Remote Sensing Systems

International research efforts have been undertaken to study the complex and interconnected processes that affect Earth's atmosphere, oceans, and land. Essential information for this research is provided by fleets of satellites and aircraft equipped with sensors that collect enormous amounts of Earth data. These systems are called remote sensing systems.

The variety of data that can be obtained through remote sensing systems is vast. The world scientific community uses remote sensing systems to obtain information about ocean temperature, water levels and currents, wind speed, vegetation density, ice sheet size, the extent of snow cover, rainfall amounts, aerosol concentrations in the atmosphere, ozone levels in thestratosphere , and many other important variables to better understand how natural phenomena and human activities impact global climate. Remote sensing systems may also be used by decision makers such as environmental resource managers, city planners, farmers, foresters and many others to better run their businesses and to improve people's quality of life. This article presents a number of applications of remote sensing systems to farming, water quality analysis, water resources in arid regions, noxious weed detection, urban sprawl, urban heat islands, and many others. This field is in rapid evolution and many new applications may appear in the years to come.

Satellite Remote Sensing

Space is an excellent vantage point from which to study air, sea, and land processes both locally and globally. It provides the bird's-eye view that captures all the information in a single image. Satellite observations have definite advantages over ground or aircraft observations. Ground observations are labor intensive, time consuming, and costly. Aircraft observations require less labor and time but are still costly. In spite of their high initial cost, satellites are a cheaper way to do observations as they may take data continuously during their lifetime over the whole globe. Satellites can also observe areas difficult to access on the ground and provide regular revisits of the same areas showing surface feature changes over time.

Satellite observations are made with sensors that measure the brightness of electromagnetic radiation either reflected or emitted by ground features. Electromagnetic radiation includes not only visible light with its various colors but also many colors invisible to the human eye such as ultraviolet and infrared , as well as radar and radio waves.

Resolution refers to the smallest size object that can be identified. A 1-kilometer (0.6-mile) resolution satellite will produce images made of small squares with uniform brightness representing 1-kilometer by 1-kilometer squares on the ground. In general, objects smaller than 1 kilometer cannot be distinguished in such an image.

Whereas visible and near-infrared radiation observed by sensors is actually solar radiation reflected by ground features, thermal infrared radiation is radiation emitted by ground features. Thermal infrared radiation provides information about the temperature of the emitting objects. Other sensors actively illuminate Earth and measure the reflected signal. Radar is an active remote sensing system that is very useful in areas that are often covered with clouds. Whereas visible and infrared radiation is blocked by clouds, radar waves penetrate clouds, thus enabling observations of Earth from space in almost all weather conditions.

Remote sensing data can be compared to an ore that contains gold (information) from which a piece of jewelry can be made (knowledge). Remote sensing is at its best when it is used to answer specific and well-posed questions. The end result of the processing of data, information extraction, and analysis is the answer to these questions.

Many remote sensors are placed onboard aircraft. Satellites may take several days, even weeks before revisiting a specific area on Earth, whereas aircraft can be commissioned to take remote sensing data over that area on a moment's notice. They also operate at significantly lower altitudes and produce higher resolution data than satellites when fitted with the same type of sensor. Finally, many new National Aeronautics and Space Administration (NASA) sensors are tested on aircraft before being put on satellites. Aircraft remote sensing has an important role to play both for global climate change studies and for more immediate applications such as the ones described below.

Global Environmental Observations

We live in a rapidly changing world facing major global challenges. A rapidly increasing world population demanding accelerated economic development strains Earth's resources. Remote sensing systems are being used to investigate a number of areas related to the global environment, including global climate change, rain forest deforestation, the health of the oceans, the size of polar ice covers, and coastal ecosystem health.

Global Climate Change.

One of the most ambitious and far-reaching programs of environmental investigation is the U.S. Global Change Research Program. This effort is part of a worldwide program to study global climate change, which involves changes in the global environment that could affect Earth's ability to support life.

A strongly debated climate change issue is global warming, which results from increased atmospheric levels of greenhouse gases—such as carbon dioxide—that trap heat in the lower atmosphere, preventing it from escaping into space.

