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Though numerous remote sensors exist, only five types are widely employed. They are: 1. Photography 2. Infrared Scanners 3. Radar Imagery 4. Multi-Band Spectral Photography 5. Land Sat Imageries.
Type # 1. Photography:
Photography is the most widely used and highly developed methods of remote sensing. Photographic cameras operate in the visible and the near infrared portions of the electromagnetic spectrum. Three basic types of cameras are employed for taking photographs. They are the frame, slit and panoramic type of cameras.
1. The frame type of camera takes the picture of the field of view on a single frame. There is a shutter that is capable of closing and opening. The frame is usually overlapped about 60 per cent to provide a stereoscopic effect.
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2. The slit camera focuses a strip of the area on a narrow slit. Behind this slit, film is placed. As the aircraft flies, the film moves at a rate that is proportional to the ratio of velocity to altitude of the aircraft. In this method, the rate of movement of the film is accurately controlled in order to obtain good pictures.
3. In the panoramic type of camera, the field of view of the lens is swept through 180° across the path of flight of the aircraft. The picture is recorded on the film that moves simultaneously with the sweep of the lens. Large amount of information can be collected with these cameras, but the picture is not suitable for stereoscopic vision.
Photographs are taken on different kinds of films.
They are:
i. Black and white films,
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ii. Black and white infrared films,
iii. True colour films, and
iv. False colour films.
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v. Black and white infrared films are particularly sensitive to visible and infrared rays. By using minus blue filters, only red and infrared parts of the spectrum can be recorded. The red and infrared parts of the spectrum can penetrate haze and clear pictures can be made in wet as dry areas.
vi. The colour films are made up of three layers of emulsions. These three layers are recorded blue, green and red special bands. These three bands together give rise to true picture because these colours are complimentary colours.
vii. In false colour films also there are three layers but in these the infrared emulsion substitutes the blue emulsion owing to this unusual representation of colours result. Green appears blue, red becomes green and photographic infrared is recorded as red. Therefore, these films are called false colour films.
False colour films have the ability to penetrate haze, further they are sensitive to charges of reflectance of vegetation. As chlorophyll reflects strongly in the infrared region, living vegetation is strongly reveals in false colour photography.
Although photography is an extremely useful and widely employed method for sensing the resources of the earth its usefulness is limited as a space sensor. This is because it presents difficulties in digitizing the film-recorded data and in returning the processed hardcopy to earth.
Type # 2. Infrared Scanning:
Infrared scanning is otherwise called Thermal Mapping. It consists of detecting remote objects by recording the amount of infrared energy or heat energy that is emitted by any material having a temperature above absolute zero, that is greater than -273°C.
The primary factors that determine the emission of infrared radiations are:
Temperature and emissivity:
Emissivity is the ratio of the energy from an object to the energy radiated by a blackbody. A blackbody is an object that completely absorbs all radiation incidents upon it. Transparent to infrared on the other hand bodies of water act as a screen that blocks infrared radiations.
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Variations in infrared radiation emitted by various the emissive of blackbody are unity. A highly polished surface is an extremely poor absorber its emissive is close to zero. Most materials lie between these two extremes in emissivity. Some materials like silicon that are opaque to visible light are relatively a feature of the terrain is due to differences in emissivity or temperature or both.
Most infrared sensors are designed to operate in the spectral bands corresponding to the “infrared windows” within the ranges of 2 to 5 microns and 8 to 14 microns. Shorter infrared wavelengths that are less than one micron can be recorded by conventional photography. But for recording infrared wavelength longer than one micron, specialized sensing devices are required for registration of infrared wavelength.
An infrared sensor is a scanning device. It produces a continuous image from a series of line scans, because the terrain is not photographed directly, the term imagery is used to describe the final image that is printed on a photographic film.
The line scanning is done by means of rotating mirrors. The mirrors scan the terrain in continuous strips perpendicular to the line of flight. The image from the mirror strikes an element like indium antimonite or mercury doped germanium, which are sensitive to infrared radiation. The signal from the element is electrically amplified.
When the amplified signal strikes cathode ray tube or flow-tube a visual image is produced. A final photographic record is made from this visual image. After photographic development, a thermal radiation image of the area flown over is obtained film density represents effective radiation temperature.
Dark areas indicate cool thermal signals. Light areas suggest warm or hot areas on positive print. The width of the scanned strip in the direction of flight is directly proportional to the altitude of the aircraft above the terrain and the total scanned angle.
Infrared images are characterized geometrical distortions owing to line scanning. Except along the nadir line directly beneath the flight line, the final image is an oblique view of the terrain and scale varies with distance from the nadir line. Therefore, precise measurements on these images are not possible.
As the information recorded on this imagery is based on thermal radiation characteristics of surface and not in the light reflecting photographic quantities. These images cannot be compared with visual photography.
