Satellite remote sensing data can provide detailed, precise geographical information concerning major changes to the surface of the earth. These photographs show the landscape at Caolingtan before the massive Sept. 21, 1999 earthquake and at several later times, and reveal the changes that occurred. (Source: Center for Space and Remote Sensing Research, National Central University)
When electromagnetic radiation is mentioned, the first applications that come to mind are communications products such as cell phones. Nevertheless, apart from these ubiquitous electronic devices, remote sensing instruments carried on satellites far above our heads also rely heavily on electromagnetic radiation to observe the earth.
Making distant observations from novel viewing angles
According to Associate Professor Lin Tang-huang of National Central University's Center for Space and Remote Sensing Research, the term "remote sensing" was first used in the United States during the 1960s. Nevertheless, even at that time, the concept of remote sensing was already part of people's lives. Remote sensing implies the use of technology that can observe certain physical characteristics of a target object without any direct contact. According to this definition, human vision and hearing can both be considered innate kinds of remote sensing technology. Our eyes and ears are both types of sensors, and are able to receive signals from distant objects and events, allowing us to form judgments about them.
The first artificial satellite, the Soviet Union's Sputnik 1 (which means "travelling companion" in Russian), was launched in October 1957. Actually, as early as 1947, in order to make meteorological observations, scientists had mounted cameras on V2 high-altitude rockets in order to photograph cloud systems from an altitude of approximately 160 km. That experimental project was the first time that people had been able to observe the earth's uniqueness and importance from a novel viewing angle far above the surface.
In 1958, with the launch of the United States' "Explorer 1" satellite, a satellite race broke out between the US and USSR. Afterwards, satellite technology advanced steadily, and a series of satellite-borne remote sensing instruments were developed and improved. In addition, the unique data obtained by satellite remote sensing was gradually applied to various types of scientific research, which was one of the positive results of the Cold War in space.
Visible light and infrared
Remote sensing technology involves applications of electromagnetic radiation. After instruments receive electromagnetic radiation in certain frequency bands emitted by or reflected by target objects, the physical characteristics of the target object can be determined through analysis of the intensity of radiation at specific wavelengths. The electromagnetic spectrum is very broad, and different wavelengths of electromagnetic radiation carry different amounts of energy. Currently, visible light, infrared, and microwave are the most common wavebands used by remote sensing instruments. By examining how these three types of electromagnetic radiation interact with the atmosphere, ocean, and ground, remote sensing systems can infer various types of information concerning target objects on the ground.
Visible light is most commonly used when performing remote sensing of the earth's resources. The surfaces of most objects possess unique reflective characteristics in the visible light waveband (wavelength: 400~700nm), which implies that most objects have different reflectivity in the visible light portion of the spectrum. For instance, an apple is red because the surface of an apple reflects red light far better than other wavelengths when it is illuminated with visible light. As a result, more red light than other types of light enters our eyes when we look at an apple, and the apple thus appears red. Analogously, tree leaves are green because the leaves have greater reflectivity in the green light waveband. We can use this principle to identify features on the Earth's surface by analyzing the visible light reflection characteristics of various surface objects. We can also create detailed surface images by analyzing the strength of radiation in the visible light waveband, which has relatively high radiation intensity (shorter wavelength); the use of visible light remote sensing is therefore very suitable for monitoring land use and detecting surface objects and various resources. But because visible light ultimately comes from the sun, one disadvantage of using this waveband is that information cannot be obtained at night.
Responding to the fact that the visible light waveband cannot be used at night, scientists found that the infrared waveband—which has its own distinctive emission characteristics—can be used to provide assistance. Following Max Planck's black-body radiation experiment, it has been known that as long as the surface temperature of an object is above absolute zero (-273.5℃), that object will naturally radiate energy. A consequence of this is that we can derive the surface temperature of an object by determining the strongest wavelength of electromagnetic radiation emitted from the surface of that object (the strongest wavelength of radiation emitted from objects on the earth's surface is usually located in the infrared band). Infrared remote sensing is based on the principle that objects' surfaces emit electromagnetic radiation with different spectral characteristics at different temperatures. Because of this, even though visible light cannot be used for remote sensing at night, the infrared waveband can be used instead to identify those objects by determining the temperature difference between various kinds of objects, and thereby.
