Remote sensing

From Open Encyclopedia

Image:Chateau Beaugency ballon.jpgIn the broadest sense, remote sensing is the measurement or acquisition of information of an object or phenomenon, by a recording device that is not in physical or intimate contact with the object. In practice, remote sensing is the utilization at a distance (as from aircraft, spacecraft, satellite, or ship) of any device for gathering information about the environment. Thus an aircraft taking photographs, Earth observation and Weather satellites, monitoring of a fetus in the womb via ultrasound, and space probes are all examples of remote sensing. In modern usage, the term generally refers to techniques involving the use of instruments aboard aircraft and spacecraft rather than other fields which are considered to be under medical imaging.

While all astronomy could be considered remote sensing (in fact, extremely remote sensing) the term "remote sensing" is normally only applied to terrestrial observations.

1 Data aquisition techniques
2 Data processing
3 History
4 Further readings
5 See also
6 External links

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Data aquisition techniques

Data may be aquired through a variety of devices depending upon the object or phenomina being observed. Most remote sensing techniques make use of emitted or reflected electromagnetic radiation of the object of interest in a certain frequency domain (infrared, visible light, microwaves). This is possible due to the fact that the examined objects (plants, houses, water surfaces, air masses ...) reflect or emit radiation in different wavelengths and in different intensity according to their current condition. Some remote sensing systems use sound waves in a similar way, and others measure variations in gravitational or magnetic fields.

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Radiometry

Image:Radar antenna.jpg

Radar may be used for ranging and velocity measurements of hard targets (i.e. an aircraft) or distributed targets (such as a cloud of water vapor, or plasmas in the ionosphere). Synthetic aperture radar can produce precise Digital elevation models of terrain (See RADARSAT, Magellan).
Laser and radar altimeters on satellites have provided a wide range of data. By measuring the bulges of water caused by gravity, they map features on the seafloor to a resolution of a mile or so. By measuring the height and wave-length of ocean waves, the altimeters measure wind speeds and direction, and surface ocean currents and directions.
Lidar may be used to measure the concentration of various chemical species in the atmosphere as well as ranging.
Radiometers and photometers may be used to detect the emission spectra of various chemical species, thus providing information on chemical concentrations in the atmosphere.
Stereographic pairs of aerial photographs have often been used to make Topographic maps. Satellite imagery has also been used.
Thematic mappers take images in multiple wavelengths of electro-magnetic radiation (multi-spectral) and are usually found on earth observation satellites, including (for example) the Landsat program or the IKONOS satellite. Maps of land cover and land use from thematic mapping can be used to prospect for minerals, measure land usage, and examine the health of plants, including entire farming regions or forests.

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Geodesy

Satellite measurements of minute perturbations in the Earth's gravitational field (geodesy) may be used to determine changes in the mass distrubution of the Earth, which in turn may be used for geological or hydrological studies.

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Acoustics

Image:Fishfinder.jpg

Sonar may be utilized for ranging and measurements of underwater objects and terrain.
Seismograms taken at different locations can locate and measure Earthquakes (after the fact) by comparing the relative intensity and precise timing.

In order to coordinate a series of observations, most sensing systems need to know where they are, what time it is, and the rotation and orinetation of the instrument. High-end instruments now often use positional information from satellite navigation systems. The rotation and orientation is often provided within a degree or two with electronic compasses. Compasses can measure not just azimuth (i.e. degrees to magnetic north), but also altitude (degrees above the horizon), since the magnetic field curves into the Earth at different angles at different latitudes. More exact orientations require gyroscopic pointing information, periodically realigned in some fashion, perhaps from a star or the limb of the Earth.

The resolution determines how many pixels are available in measurement, but more importantly, higher resolutions are more informative, giving more data about more points. However, more resolution occasionally yields less data. For example, in thematic mapping to study plant health, imaging individual leaves of plants is actually counterproductive. Also, large amounts of high resolution data can clog a storage or transmission system with useless data, when a few low resolution images might be a better use of the system.

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Data processing

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Generally speaking, remote sensing works on the principle of the inverse problem. While the object or phenomina of interest (the state) may not be directly measured, there exists some other variable that can be measured (the observation), which may be related to the object of interest via some (usually mathematical) model. The common analogy given to describe this is trying to determine the type of animal from its footprints. For example, while it is impossible to directly measure temperatures in the upper atmosphere, it is possible to measure the spectral emissions from a known chemical species (such as carbon dioxide) in that region. The frequency of the emission may then be related to the temperature in that region via various thermodynamic relations.

In order to generate maps, most remote sensing systems expect to convert a photograph or other data item to a distance on the ground. This almost always depends on the precision of the instrument. For example, distortion in an aerial photographic lens or the platen against which the film is pressed can cause severe errors when photographs are used to measure ground distances.

Interpretation is the critical process of making sense of the data. Traditionally, this was a human being, perhaps with a few measurement tools and a light table. In modern systems that produce digital data, often the tool is a family of computer programs that interpret the data to form maps, or statistical analyses.

Old data from remote sensing is often unreasonably valuable because it may provide the only long-term data for a large extent of geography. At the same time, the data is often complex to interpret, and bulky to store. Modern systems tend to store the data digitally, often with lossless compression. The difficulty with this approach is that the data is fragile, the format may be archaic, and the data may be easy to falsify. One of the best systems for archiving data series is as computer-generated machine-readeable ultrafiche, usually in typefonts such as OCR-B, or as digitized half-tone images. Ultrafiches survive well in standard libraries, with lifetimes of several centuries. They can be created, copied, filed and retrieved by automated systems. They are about as compact as archival magnetic media, and yet can be read by human beings with minimal, standardized equipment.

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History

Image:Usaf.u2.750pix.jpg Image:Mars-odyssey-sm.jpg Beyond the primitive methods of remote sensing our earliest ancestors used (ex.: standing on a high cliff or tree to view the landscape), the modern discipline arose with the development of flight. The balloonist G. Tournachon (alias Nadar), who made photographs of Paris from his balloon in 1858, is considered to be the first aerial photographer. Messenger pigeons, kites, rockets and unmanned balloons were also used for early images. These first, individual images were not particularly useful for map making or for scientific purposes.

Systematic aerial photography was developed for military purposes beginning in World War I and reaching a climax during the Cold War with the development of reconnaissance aircraft such as the U-2.

The development of artificial satelites in the latter half of the 20th century allowed remote sensing to progress to a global scale. Instrumentation aboard various Earth observing and weather satelites such as Landsat, the Nimbus and more recent missions such as RADARSAT and UARS provided global measurements of various data for civil, scientific, and military purposes. Space probes to other planets have also provided the opportunity to conduct remote sensing studies in extra-terrestrial enviroments, synthetic aperture radar aboard the Magellan spacecraft provided detailed topographic maps of Venus, while instruments aboard SOHO allowed studies to be performed on the Sun and the solar wind, just to name a few examples.

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