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Remote sensing

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For the purported psychic ability to sense remotely, see Remote viewing

Synthetic aperture radar image of Death Valley colored using polarimetry

In 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 pregnancy via ultrasound, MRI,PETscan, 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, and is distinct from other imaging-related fields such as medical imaging.

There are two kinds of remote sensing. Passive sensors detect natural energy (radiation) that is emitted or reflected by the object or scene being observed. Reflected sunlight is the most common source of radiation measured by passive sensors. Examples of passive remote sensors include film photography, inra-red charge-coupled devices and radiometers. Active sensors, on the other hand, provide their own source of energy to illuminate the objects they observe. An active sensor emits radiation in the direction of the target area to be scanned. The sensor then detects and measures the radiation that is reflected or backscattered from the target. RADAR is an example of active remote sensing wher the time delay between emission and return is measured, the distance to the target can be determined.

Remote sensing makes it possible to collect data on dangerous or inaccessable areas. The use of Remote sensing is in use for the purposen of monitoring deforestation in areas such as the amazon basin, the effects of polar warming in glaciers and arctic and antarctic regions, and depth soung of coastal areas and ocean depths. Remote sensing also replaces costly and slow collection on the ground, ensuring in the process that areas or objects are not diturbed.

Orbital collections transmit different parts of the electromagnetic spectrum, in conjunction with smaller scale aerial or ground-based sensing and analysis provides researchers with enough information to monitor trends such as ell nino and other natural long and short term phenomena. Other uses include different areas of earth sciences, to include natural resource mangement and agricultural fields such as land usage and conservation, and national security, both overhead, ground-based and stand-off collection of border areas.[1].


Data acquisition techniques

The basis for multi-spectral collection and analysis is that of examined areas or objects that reflect or emit radiation that stand out from surrounding areas.

Applications of Multi-spectral data

  • Radar Conventional radar is mostly associated with aerial traffic control, early warning, and certain large scale meteorical data. Doppler radar is used by local law enforcements' monitoring of speed limits, enhanced meteorolical collection to include wind speed and direction within weather sytems. Other types of active collection includes plasmas in the ionosphere). Interferometric synthetic aperture radar is used to produce precise digital elevation models of large scale 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 Light Detection And Ranging - is well known in the examples of weapon ranging laser illuminated homing of projectiles. LIDAR is used to detect and measure the concentration of various chemicals in the atmosphere, while airborne LIDAR can be used to measure heights of objects and features on the ground more accurately than with radar technology.
  • Radiometers and photometers are the most common instrument in use, collecting reflected and emitted radiation in a wide range of frequencies. The most common are visible and infrared sensors, followed by microwave, gamma ray and rarely, ultraviolet. They may also be used to detect the emission spectra of various chemicals, providing data on chemical concentrations in the atmosphere.
  • Stereographic pairs of aerial photographs have often been used to make Topographic maps by Imagery Analysts, Terrain Anaysts in trafficabilty and highway departments for potential routes.
  • Simultaneaous multi-spectral platforms such as Landsat have been in use since the 70's.these 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 indigeanous plants and crops, including entire farming regions or forests.

Geodetic

  • Overhead geodetic collection was first used in aerial submarine detection and gravitational data used in military maps. This data reveal minnute perturbations in the Earth's gravitational field (geodesy) may be used to determine changes in the mass distribution of the Earth, which in turn may be used for geological or hydrological studies.

Acoustic and near-acoustic

  • Passive;Sonar is used 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.
  • Active; pulses are used by geologists to detect oil fields.

In order to coordinate a series of large-scale observations, most sensing systems need to know where they are, what time it is, and the rotation and orientation 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.

Resolution impacts collection and is best explained with the following relationship; less resolution=less detail & larger coverage, More resolution=more detail, less coverage. One example of the misuse of resources is that of using multiple high resolution data which tends to clog transmission and storage infrastructure.

Data processing

See also: Inverse problem

Generally speaking, remote sensing works on the principle of the inverse problem. While the object or phenomenon 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.

