Landsat satellites and satellite technologies

In 1972, a series of Earth resources satellites called Landsat began collecting images of Earth. They gather information about various surface or near-surface phenomena, including weather, landforms, and land-use patterns. Satellites are used for crop forecasting, mineral and energy resource exploration, navigation and survey applications, and the compilation of resource inventories.

Background

Landsat satellites and similar satellite technologies designed for collecting information about Earth use a process known as remote sensing. R is the collection of data concerning an object or area without being near or in physical contact with it. Landsat satellites occupy various orbits above Earth. Some orbit from pole to pole, some circle around the equator, and others remain fixed above a specific geography.

The first remotely sensed images may have been acquired in 1858 by Gaspard-Félix Tournachon, who mounted a camera to a balloon and raised it 80 meters above Bièvre, France, thereby taking the first aerial photograph. The first attempt at remote sensing from rockets was made by Ludwig Rahrmann, who was granted a patent in 1891 for “obtaining bird’s eye photographic views.” Rahrmann’s rocket-launched camera, recovered by parachute, rarely exceeded 400 meters in height. The first cameras carried by modern rockets were mounted on captured German V-2 rockets launched by the US Army over White Sands, New Mexico, shortly after World War II.

Comprehensive imaging of Earth’s surface from a platform in space began with the development of a series of meteorological satellites in 1960. These first efforts, crude by later standards, were exciting at the time. However, scientists wanted to see more than cloud patterns. Later, during the manned space program, Gemini IV took a series of photographs of northern Mexico and the American southwest that guided geologists to new discoveries. The success of these and other attempts at space photography led to a program to develop satellites that could provide systematic repetitive coverage of any spot on Earth.

Early Landsat Satellites

In 1967, the National Aeronautics and Space Administration (NASA) began to plan a series of Earth Resources Technology Satellites (ERTS). The first, ERTS-1, was launched on July 23, 1972. ERTS-1 was a joint mission of NASA and the US Geological Survey (USGS), was the first satellite dedicated to systematic remote sensing of Earth’s surface, and used a variety of medium-resolution scanners. Perhaps most important, all images collected were treated according to an “open skies” policy; that is, the images were accessible to anyone. This policy created some concern in the government because of the Cold War tensions of the time. However, scientists realized that the advantages of worldwide use and evaluation of remotely sensed data far outweighed any concerns of disclosure. The project was judged to be a tremendous success by researchers worldwide.

A second ERTS satellite, launched on January 22, 1975, was named Landsat, for “land imaging satellite,” to distinguish it from Seasat, an oceanographic satellite mission then in the planning stages. Therefore, ERTS-1 was retroactively renamed Landsat 1, the 1975 satellite was designated Landsat 2, and the next satellite in the series, launched on March 5, 1978, was named Landsat 3.

The early Landsat satellites orbited Earth, north to south, about every 103 minutes at an approximate altitude of 920 kilometers. Orbiting in near-polar, sun-synchronous orbits, they crossed each latitude at the same time each day. This rendered every image with the same Sun angle (shadows) as recorded in previous orbits. The onboard scanners recorded a track 185 kilometers wide and returned to an adjacent western track twenty-four hours later. For example, if the satellite’s target was the state of Iowa, eastern Iowa would be scanned on Monday, central Iowa on Tuesday, and the western part of the state on Wednesday. This cycle of images could then be repeated every eighteen days, or about twenty times per year. The early Landsat satellites carried two imaging systems, each designed to record different parts of the electromagnetic spectrum: a return beam vidicom (RBV) system and a multispectral scanner system (MSS). The satellites’ data were sent back to Earth in a manner similar to television transmission.

The RBV system for Landsats 1 and 2 involved three television-type cameras aimed at the same ground area, while Landsat 3’s RBV system used two side-by-side panchromatic cameras (that is, cameras sensitive to the broad visible wavelength range) with a spatial resolution higher than that of RBV systems aboard the earlier Landsat platforms. Each camera recorded its image in a different frequency of light. Data obtained via the RBV were in the form of images similar to those of a television.