The Carbon Dioxide Information Analysis Center estimates that fossil fuel use and other industrial activities have resulted in the release of 265 billion tons of carbon into the atmosphere since 1751, with half of the total occurring since the mid-1970s. Worldwide levels of carbon dioxide in the atmosphere have increased by 25 to 30 percent since 1850. The average global surface temperature of Earth is up. The year 1997 was the warmest of the twentieth century and possibly the warmest of the past 1,000 years. The question that some people debate is whether this warming is directly related to the human production of carbon dioxide or due to natural processes. Whatever the answer to this question, the trend is clear and the consequences may be severe for the human species.

Plants grow by absorbing atmospheric carbon dioxide, storing the carbon in their tissue. Rain forests and ocean phytoplankton are great carbon dioxide absorbers. So are corals and shellfish, which make calcium carbonates that end up in the bottoms of oceans.

Trees and plankton may grow faster—left to themselves—if the level of carbon dioxide in the atmosphere increases. This could provide a mechanism limiting atmospheric carbon dioxide concentrations. Unfortunately, people pollute the oceans, which kills phytoplankton and coral reefs, and destroy tropical rain forests.

Scientists worldwide inventory and monitor rain forests, phytoplankton, and coral reefs in an effort to estimate their impact on the concentration of carbon dioxide in the atmosphere. Their main sources of information are from satellite remote sensing data. The warming of Earth's lower atmosphere results in the melting of glaciers and polar ice sheets. The extra liquid water produced raises ocean water levels. Indeed, sea level rose 10 to 25 centimeters (4 to 10 inches) during the last century and glaciers are melting. Data from a number of satellites are used by the National Oceanic and Atmospheric Administration (NOAA) to measure the rate of ice melting in Antarctica and Greenland, two major causes of sea level rising.

Rain Forests.

The Global Observations of Forest Cover is an international effort to inventory worldwide forest cover and to measure its change over time. From these observations, which are based on high-resolution satellite remote sensing, scientists produce digital deforestation maps.

Deforestation is a politically sensitive topic. Developed nations pressure developing countries such as Brazil and Indonesia to stop the deforestation process, arguing that the rain forests in these countries are virtual lungs for the world's atmosphere. Developing countries with tropical rain forests argue that the deforested areas are important and necessary sources of revenue and food as they are used for agricultural activities. The debate prompted an international meeting, the United Nations Conference on Environment and Development in Rio de Janeiro, Brazil, in June 1992. This conference resulted in the Rio Declaration on Environment and Development that sets the basis for a worldwide sustainable development—an economic development that does not deplete natural resources and that minimizes negative impact on the environment.

Oceans.

Phytoplankton and coral reefs in the oceans significantly contribute to the removal of atmospheric carbon dioxide. Acid rain and other pollutants adversely affect coral reefs and phytoplankton. Satellite remote sensing is used to inventory coral reefs and phytoplankton worldwide.^* Indeed, because of their wide distribution and remote locations, coral reefs can practically be inventoried and monitored only from space.

Satellite sensors are also used to measure other ocean characteristics such as topography and ocean temperature. For example, a partnership between the United States and France developed TOPEX-Poseidon, a satellite that monitors global ocean circulation and global sea levels in an effort to better understand global climate change, specifically the links between the oceans and the state of the atmosphere.

Ocean monitoring by satellites enables NASA and NOAA to predict the El Niño weather patterns. El Niño is a global weather pattern that is driven by conditions in the Pacific Ocean. During an El Niño, countries in the western Pacific experience severe droughts, whereas the eastern Pacific is drenched by torrential rains, leading to mudslides in California and South America.

Ocean observations are undertaken not only to estimate pollution and ocean health but also for commercial purposes. The OrbImage company, for example, provides fish finding maps to fishing companies based on plankton concentration information from their OrView-2 satellite and on sea surface-temperature information from U.S. weather satellites. This information is radioed to the boats that use it for their fishing operations.

Radar remote sensing has several important uses over oceans. Reflected signals from radar are sensitive to water surface roughness. The rougher areas reflect the radar signal better and appear brighter. Smooth areas are dark as they barely reflect radar signal. This feature helps locate and monitor oil spills on the ocean surface because oil makes the ocean surface smooth and thus appears dark on radar images .

