Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Earth and other planets. Remote sensing is used in numerous fields, including geophysics, geography, land surveying and most Earth science disciplines (e.g. exploration geophysics, hydrology, ecology, meteorology, oceanography, glaciology, geology). It also has military, intelligence, commercial, economic, planning, and humanitarian applications, among others.
In current usage, the term remote sensing generally refers to the use of satellite- or airborne-based sensor technologies to detect and classify objects on Earth. It includes the surface and the atmosphere and oceans, based on propagated signals (e.g. electromagnetic radiation). It may be split into "active" remote sensing (when a signal is emitted by a sensor mounted on a satellite or aircraft to the object and its reflection is detected by the sensor) and "passive" remote sensing (when the reflection of sunlight is detected by the sensor).
Overview
Remote sensing can be divided into two types of methods: Passive remote sensing and active remote sensing. Passive sensors gather radiation that is emitted or reflected by the object or surrounding areas. Reflected sunlight is the most common source of radiation measured by passive sensors. Examples of passive remote sensors include film photography, infrared, charge-coupled devices, and radiometers. Active collection, on the other hand, emits energy in order to scan objects and areas whereupon a sensor then detects and measures the radiation that is reflected or backscattered from the target. Radar and lidar are examples of active remote sensing where the time delay between emission and return is measured, establishing the location, speed and direction of an object.
Remote sensing makes it possible to collect data of dangerous or inaccessible areas. Remote sensing applications include monitoring deforestation in areas such as the Amazon Basin, glacial features in Arctic and Antarctic regions, and depth sounding of coastal and ocean depths. Military collection during the Cold War made use of stand-off collection of data about dangerous border areas. Remote sensing also replaces costly and slow data collection on the ground, ensuring in the process that areas or objects are not disturbed.
Orbital platforms collect and transmit data from different parts of the electromagnetic spectrum, which in conjunction with larger scale aerial or ground-based sensing and analysis, provides researchers with enough information to monitor trends such as El Niño and other natural long and short term phenomena. Other uses include different areas of the earth sciences such as natural resource management, agricultural fields such as land usage and conservation, greenhouse gas monitoring, oil spill detection and monitoring, and national security and overhead, ground-based and stand-off collection on border areas.
Data acquisition The basis for multispectral collection and analysis is that of examined areas or objects that reflect or emit radiation that stand out from surrounding areas. For a summary of major remote sensing satellite systems see the overview table. Applications of remote sensing
Radar
Conventional radar is mostly associated with air traffic control, early warning, and certain large-scale meteorological data. Doppler radar is used by local law enforcements' monitoring of speed limits and in enhanced meteorological collection such as wind speed and direction within weather systems in addition to precipitation location and intensity. 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, TerraSAR-X, 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 wavelength of ocean waves, the altimeters measure wind speeds and direction, and surface ocean currents and directions. Ultrasound (acoustic) and radar tide gauges are used to measure sea level, tides and wave direction in coastal and offshore tide gauges.
Light detection and ranging (LiDAR) Light detection and ranging (LiDAR) is used for weapon ranging, laser illuminated homing of projectiles, and to detect and measure the concentration of various chemicals in the atmosphere while airborne LiDAR can be used to measure the heights of objects and features on the ground more accurately than radar technology. LiDAR can be used to detect ground surface changes typically by creating Digital Surface Models (DSMs) or Digital Elevation Models (DEMs). Vegetation remote sensing is a principal application of LIDAR.
Radiometers and Photometers
The most common instruments in use are radiometers and photometers, which collect reflected and emitted radiation in a wide range of frequencies. The most prevalent of these frequencies 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. Radiometers are also used at night, as artificial light emissions are a key signature of human activity. Applications include remote sensing of population, GDP, and damage to infrastructure from war or disasters. Radiometers and radar onboard of satellites can be also used to monitor volcanic eruptions.
Spectropolarimetric Imaging Spectropolarimetric Imaging has been reported to be useful for target tracking purposes by researchers at the U.S. Army Research Laboratory. They determined that manmade items possess polarimetric signatures that are not found in natural objects. These conclusions were drawn from the imaging of military trucks, like the Humvee, and trailers with their acousto-optic tunable filter dual hyperspectral and spectropolarimetric VNIR Spectropolarimetric Imager.Goldberg, A.; Stann, B.; Gupta, N. (July 2003). "Multispectral, Hyperspectral, and Three-Dimensional Imaging Research at the U.S. Army Research Laboratory" (PDF). Proceedings of the International Conference on International Fusion [6th]. 1: 499–506.