The MSS, which collected its multispectral data in digital form, proved to be more versatile. An MSS is a collection of scanning sensors, each of which gather data from a different portion of the spectrum. In Landsats 1 and 2, two cameras collected images in the visible spectrum: green light and red light; the other two collected in the near infrared. Landsat 3 added a fifth camera, which recorded thermal infrared wavelengths; however, it failed shortly after launch.

Each MSS image covers an area of about 185-by-185 kilometers. This renders a scale of 1:1,000,000 and an area of 34,000 square kilometers per frame. The resolution of the scanners was largely dependent on the atmospheric conditions and the contrast of the target, but under ideal conditions, they could resolve an area about 80 square meters. Therefore, any objects “seen” by the scanner had to be the size of a football field or larger. In the early to mid-1970s, this was considered medium-resolution capability. It was sufficient to resolve various natural phenomena but not detailed enough to compromise security-sensitive areas and activities such as military bases and operations.

Once transmitted to Earth, MSS data were retained in digital format and/or scanned onto photographic film. On film, they became black-and-white images that could be optically registered to create a single image. Then a color image could be created by passing red, blue, and green light through each negative. This color was not intended to re-create the natural scene but rather to enhance the contrast between various features recorded in different wavelengths.

The early Landsat satellites all continued to operate past their minimum design life of one year. Landsat 1 ended its mission on January 6, 1978, Landsat 2 on February 25, 1982, and Landsat 3 on March 31, 1983. By the time Landsat 3 stopped transmitting data, a new generation of Landsat satellites had taken to the skies.

Later Landsat Missions

Like their predecessors, the later Landsat satellites follow a near-polar, Sun-synchronous orbit to acquire data from a 56-meter-wide swath, but at a lower altitude of approximately 705 kilometers. These satellites orbit Earth about every 99 minutes, so that their repeat cycle is every sixteen days.

With Landsat 4, the National Oceanic and Atmospheric Administration (NOAA) and the private Earth Observation Satellite Company (EOSAT) joined NASA and the USGS as mission participants. Launched on July 16, 1982, Landsat 4 employed a four-band MSS like the ones aboard Landsats 1 and 2 but replaced the RBV (which had experienced a number of technical problems) with the more sophisticated thematic mapper (TM). The TM system, a multispectral imaging sensor similar to the MSS, added improved spatial resolution and midrange infrared to the data; three of its seven bands were dedicated to visible wavelengths, two to near-infrared, one to thermal infrared, and one to midinfrared. Landsat 4 ended its mission on December 14, 1993, with the failure of its last remaining science data downlink capability. Landsat 5 launched on March 1, 1984, with the same type of MSS and TM sensors used on Landsat 4. Like Landsat 4, it was a joint mission of NASA, the USGS, NOAA, and EOSAT. Although its MSS was powered off in August, 1995, as of 2009, Landsat 5 continued to collect and transmit data using only its TM system.

EOSAT’s participation in Landsats 4 and 5 was a result of the Land Remote Sensing Commercialization Act of 1984, legislation that opened up Landsat program management to the private sector. EOSAT began managing the program in 1985; however, within a few years it was apparent that the market for Landsat images could not offset operational costs. The Land Remote Sensing Policy Act of 1992 ended privatization and restored program management of future Landsat missions to the federal government. In 2001, operational responsibility for Landsats 4 and 5 returned to the government, along with rights to the data these satellites collected.

Landsat 6, launched on October 5, 1993, failed; it did not achieve orbit. With Landsat 7, a joint mission of NASA, the USGS, and NOAA, a new generation of sensor began to gather data. Landsat 7 was launched on April 15, 1999, equipped with an Enhanced Thematic Mapper Plus (ETM+). This sensor, the only one carried aboard the satellite, uses an oscillating mirror and detector arrays to make east-west and west-east scans as the satellite descends over Earth’s sunlit side. Of the sensor’s eight bands, three are devoted to visible wavelengths, one to near-infrared, two to short-wave infrared, and one to thermal infrared. The remaining band is panchromatic.