The quality of remote sensing data consists of its spatial, spectral, radiometric and temporal resolutions. Spatial resolution refers to the size of a pixel that is recorded in a raster image - typically pixels may correspond to square areas ranging in side length from 1 to 1000 metres. Spectral resolution refers to the number of different frequency bands recorded - usually, this is equivalent to the number of sensors carried by the satellite or plane. Current Landsat collection is that of seven bands, including several in the infra-red spectrum. The MODIS satellites are the highest resolving at 31 bands. Radiometric resolution refers to the number of different intensities of radiation the sensor is able to distinguish. Typically, this ranges from 8 to 14 bits, corresponding to 256 levels of the gray scale and up to 16,384 intensities or "shades" of colour, in each band. The temporal resolution is simply the frequency of flyovers by the satellite or plane, and is only relevant in time-series studies or those requiring an averaged or mosaic image as in deforesting monitoring. This was first used by the intelligence community where repeated coverage revealed changes in infrastructure or the deployment of units. Cloud cover over a given area or object makes it necessary to repeat collection of said location. Finally, some people also refer to the "economic resolution", that is, the cost-effective way to manage the collection of data.

In order to photographic-based maps, most remote sensing systems expect to convert a photograph or other data item to a distance on the ground. This depends on the type of sensor used. For example, in conventional photographs, distances are accurate in the center of the image, with the distortion of measurements increasing the farther you get from the center. Another factor is that of the platen against which the film is pressed can cause severe errors when photographs are used to measure ground distances. The step in which this problem is resolved is called georeferencing, and involves computer-aided matching up of points in the image (typically 30 or more points per image) which is extrapolated with the use of an established benchmark, "warping" the image to produce accurate spatial data. As of the early 1990s, most satellite images are sold fully georeferenced.

In addition, images may need to be radiometrically and atmospherically corrected. Radiometric correction gives a scale to the pixel values, e.g. the monochromatic scale of 0 to 255 will be converted to actual radiance values. Atmospheric correction eliminates atmospheric haze by rescaling each frequency band so that its minimum value (usually realised in water bodies) corresponds to a pixel value of 0.

Interpretation is the critical process of making sense of the data. The first application was that of aerial photographic collection which used the following process; spatial measurement through the use of a light table in both conventional single or stereographic coverage, added skills such as the use of photogrammetry, the use of photomosaics, repeat coverage, Making use of objects' know dimentions in order to detect modifications. Image Analysis is the recently developed automated computer-aided application which is in increasing use.

Old data from remote sensing is often 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-readable 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.

History

The U-2 reconnaissance aircraft.
The 2001 Mars Odyssey Spacecraft used spectrometers and imagers to hunt for evidence of past or present water and volcanic activity on Mars.

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 use of modified combat aircraft such as the P-51, P-38, RB-66, F4-C and the SR-71 or specifically designed collection platforms such as the U2/TR-1 and the OV-1 series both in overhead and stand-off collectionevelopment of reconnaissance aircraft such as the U-2 and the A-5. A more recent development is that of increasingly smaller infra-red sensor turrets such as those used by law enforcement and the military, both manned and unmanned platforms. The advantage is that this approach requires minimal modification to a given airframe. Later imaging technologies would include Infra-red, conventinal, doppler and synthetic aperture radar

The development of artificial satellites in the latter half of the 20th century allowed remote sensing to progress to a global scale as of the end of the cold war. Instrumentation aboard various Earth observing and weather satellites such as Landsat, the Nimbus and more recent missions such as RADARSAT and UARS provided global measurements of various data for civil, research, and military purposes. Space probes to other planets have also provided the opportunity to conduct remote sensing studies in extra-terrestrial environments, 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.

Further strides were made in the 1960s and 1970s with the development of image processing of satellite imagery, beginning in the United States. Several research groups in Silicon Valley including NASA Ames Research Center, GTE and ESL Inc. developed Fourier transform techniques leading to the first notable image enhancement of aerial photographs.

Further reading

  • Campbell, J.B. (2002). Introduction to remote sensing (3rd ed. ed.). The Guilford Press. ISBN 1-57230-640-8. {{cite book}}: |edition= has extra text (help)
  • Jensen, J.R. (2000). Remote sensing of the environment: an Earth resource perspective. Prentice Hall. ISBN 0-13-489733-1.
  • Lillesand, T.M. (2003). Remote sensing and image interpretation (5th ed. ed.). Wiley. ISBN 0-471-15227-7. {{cite book}}: |edition= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  • Richards, J.A. (2006). Remote sensing digital image analysis: an introduction (4th ed. ed.). Springer. ISBN 3-540-25128-6. {{cite book}}: |edition= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  • US Army FM series.
  • US Army military intelligence museum, FT Huachuca, AZ

See also

Associations and Societies

Spatial Agencies

Remote Sensing Centres

Satellites (missions, platforms and sensors)

Tutorials

Other relevant sites

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