It is worth noting that visible light and infrared remote sensing information can be used in combination. When surface temperatures determined using the infrared waveband are paired with visible light information, surface objects can be precisely identified. For instance, when cloud masses are viewed from a high altitude, the color and brightness of high altitude clouds and clouds near the Earth's surface are often very similar, so the clouds cannot be distinguished using only the intensity of reflected light. If infrared remote sensing data is also used, however, the temperature difference caused by the difference in the height of the clouds will allow them to be clearly distinguished. In this case, the use of visible light and infrared remote sensing data in tandem allows us to make precise judgments.
Even more active microwave remote sensing
The use of electromagnetic radiation in the visible light and infrared wavebands as a means of remote sensing involves calculation, inference, and interpretation of electromagnetic radiation signals reflected from or emitted by the surfaces of target objects. This approach, with its passive reception and no time series parameters, chiefly provides bulk data, and may not meet certain observing needs, such as when there are restrictions on the distance from which a target object can be observed.
Scientists have chosen to use the microwave waveband to overcome this kind of restrictions. In the electromagnetic spectrum, radiation in the microwave band has a longer wavelength than infrared, and carries less energy. As a result, if remote-sensing instruments have sufficient power, they can actively emit electromagnetic radiation, and receive the reflected signals from a target area. Ordinary radar employs this type of operating principle. Because this active remote sensing approach allows us to determine the time and strength of reflected electromagnetic radiation echoes, mathematical and physical formulas can be used to derive the distance, height, and elevation of target objects, as well as how these parameters change with time. This method can be used to determine changes in the height of sea level and the rise or subsidence of the ground surface, etc.
While the use of microwaves for this purpose is very convenient, it also comes with certain disadvantages. Because most ordinary objects also emit low levels of microwave radiation, remote-sensing using microwaves entails a complicated situation in which both reflected microwaves and straight microwave emissions are detected. Secondly, compared with visible light and infrared, microwave energy is relatively weak. In view of these two issues, considerable theoretical knowledge and technology is needed to distinguish useful signals from noise.
Remote sensing applications derived from weather monitoring
In the beginning, satellite-borne remote sensing instruments had relatively poor spatial resolution, and few types of instruments were used. Nevertheless, this was sufficient for the meteorological observation that was the predominant use of space remote sensing at the time. Because of the great distance from which the observations are made, satellite remote sensing can obtain data for vast areas of the earth's surface at one time. This characteristic enables satellites to provide extremely important supplementary surface weather data, especially for places where surface data is lacking or absent, such as in the high mountains and at sea.
With the advance of technology, the types of remote sensing instruments, and their resolution, have increased steadily, and they are being used in more diverse ways than ever before. Apart from extremely high-resolution military applications, the two most important applications of remote sensing are surface resource monitoring and natural disaster monitoring.
When the United States launched the Landsat-1 resources technology satellite in 1972, humanity first had a means of performing large-scale real-time resource monitoring and surveys of the earth's surface. As an example, while forest conservation and monitoring work could previously be accomplished only with manual surveys and patrols, with the availability of remote sensing instruments on resource satellites, we can not only use reference information to calibrate remote sensing data, including visible light and infrared data, which can be used to identity tree species and monitor their state of growth and stand area, but also precisely grasp the locations of stands of trees and total lumber volume. Similarly, we can also use various kinds of remote sensing data to perform precise, regular monitoring of the changes in the areas of lakes, rivers' water volume and channel locations, and even the locations of coastlines.
Furthermore, when major natural disasters strike, satellite remote sensing can provide the most immediate and precise information concerning the earth's surface. As an example, when earthquakes, torrential rains, and mudflows occur, it is often difficult for persons on the spot to precisely assess the huge changes to the earth's surface that have taken place. In this type of situation, comparing satellite remote sensing images taken before and after the event can provide an objective, precise comparative look at changes, allowing a quick assessment of the scope of a disaster. At the same time, because it can precisely determine the scope of and magnitude of natural disasters, remote sensing data can also provide more reliable information concerning search & rescue, damage assessment, and reconstruction.
More than half a century has gone by since various bands of the electromagnetic spectrum have first been used for remote sensing purposes, and observations made by satellite-borne remote sensing instruments are providing much valuable data. However, the uses of remote sensing are not limited to these applications; the body temperature sensors at airports and train stations provide another example of the use of remote sensing. We can foresee that remote sensing technology will be applied even more directly and broadly in our everyday lives as time passes, and will be especially important in monitoring the changes affecting the global environment.
Translated by Glen E. Lucas
Date:13 Jun 2016