Both Landsats 5 and 7 have exceeded their life expectancies by several years. NASA and the USGS planned to launched the next satellite in the series, the Landsat Data Continuity Mission (LDCM), in 2013. NASA led the design, construction, and launch. After this, USGS oversaw its operation and renamed it Landset 8, which celebrated its tenth anniversary in 2023. Landsat 9 launched on September 27, 2021. NASA scheduled the launch of Landsat Next, also called Landsat 10, in late 2030 or early 2031.

Uses and Benefits

Generally, TM images can be used for a wider range of applications than MSS images can. The reason is that the TM records through more spectral bands with a greater spatial resolution. The MSS images are most useful describing and delineating large-scale phenomena such as geologic structures and land cover. The TM is perhaps more beneficial for land-use description and planning.

The ability of Landsat images to contrast target phenomena to the background or “noise” is what makes this research tool so powerful. Once the target has been delineated, a computer can inventory and/or map the target phenomena. The usefulness of Landsat images has been demonstrated in many fields, among them agriculture and forestry, and geography, and land-use planning. The World Bank uses these images for economic geography studies. A distinct advantage of this database is the “big picture” perspective afforded by the format: A single Landsat image can replace more than sixteen hundred aerial photographs of 1:20,000 scale. However, with the increase of aerial coverage comes a decrease in resolution. Therefore, these images may best be used as a complementary or confirming database to be used with other aerial imagery and ground surveys. Identifying the appropriate season for viewing a phenomenon or target is critical. For geographic features, the low Sun angle and “leaf-down” conditions of winter are an advantage. For biological phenomena, wet-dry seasons and time of year are critical. A riverbed or lake can disappear in dry conditions or be misinterpreted as a pasture if covered with green moss or algae. Therefore, matching the target to time of year and seasonal conditions must be a consideration when selecting a time window for observation.

The power of this perspective is revealed when satellite images are used to examine regional or area formations, structures, and trends. The extent of many geologic structures has been delineated with satellite imagery. For example, Landsat imagery has clearly identified impact craters, such as the Manicouagan ring in east-central Quebec, Canada, and fault systems, such as those of California’s San Andreas fault and Georgia’s Brevard fault zone. These systems extend hundreds of kilometers and are difficult, if not impossible, to perceive from the ground.

Additionally, satellite imagery has suggested areas for and exploration by decoding structure, potential oil and gas traps, and fault lines. Many of the areas involved are relatively inaccessible, and remote sensing has provided a map base and assisted in decoding the structures. Examples include the complex structures on the east side of the Andes, ranging from Brazil to Argentina, and a number of structures in countries of the former Soviet Union: the Caspian Sea states of Azerbaijan, Kazakhstan, and Turkmenistan; northern Russia’s tundra; the Timan-Pechora region near the Barents Sea; and western Siberia’s Priobskoye region. Satellite imaging is assisting the exploration of these remote areas, for which reliable topographic and geologic maps are scarce or nonexistent.

The usefulness of remote sensing is by no means restricted to energy exploration. The imagery has been used to inventory agriculture cropland and crop yields and to monitor irrigation and treatment programs. Therefore, it aids in commodities analysis. It also aids in environmental monitoring. Different plants reflect different spectral energies, and sensors can differentiate these wavelengths. In this way, the distribution and health of forests and wetlands can be mapped. Extreme environmental impacts can be assessed as well: The effects of disasters such as volcanic eruptions, earthquakes, droughts, forest fires, floods, hurricanes, cyclones, and oil spills can be mapped and inventoried via the satellite platform. Technological advances in data processing, integration, and dissemination have allowed the Landsat program to become a valuable source of real-time data, so that, in the wake of disasters, satellite imagery can support cleanup and relief efforts and hazard assessments.

As the longest-running program for remote sensing of Earth’s surface from orbit, Landsat provides an unparalleled view of the planet over time. Satellite images have proven to be an outstanding tool for observing changes to vegetation, coastal areas, and the land surface brought on by natural processes and human activity. They can be used to study everything from seasonal variations in vegetative cover to long-term trends in urban growth, wetlands loss, movement and melting, and desert encroachment.

