Remote-sensing satellites

Remote-sensing satellites use instruments for gathering data about Earth's atmosphere, climate, weather, and geography, and also for exploring celestial bodies.

The Meaning and History of Remote Sensing

While remote sensing has applications for greater discovery on Earth and the celestial bodies beyond, a person can be engaged in remote sensing, at its most basic level, at any given moment. Remote sensing is utilizing instruments to obtain information about objects without coming into contact with those objects. (By reading this text, one engages in remote sensing, using an “instrument” of the body.)

The history of remote-sensing satellites significantly precedes that of space travel. Early attempts at aerial surveillance go as far back as the eighteenth century. In 1794, French troops utilized hot air balloons to obtain information on enemy troop formations at the Battle of Fleurus, during the French Revolutionary Wars.

In the nineteenth century, with the invention of photography, balloons were used to assist in mapping. Later, aerial reconnaissance was used by the Union and Confederacy forces during the American Civil War to learn more about enemy troop positions. Similar balloons were used for mapping in the Spanish-American War at the close of the 1890s.

The twentieth century saw a boom in aerial photography. In 1903, German engineer Julius Neubronner invented a camera to be placed on the breast of a pigeon to take photographs at set intervals. In 1911, Italian airplane racer and air force commander Carlo Piazza flew probably the first reconnaissance mission using photography during the Italo-Turkish War. Aerial photography accompanied the widespread use of airplanes in World War I. The technology spread afterward, allowing pilots to photograph difficult-to-access areas like Antarctica from above.

During World War II, Nazi Germany developed V-2 rockets, missiles with a range of about 305 kilometers (190 miles) and designed to strike land targets. Following the war, Wernher von Braun, the physicist who headed the team that had developed the V-2, came to the United States and worked with the U.S. Army and later with the National Aeronautics and Space Administration (NASA) in developing rockets. His research at White Sands, New Mexico, involved the use of rockets, often outfitted with cameras or video recorders.

A key step from aerial photography and surveillance to remote sensing took place with the launch of the first satellite, Sputnik I, by the Soviet Union on October 4, 1957. While later satellites would conduct remote sensing to gather data about the planet, Sputnik I was effectively different. It sent out a radio signal that could be picked up on Earth; remote sensing of space, as it applied to this satellite, was conducted by operations on Earth.

The launch of Sputnik triggered strong public opinion and is cited as a primary reason for the founding of NASA in 1958. A number of Russian and later U.S. satellites followed, many of which were equipped with instruments to gather data. Some were sent to make measurements of outer space, but others were designed to observe the earth.

Researchers recognized the limits of conventional photography when it came to observing the earth and reached out for newer tools, including multispectral, infrared, and ultraviolet photography. Recognizing a change in the field, Evelyn Pruitt, a geographer working for the U.S. Office of Naval Research, coined the term “remote sensing” in 1960. Pruitt explained that a new term was needed to change the notion of what the field involved.

Beyond aerial photography, remote sensing incorporated photography methods outside the visual light spectrum, ultimately “seeing” the earth in different ways. The great amount of data being collected by remote-sensing satellites also required a better processing system for the data. Geographic information systems (GISs) were used beginning in the 1970s to synthesize all the data, enabled by the increasing memory and processor speeds of computers becoming available.

GIS allows for the development of many different “maps,” including, for example, a map that would include details of a location's topography, soil, vegetation, and temperature. Maps not generated by satellite data also can be incorporated into such a system (for example, a census of a region's population). GIS, like other remote-sensing satellite technologies, is now available to consumers. Starting in the early twenty-first century, Web-based programs such as Google Earth have allowed for the observation of satellite data on personal computers.

Remote-sensing satellites are used in a wide variety of applications. These applications include determining the location of objects on the earth's surface and monitoring pollution, weather systems, oceanic activity, and the changing geographic terrain of areas such as glaciers, rainforests, and crop lands. These satellites also are used for space exploration.

Observing the Earth from Above

In 1972, NASA and the U.S. Geological Survey started the Landsat program. This program, originally called the Earth Resources Technology Satellite I, was designed to monitor the natural resources of the planet. The program is ongoing, and seven Landsat satellites have been launched, the most recent in 1999. However, only Landsat 5 and Landsat 7 are operational.

Landsat satellites photograph the earth's surface, and the images are stored to give a picture of changing geography over time. Landsat's imaging is at a medium resolution, which allows the viewer to see major geographic features and larger roads but not individual houses.

The European Space Agency also monitors Earth's climate and natural resources, with programs such as Envisat, which launched a satellite in 2002. Envisat provides data on Earth's land surface, oceans, and atmosphere.

Many satellites also are equipped with advanced instruments that can obtain data beyond visible light frequencies. The National Oceanic and Atmospheric Administration, for example, launched the Advanced Very High Resolution Radiometer satellite program in 1998. The satellite captures images at infrared and near-infrared frequencies.