Polar Ice Covers.

The U.S. Landsat satellites, the Canadian RADARSAT spacecraft, and the European radar satellite ERS have been actively used to monitor the ice sheets in Antarctica and Greenland. The Landsat program has been ongoing since the early 1970s and has shown a significant modification of coastal regions in Antarctica during that period. Although it is not clear if there is a net gain or loss of ice volume in Antarctica, some of the ice shelves present in 1970s images have since disintegrated.

The Land-Sea Interface.

Beaches provide a lively, productive habitat for wildlife and a buffer against coastal storms. Salt marshes produce nutrient-rich "sludge" as a basis for the food chain while providing nurseries for juvenile fishes and habitat for shrimp, crabs, shellfish, turtles, and waterfowl. Coastal habitats are essential to the feeding, reproduction, and migration of fish and birds. But development, and the sand pumping, jetties, and seawalls that come with it, is overwhelming beaches. Salt marshes are under constant threat from short-sighted development schemes that require they be drained and filled.

NOAA has a Coastal Remote Sensing program that is using remote sensing, along with other technologies, to help coastal resource managers improve their management of aquatic and coastal ecosystems. The data sets and products provided by this program include ones dealing with ocean color, coastal topography and erosion, water quality, and the monitoring and tracking of harmful algal bloom.

Satellite remote sensing can thus play a central role in monitoring the health of coastal waters. The challenge is to provide decision makers with the knowledge derived from the remotely sensed data and to educate them about the mechanisms at work in coastal waters using satellite images.

Several commercial companies also provide remote sensing images and data from satellites for littoral water and ocean monitoring. The Digital-Globe company will launch 1-meter (39-inch) resolution satellites that are intended to show detailed coastal features, including beach structure, sand-bars, and wave patterns.

OrbImage has launched a commercial satellite, OrbView-2, to measure phytoplankton and sediment concentration in oceans and inland lakes, data that are useful for environmental applications such as coastal pollution monitoring and "red tide" tracking. Red tides are the result of dying algae producing a rapid multiplication of the bacteria that feed on them. These bacteria in turn deplete the water of its oxygen, killing marine life. Red tides can make mussels and oysters dangerous to eat as they produce toxins that can be life threatening to consumers.

These examples are by no means exhaustive of the many applications of satellite remote sensing, the numerous satellites in orbit, or the large number of new satellites planned. Satellite remote sensing is a business in rapid expansion, particularly on the commercial side. Earth data have been provided mainly by government-sponsored satellites until recently, but commercial satellite providers have entered the scene and will play an increasingly important role. This in turn has spurred the geographic information business.

Land Features

Satellite remote sensing was first used by the intelligence communities of the United States and the Soviet Union to spy on each other's military targets, starting in the early 1960s. In the 1970s, the United States initiated the Landsat program—a civilian program monitoring Earth's land resources—and in the 1980s NASA launched the Mission to Planet Earth program with an emphasis on understanding the global climate and monitoring the human impact on it.

In the late 1970s and in the 1980s, several other countries—such as India and France—launched remote sensing satellites to gather land surface data in an effort to monitor their agriculture and land use processes. Remote sensing information helps these countries establish national policies and monitor compliance. Since these satellites orbit over the whole Earth, they can provide data about many other locations. Both Indian Remote Sensing and the French Satellites Pour l'Observation de la Terre (SPOT) data are sold in the United States. Radar remote sensors have been put into space by Canada, the European Union, and Japan. Recently, several U.S. companies (Space Imaging, OrbImage, and DigitalGlobe being the leading companies) have obtained permission from the U.S. government to launch very-high-resolution satellites capable of seeing objects on the ground as small as 1 meter (39 inches). This may spur another information revolution similar to the personal computer explosion of the 1980s and the burgeoning of the Internet in the 1990s.

The enabling factors this so-called spatial information revolution include higher resolution, more reliable sensors, more powerful personal computers, the Internet, the civilian use of the Global Positioning System (GPS), and significantly improved geographic information system (GIS) software. Very-high-resolution, color images of any part of the world are predicted to become available on the Internet in almost real time for a modest fee. Anyone with a computer connected to the Internet would then be able to monitor his or her crops in a field, observe traffic jams in a big city in real time, and so forth.