Other Satellite Programs

Landsat 9 was part of the Earth Observing System (EOS), a program involving a series of polar-orbiting satellites and related interdisciplinary investigations looking into global change. Other EOS missions included the Quik Scatterometer, or QuikSCAT (launched June 19, 1999), which collected data on near-surface wind directions and speeds over Earth’s oceans; Terra (launched December 18, 1999), the first satellite designed to look at Earth’s air, oceans, land, ice, and life as a global system; the Active Cavity Radiometer Irradiance Monitor Satellite, or ACRIMSAT (launched December 20, 1999), which measured how much of the Sun’s energy reached Earth’s atmosphere, oceans, and land surface; Jason-1 (launched December 7, 2001), a joint US-French mission that studied global ocean circulation; Aqua (launched May 4, 2002; still in operation in 2024), which gathers data on clouds, precipitation, atmospheric moisture and temperature, terrestrial snow, sea-ice and sea-surface temperature; the Ice, Cloud, and land Elevation Satellite, or ICESat (launched January 12, 2003), which monitored the elevations of ice sheets, clouds, and the land surface; the Solar Radiation and Climate Experiment, or SORCE (launched January 25, 2003), which measured irradiance from the Sun; Aura (launched July 15, 2004; still in operation as of 2024), which investigates atmospheric dynamics and chemistry; and the Ocean Surface Topography Mission, or OSTM (launched June 20, 2008), which measured ocean surface topography.

In 1986, the French government, with Sweden and Belgium as partners, launched the first of a series of Système Probatoire d’Observation de la Terre (SPOT) satellites. This commercial system, designed to compete with the American Landsat program, featured 10-meter resolution for its black-and-white imagery and 20-meter resolution for color imagery. SPOT had the further advantageous ability to create stereoscopic images. As of 2023, two of the seven satellites launched in the SPOT series remained operational.

Other satellite systems are also scanning the surface of Earth. For example, there are meteorological satellites serving the needs of the US National Oceanographic and Atmospheric Administration (NOAA). Another large-scale satellite endeavor is the Geostationary Operational Environmental Satellite (GOES) series. A geostationary satellite is one that can remain stationary over a specific point above Earth and observe it twenty-four hours a day. A third class of meteorological satellite is the US Defense Meteorological Satellite Program (DMPS). Another satellite program, Seasat, monitors the oceans. These satellites scan in the microwave wavelengths and have proven to be reliable in mapping temperatures and detecting chlorophyll and suspended solids.

While not revealing any information about Earth itself, a class of navigation satellite known as the Navstar Global Positioning System (GPS) assists in resource development in a different way. This system began in March, 1994, and is funded by the US Department of Defense (DOD) and managed by the United States Air Force Fiftieth Space Wing. The GPS system consists of twenty-four to thirty-two satellites spaced so that between five and eight are visible from any point on Earth. By triangulation of a radio signal broadcast from each satellite, users equipped with a receiver may accurately locate their position on the ground in three dimensions. When the military first introduced global positioning via satellite, it intentionally degraded the signal so that civilian users could be accurate to only 100 meters or so, while DOD users could locate a position to within 20 meters for military operations. In 2000, after the military had demonstrated that regional signal degradation could provide sufficient protection for security-sensitive locations, civilian and commercial access to the higher-resolution data was enabled. GPS initially gained popularity among nonmilitary users as a valuable tool for people working in areas where maps were of poor scale or nonexistent—for instance, in remote oil or mineral exploration operations or environmental surveys or mapping efforts in the wild. Afterward, and particularly after the improvement of signal accuracy, GPS has found many commercial applications; civilians can access GPS signals from their cell phones, smart phones, car computers, and other wireless devices.

Remote sensing from near-space orbital platforms has revolutionized how humans see Earth and contributed greatly to the disciplines of agriculture, cartography, environmental monitoring, forestry, geology and geography, land-use planning, meteorology, and oceanography. Its impact has been not only scientific but also political and sociological. As other countries launch satellites, information concerning Earth becomes more democratic, and political boundaries become more artificial. Remote sensing has become an invaluable tool for scientific investigation, but its data must be used and interpreted appropriately and in conjunction with other research tools and databases.

Bibliography

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