Aerial platforms and Satellites Stereoscopic pairs of aerial photographs have often been used to make topographic maps by imagery and terrain analysts in trafficability and highway departments for potential routes, in addition to modelling terrestrial habitat features.
Simultaneous multi-spectral platforms such as Landsat have been in use since the early 1970s. These thematic mappers take images in multiple wavelengths 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, detect or monitor land usage, detect invasive vegetation, deforestation, and examine the health of indigenous plants and crops (satellite crop monitoring), including entire farming regions or forests. Prominent scientists using remote sensing for this purpose include Janet Franklin and Ruth DeFries. Landsat images are used by regulatory agencies such as KYDOW to indicate water quality parameters including Secchi depth, chlorophyll density, and total phosphorus content. Weather satellites are used in meteorology and climatology.
False Color Composites (FCCs) and Pseudocolor A major remote sensing application, used broadly across fields such as urban planning and environmental studies, is False Color Composites (FCCs) and Pseudocolor maps. An FCC is a color-rendering method in which a true-color image (a photograph or image that the human eye would render) is altered to appear in different colors. The purpose is typically to identify patterns the human eye cannot normally detect, or highlight certain land cover uses. Pseudocolor uses one spectral band and maps it to a new color spectrum rather than the original gray band. FCCs and Pseudocolor mapping remain vital tools for visualizing data spatially by allowing researchers to treat non-visible wavelengths as visible.
How it works
False Color Composite Humans use three spectral bands to perceive color: red, green, and blue. Remote sensing platforms such as ArcGIS and QGIS similarly compose images using three color bands. Therefore, a true color image assigns red light wavelengths (620-750 nm) to the red band, and green and blue light wavelengths (495-570 nm and 450-485 nm) to the green and blue bands, respectively. FCCs are easily made by rearranging which wavelengths on the electromagnetic spectrum are assigned to certain color bands. For example, a Color Infrared composite assigns Near Infrared (NIR) light to the red band, red light to the green band, and green light to the blue band, identifying vegetation as bright purple because vegetation strongly reflects NIR and green light. Even if a human cannot see NIR light, sensors from satellites and other remote sensing platforms still detect the reflectance of non-visible wavelengths, so simply assigning such data to a visible color on software platforms allows the visualization of previously imperceptible data.
Pseudocolor Unlike three-band images, pseudocolor images use only one band. The intensity of light within certain wavelength ranges (or one spectral band on a satellite) is initially measured as a two dimensional numerical data array. The data pixels are each associated with an 8-bit intensity value, which is then normalized to a grayscale from 0 to 1. This scale can also apply to other data, such as temperature and elevation, not just light intensity. These data ranges can then be mapped to different color ranges, such as red to blue, yellow to purple, etc. Different color ranges better highlight data variations that are less obvious in grayscale. Raw grayscale images often lack contrast, so more vibrant colors and sharper changes make patterns easier to identify.
Hyperspectral Imaging Hyperspectral imaging produces image cubes where each pixel has full spectral information with imaging narrow spectral bands over a contiguous spectral range. Hyperspectral imagers are used in various applications including mineralogy, biology, defence, and environmental measurements. Within the scope of the combat against desertification, remote sensing allows researchers to follow up and monitor risk areas in the long term, to determine desertification factors, to support decision-makers in defining relevant measures of environmental management, and to assess their impacts. Remotely sensed multi- and hyperspectral images can be used for assessing biodiversity at different spatial scales. Since the spectral properties of different plants species are unique, it is possible to get information about properties that relates to biodiversity such as habitat heterogeneity, spectral diversity and plant functional trait. Remote sensing has also been used to detect rare plants to aid in conservation efforts. Prediction, detection, and the ability to record biophysical conditions were possible from medium to very high resolutions. Remote sensing is often utilized in the collection of agricultural and environmental statistics, usually combining classified satellite images with ground truth data collected on a sample selected on an area sampling frame.
Geodetic