Infrared is key in measuring polar ice, glaciers, and snow fall, all of which are crucial to scientific inquiry into global climate trends. Infrared sensing allows for gauging snowfall and the density of ice. The earth's reduced ice density indicates a warming trend for the planet, by which ice melts and does not refreeze as the temperature cools. This reduced density is evident not only in the polar regions but also in areas where glaciers have been shrinking with the passing years.

Other monitoring issues are more localized. Many of the satellites used to gauge land usage are accurate to within about 1 kilometer (0.6 mile). This accuracy is enough to gauge the effect of human activity on local ecosystems. A common issue is the conversion of other types of land into cropland. Two problems that often accompany cropland development are deforestation and soil erosion. Satellite observation has shown that tens of thousands of square kilometers of rainforest are lost each year in Southeast Asia, Africa, and Latin America. In addition to a loss of wildlife and other parts of the rainforest ecosystem, the loss of rainforest can lead to soil erosion.

Monitoring the earth through satellites goes beyond landforms. One method of mapping that has been distinctly changed by use of satellites is bathymetry. From ancient times, sailors have “sounded” the ocean floor by using poles to determine water depth. Over time, instruments such as sonar were developed to help measure the depths of the ocean floor—measurements that could provide three-dimensional maps.

In the 1850s, experts determined that height analysis at the water's surface could indicate depth. Height analysis was further tested in the late 1970s. Later, altimetry from orbiting satellites was used to gauge the depths of the oceans.

Another important measurement of altimeters on satellites is that of sea levels. Rising global temperatures are leading to a rise in ocean levels, which is caused by melting ice caps in the polar regions. Satellite altimetry, which is thought to be more accurate than land-based measurement, in which movement of the land may confound the measurements, has shown a rise in ocean levels of 1 to 2 millimeters per year. The small changes involved in detecting changes in sea level require a fairly sensitive measurement.

Monitoring the Atmosphere

A key satellite component in measuring substances in Earth's atmosphere is NASA's Moderate-Resolution Imaging Spectroradiometer (MODIS), which is included on several satellites. MODIS instruments observe visible and infrared radiation. Although designed to make surface observations, MODIS also can collect data on precipitation, clouds, and surface temperatures.

Other satellites involved in monitoring the atmosphere use similar technologies to observe the levels of pollutants. Some of the molecules monitored are carbon dioxide, methane, and ozone. Carbon dioxide and methane are special concerns because they can absorb heat, contributing to increased temperatures on Earth. Ozone, a component of the atmosphere, is reactive. Certain pollutants, when released into the atmosphere, will break the bonds of the ozone. This effect is leading to concerns about increased greenhouse gases with lower levels of ozone in the atmosphere.

Weather systems also can be tracked by satellite. However, for local weather, a key component of remote sensing is on the ground. Doppler radar utilizes the Doppler principle, according to which radiation from incoming objects arrives at a higher frequency. The stationary towers with Doppler radar are thus able to predict incoming storms and other systems. For this reason, they are often located near airports, where knowledge of local wind patterns is critical for planes departing and landing.

Remote Sensing and Space Exploration

While humans traveling to other planets has been a regular theme of science fiction, for the foreseeable future satellites will be doing the majority of space exploration. The same instruments used to gather information about the topographic features of Earth have been utilized for gathering data about the geographic features of other planets.

Europa, Jupiter's smallest moon, has been found to be covered in a layer of ice, beneath which there is flowing water (the only known extraterrestrial ocean in the solar system). This ocean was discovered through remote sensing—specifically, through infrared photography from the Galileo spacecraft, which was launched by the United States in 1989. The Hubble Space Telescope, launched by NASA in 1990, is perhaps the best-known satellite for space exploration. Orbiting Earth, its cameras capture previously unseen images of outer space.

Other Remote-Sensing Applications

The products of remote sensing have had perhaps their greatest influence in the consumer marketplace. The U.S. Department of Defense has a series of more than thirty satellites orbiting Earth, satellites that communicate with one another about their respective position. Although this system was originally created to help the U.S. military with global positioning, a second, civilian signal was introduced in 1983. Initially much less accurate than the encrypted military signal, it has improved in quality, with positioning accurate to within a few meters of a location.

In addition to helping determine the current state of the planet, remote-sensing satellites play a role in discovering more about Earth's past. Remote sensing has long been used in archaeology. Some of the earliest aerial photographs of archaeological sites came just after the first aerial photographs were taken. Satellites allow for a wider survey of archaeological sites, assisting in the understanding of the topology of a region and whether certain changes over time are caused by human activity or by natural, nonhuman processes. Multispectral photography is one tool that can display human-caused disturbances in the soil or other geographic features. Remote-sensing satellites also allow for the study of areas, such as rainforests, that are remote and difficult for humans to reach.