Environmental Observations.

Land observations from space have an endless list of applications. A few examples are watershed analysis (including water resources inventory and water quality analysis), noxious weed detection, monitoring land use change over time, tracking urban sprawl and the loss of agricultural land, erosion monitoring, observing desertification in semiarid lands, tracking natural hazards such as floods and fires, agricultural land inventory, and crop yield prediction. Many of these observations have a significant economic impact and enhance the quality of life of citizens and user communities.

In the Middle East and in Africa water scarcity has become a serious geopolitical issue. Ecosystems rarely recognize political boundaries and several countries share common water resources. The Nile River, for example, flows through eight countries before reaching Egypt, a country that experiences very little rainfall and relies almost entirely on the waters of the Nile for its agriculture and drinking water resources. Actions upstream by other governments can severely impact Egypt. Similar problems exist in the Middle East between Syria, Jordan, and Israel, countries that share common aquifers (natural underground water reservoirs) and other water resources.

Satellite remote sensing may play an essential role for peace by providing information about new water resources as well as accurate maps of existing known resources and a means to monitor use. Radar remote sensing in particular can be helpful in discovering new water resources in arid areas. For instance, the Canadian RADARSAT system has discovered new underground water flows in the African desert.

In the American West, noxious weeds spread at an alarming rate, overcoming other species of vegetation, destroying ecological balance, and even killing livestock. It is very impractical and costly to locate these weeds from the ground in the semiarid expanses of Arizona, New Mexico, and Utah. Satellite remote sensing helps locate these weeds either through their spectral reflectance pattern or by observing their blooming at specific times of the year when no other vegetation blooms. This information can then be used to eradicate the weeds.

Agricultural Applications.

The agricultural applications of remote sensing are particularly useful. French SPOT satellites are used to determine what crops are planted where and how many acres of a given crop are planted in a region. Crop health is monitored over time, and claims of crop loss to drought or other natural disaster can be verified using satellite images.

The Earth Satellite Corporation uses remote sensing data to provide weekly information about worldwide crop conditions on the Internet. The company, for example, claims to make 95 percent correct yield predictions for cocoa, sugar, and coffee crops, two months ahead of harvest. Information such as this is extremely useful to growers needing to decide what crops to plant. If a wheat glut is predicted in South America in winter, informed farmers in the Northern Hemisphere will not plant wheat in early spring. Spatial technologies also give farmers new tools to better manage crops. A yield map over a field shows areas of higher and lower productivity.

In precision farming, a field is not treated as a homogeneous whole. Rather, as conditions—such as soil composition and soil fertility—vary across fields, the farmer's treatment of the field also varies. Thus irrigation, liming, fertilizers, and pesticides are not applied uniformly across a field but are varied according to need using a variable spreader with a GPS antenna and a computer program that has in its memory information about the local needs of the field. High-resolution satellite images can provide information about crop health. This information is put on a GIS used by the farmer to divide his fields into zones, each zone being treated differently. Precision farming has several advantages over traditional farming. As a result of differential treatment of field zones, there is less fertilizer, water, and/or pesticides used because applications are made in response to local needs only. There is thus economic benefit to the farmer. There is also less impact on the environment because fertilizer is not squandered in areas where it is not needed, reducing leaching into runoff water.

In the late 1990s thermal remote sensing data of fields in Alabama and Georgia showed a strong correlation between temperature maps of corn fields in June and yield maps of the same fields at harvest, at the end of August. These data were obtained using the NASA ATLAS sensor onboard a Stennis Space Center Lear Jet. Results indicate that thermal infrared remote sensing may predict crop yields with high accuracy several months before harvest. Thermal infrared emission from plants is a measure of their temperature. A healthy plant pumps water from the ground, vaporizes it (perspires), and stays cool by doing so. Less-healthy plants exposed to the hot summer Sun cannot keep cool and show a "fever."

Urban Observations.

