquisition of aerial and satellite images, let us analyze the reflected solar radiation that is measured at a satellite sensor. As sunlight initially enters the atmosphere, it encounters gas molecules, suspended dust particles, and aerosols. These materials scatter a portion of the incoming radiation in all directions, with shorter wavelengths experiencing the strongest effect. An example is the preferential scattering of blue light in comparison to green and red light, which accounts for the blue color of the daytime sky. Clouds appear opaque because of intense scattering of visible light by tiny water droplets. Although most of the remaining light is transmitted to the surface, some atmospheric gases are very effective at absorbing particular wavelengths. The absorption of dangerous ultraviolet radiation by ozone is a well-known example.
As a result of these effects, the illumination reaching the surface is a combination of highly filtered solar radiation transmitted directly to the ground and more diffuse light scattered from all parts of the sky, which helps illuminate shadowed areas. As this modified solar radiation reaches the ground, it may encounter soil, rock surfaces, vegetation, or other materials that absorb, transmit, and reflect the radiation. The amount of energy absorbed, transmitted, and reflected varies in wavelength for each material in a characteristic way, creating a spectral signature. The selective reflectance of different wavelengths of visible light determines what we perceive as a material's color. Most of the radiation not absorbed is diffusely reflected (scattered) back up into the atmosphere, some of it in the direction of the satellite. This upwelling radiation undergoes a further round of scattering and absorption as it passes through the atmosphere before finally being detected and measured by the sensor. If the sensor is capable of detecting thermal infrared radiation, it will also pick up radiation emitted by surface objects as a result of solar heating.
Scattering and absorption of EMR by the atmosphere have significant effects that impact sensor design as well as the processing and interpretation of images. When the concentration of scattering agents is high, scattering produces the visual effect we call haze. Haze increases the overall brightness of a scene and reduces the contrast between different ground materials. A hazy atmosphere scatters some light upward, so a portion of the radiation recorded by a remote sensor, called path radiance, is the result of this scattering process. Because the amount of scattering varies with wavelength, so does the contribution of path radiance to remotely sensed images. The path radiance effect is greatest for the shortest wavelengths, falling off rapidly with increasing wavelength. When images are captured over several wavelength ranges, the differential path radiance effect complicates comparison of brightness values at the different wavelengths. As detailed in Chapter 2, the atmospheric components that are effective absorbers of solar radiation are water vapor, carbon dioxide, and ozone. Each of these gases tends to ab sorb energy in specific wavelength ranges. Some wavelengths are almost completely absorbed (Figure 7.3). Consequently, most broadband remote sensors have been designed to detect radiation in the "atmospheric windows," those wavelength ranges for which absorption is minimal and, conversely, transmission is high.
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