The Politics of Remote Sensing

Remote sensing and its products have played a part in two major political debates in recent years. While GPS technologies were originally developed for use by the U.S. military, GPS has also been used as a weapon against the military. After capturing insurgents following battles, soldiers have recovered GPS units used to gather information about terrain and potential troop positions.

Perhaps greater public concern has surrounded remote sensing's involvement in acts of terrorism. Following the terrorist attacks on the city of Mumbai, India in 2008, it was revealed that terrorists had used Google Earth to map their attack route. These concerns may influence scientific investigations. Researchers have noted that in certain parts of the world, the use of some of the more sensitive remote-sensing technologies (such as LIDAR) is restricted for military use only.

Privacy also is a concern. GPS is feared by some as a tracking device. It should be noted, however, that many of these concerns arise out of a misunderstanding of GPS technology. With GPS, the receiver tracks the satellites; the satellite does not track the receiver. Privacy advocates should not be concerned with GPS and remote-sensing satellites; rather, they should focus their concerns on service providers, cell phone towers, closed-circuit television cameras, and other remote-sensing devices much closer to earth.

Climate change is another major political issue associated with remote-sensing satellites. The proliferation of satellite technologies in the late twentieth century has enabled the widespread study of Earth's climate. In addition to land-use and ecosystem changes, the overall climate of the planet is changing. Data suggest that the planet is becoming warmer, which could spell catastrophe for life on Earth. The scientific consensus is that much of this climate change is caused by humans.

Finding a solution to this issue is likely to take some time. While local environmental issues can be fixed more quickly and easily, harmful global climate change will require worldwide efforts to alleviate. Modifying human habits will likely prove difficult, and the international diplomacy required will likely lead to roadblocks, as developing nations may face conflicts between economic development and reducing pollution levels.

Principal Terms

aerial photography: geospatial observation from above; in some respects, a fairly limited predecessor of modern remote sensing

altimetry: a determination of the topography of geographic features; with satellites, it involves sending a signal and determining its time of return

bathymetry: a determination of ocean depths, previously measured through depth sounding but now measured through satellite altimetry

geographic information system (GIS): a computer system designed to store and handle the large quantity of data generated by geographic research

geospatial: pertaining to the location of objects on the surface of the earth and their relationship to each other

light detection and ranging (LIDAR): a system in which light signals are pulsed to the ground, detecting range to map an area's topography

multispectral photography: photography generated by utilizing the varying emission wavelengths of different rocks and vegetation

remote sensing: the use of instruments not in contact with an object to gain information about that object

satellite: a human-made object put into orbit around a celestial body, typically equipped with instruments for various measurements or observations

topography: a representation of the geographic features of an area, such as on a map

topology: the study of the topography of a certain area over time

Bibliography

Chuvieco, Emilio, and Chris Justice. “NASA Earth Observation Satellite Missions for Global Change Research.” Earth Observation of Global Change: The Role of Satellite Remote Sensing in Monitoring the Global Environment, edited by Emilio Chuvieco. Berlin: Springer, 2008. The authors enumerate the various methods, technologies, and specific missions NASA has undertaken to monitor global environmental change.

Garfinkel, Simson. “Google Earth: How Google Maps the World.” Technology Review 110, no. 6 (November/December 2007): 20-21. A brief overview of how Google uses a satellite network to provide high-resolution images of the earth and allows users to view specific areas using only the power of a home computer.

Kramer, Herbert J., and Arthur P. Cracknell. “An Overview of Small Satellites in Remote Sensing.” International Journal of Remote Sensing 29, no. 15 (August 10, 2008): 4285-4337. This article provides a detailed history of satellites, listing the various missions and countries of origin of satellites that have been launched since Sputnik and the technological changes that have taken place along the way.

Mayaux, Philippe, et al. “Remote Sensing of Land-Cover and Land-Use Dynamics.” In Earth Observation of Global Change: The Role of Satellite Remote Sensing in Monitoring the Global Environment, edited by Emilio Chuvieco. New York: Springer, 2008. The authors provide a view of how recent developments enable remote-sensing satellites to give a picture of resource availability. The book discusses problems such as deforestation and how the use of land for crops has affected other ecosystems.

Norris, Pat. Watching Earth from Space: How Surveillance Helps Us, and Harms Us. New York: Springer, 2010. This book provides an overview of some of the benefits of remote sensing, but also discusses some of the incidents where this information has been used to plan terrorist acts.

Pruitt, Evelyn L. “The Office of Naval Research and Geography.” Annals of the Association of American Geographers 69, no. 1 (March 1979): 103-108. In this overview, Pruitt explains, among other developments, the reasons for the changes in the field of remote sensing and why she developed a new term for this field of study.

Verbyla, David L. Satellite Remote Sensing of Natural Resources. Boca Raton, Fla.: Lewis, 1995. This short book provides an overview of how many of the technologies available in remote sensing are used to map the earth's surface.