Whereas in 1950 less than one-third of the world's population lived in urban areas, almost half the population lives in cities at the beginning of the twenty-first century. Projections indicate that in 2025 two-thirds of the growing world population will be city dwellers. Most of the city population increase will occur in developing countries where serious challenges are expected. In rich countries such as the United States, city development is characterized by urban sprawl using up an ever-increasing proportion of available land. A 1997 U.S. Department of Agriculture study reported that nearly 6.5 million hectares (16 million acres) of American forestland, cropland, and open spaces were converted to urban use between 1992 and 1997.

Rapid growth and changes of urban geography require detailed, accurate, and frequently updated maps. Such maps can be produced faster, cheaper, and with considerably less manpower by using very-high-resolution satellites such as Space Imaging's IKONOS than by using ground-based data acquisition. A number of satellite remote sensing companies, such as the French company, SPOT, provide services and products for land and urban planners and for businesses such as real estate and insurance companies.

This information may be used to decide in which region to expand urbanization, where to build roads, and how to develop transportation infrastructure. Frequently updated and accurate maps from very-high-resolution satellites will also be useful for infrastructure designs—power cables, water lines, sewer lines, urban transportation systems, and so on.

Businesses can use very-high-resolution urban satellite observations in conjunction with other data—such as demographics—to choose the right location for a franchise or a new store by extrapolating information about urban growth trends. Construction companies can use images taken by satellites, such as Space Imaging's IKONOS or Digital Globe's QuickBird to plan large-scale construction projects. These very-high-resolution satellites are able to identify and locate, with a great deal of accuracy, such surface features as buildings, parking lots, and their elevation.

Urban expansion and loss of farmland can also be monitored using radar remote sensing, such as that provided by the Canadian RADARSAT system. The advantage of radar is that it "sees" through clouds and at night. Thus, regions that are often covered with clouds and do not lend themselves to visible light and near-infrared remote sensing can be imaged using radar illumination.

Wireless communications in cities require a judicious distribution of relays atop tall buildings to avoid blind spots. A three-dimensional model of the cityscape is thus essential. Currently, such models are produced from radar and stereoscopic remote sensing from aircraft. Since cityscapes change rather quickly as new skyscrapers or other tall buildings are built, there is a need for updates. High-resolution radar or stereoscopic visible data from space-based satellites may in the future prove cheaper than aircraft for such applications.

Conclusion

This rapid tour of satellite and airborne remote sensing applications shows how useful this technology can be to resolve global, regional, or very local challenges when combined with GIS. It also gives a flavor of a future where geospatial information will permeate all activities on Earth and create tremendous business opportunities.

see also Global Positioning System (volume 1); Military Customers (volume 1); Military Uses of Space (volume 4); Natural Resources (volume 4); Reconnaissance (volume 1); Satellites, Types of (volume 1).

J.-M. Wersinger

Bibliography

Baker, John C., Kevin M. O'Connel, and Ray A. Williamson. Commercial Observation Satellites: At the Leading Edge of Global Transparency. Santa Monica, CA: RAND, and the American Society for Photogrammetry and Remote Sensing (ASPRS), 2001.

Jensen, John R. Remote Sensing of the Environment: An Earth Resource Perspective. Upper Saddle River, NJ: Prentice Hall, 2000.

National Research Council. Precision Agriculture in the 21st Century: Geospatial and Information Technologies in Crop Management. Washington, DC: National Academy of Science, 1998.

Internet Resources

Carbon Dioxide Information Analysis Center. <http://cdiac.esd.ornl.gov/home.html>.

Digital Globe Company. <http://www.digitalglobe.com>.

Earth Sat Company. <http://www.earthsat.com/>.

OrbImage Company. <http://www.orbimage.com>.

National Oceanic and Atmospheric Administration. Coastal Remote Sensing. <http://csc.noaa.gov/crs/>.

Space Imaging Company. <http://www.spaceimaging.com>.

United States Environmental Protection Agency, Global Warming. <http://www.epa.gov/globalwarming/>.

U.S. Geological Survey. Coastal-Change and Glaciological Maps of Antarctica. <http://pubs.usgs.gov/factsheet/fs50-98>.

U.S. Department of Energy, Center of Excellence for Sustainable Development. <http://www.sustainable.doe.gov/>.

RLV See Launch Vehicles, Reusable (Volume 1).

^*Phytoplankton is easy to recognize from space because it has a distinctive green color.