Solar radiant energy provides the principal source of infrared emissions. In daylight, surfaces with a high absorbency for sunlight store up large amounts of heat, while surfaces having high reflectivity for solar radiation absorbs little heat.
Bodies of water have capacity of storing heat, water surface heats up much more slowly than adjacent soil and rock surfaces and they also releases heat at much slower rate while cooling. In general cooler surfaces like water and tree appear as dark on an infrared image, while warmer surfaces like highways, industrial plants, thermal springs, etc. will appear in bright tone.
i. Infrared imagery is most suited for detecting and mapping forest fires because of the great contrast in surface temperature.
ii. Leaf temperature can be remotely infrared in the infrared region of 3.7 to 5.5 microns and such knowledge can be helpful in determining various aspects of plant health, age and relative water supply or irrigation.
iii. The texture on the terrain surface material up to gravel size can be inferred on radar display.
iv. The moisture content of terrain can be determined when temperature data are available.
v. Surface temperature is determined when the moisture content of the terrain is known.
vi. Selective radar bands can be used to determine the metallic contents of surface and near surface features.
vii. Snow cover is the features beneath snow cover can be determined.
There are two types of systems:
They are:
1. Plan positive indicator (PPI) radar, and
2. Side looking airborne radar (SLAR).
In both the systems, the principle involved is the same. All the objects of the terrain scanned by the beam reflect transmitter send a radar beam and the radar are scanned by the beam.
These reflected signal strikes the airborne receiver. These reflected signals then pass through cathode ray tube and the image of the terrain appears on a screen for direct visual interpretation. The images appearing on the screen are continuously photographed on a film for later interpretation.
1. In PPI radar system the radar beam sweeps the terrain along a circle in clockwise direction as the aircraft moves forward. All the features of the terrain within that circular beam reflect a portion of the beam and the reflected beam streaks the receiver on the aircraft.
The receiver transmits on the signal to cathode ray tube and it displays the visual image of the scene. Thus planed position indicated radar produces a succession of circular images of the ground along the track of the aircraft similar to vertical photographs.
2. In side looking airborne radar (SLAR) 2 antennas transmit energy to either side of the aircraft are right angles to the flight path.
In the field of geology, infrared imagery is valuable for mapping fault lines. For identifying rocks and minerals for conducting oil survey and in maintaining vigilance on volcanoes.
In agriculture thermal images have been utilized for the identification of crop species, crop diseases and soil types.
Infrared scanning can extract considerable amounts of information from bodies of eater. When hot effluents are discharged into streams or lakes they are easily detected because of temperature gradients. Similarly, cool underground springs that empty into warmer bodies of water can also be discovered through infrared mapping technique.
Military application of infrared or thermal mapping consists in locating thermal changes on the ground that might result due to unusual concentrations of troops, weapons and military vehicles.
Type # 3. Radar Imagery:
Radar is a remote sensor. It can be used during all weather conditions and during both day and night.
Radar system can resolve and produce tonal control contrasts of the terrain features they can also provide subsurface information. By the images produced by radar systems are not so good as in aerial photography. Therefore, radar images can only supplement conventional photographs but cannot replace them.
Radar systems employ that portion of electromagnetic spectrum in which wavelengths are approximately one centimeter to three meter long. Radar is colouring blind.
In radar images, light tones usually indicate cultural features and man-made structures. Medium gray tones indicate both areas of open country and flat and smooth terrain. Black tones usually suggest the presence of water and hydrological features. Qualities like size, shape, tone, texture, etc. that are employed for interpreting air-photos can be employed for identifying objects on radar images.
Example:
Paten work appearance can be relied upon for interpreting the image as a cultivated field. The images with bright and dark tones suggest relief features increases in bright tone indicate cultural features.
Type # 4. Multi-Band Spectral Photography and Scanning:
The simultaneous use of two or more sensors to produce image from different parts of the electromagnetic spectrum is called multi-band spectral photography. The idea behind multi-band spectral photography is that by comparing two or more photographs of the same object in different regions of the spectrum. Additional information of the object can be obtained.
Further two or more objects that possess the same terrain in one spectral band display different tones contrasts in other spectral bands. This enables to distinguish objects from one another.
Type # 5. Land Sat Imageries:
Remote sensing from space platform has become a useful tool or studying the resources of the earth.
There are three types of sensor platforms:
They are:
i. Land sat,
ii. Skylab, and
iii. Aircrafts.
Land sat:
Land sat is the satellite that investigates the earth resources. It generates images of the earth from a distance of 900 km.
Skylab:
Skylab is a satellite that produces images of the earth from a distance of 440 km.
Aircrafts:
Aircrafts are employed for producing images from a distance of 700 meter to 25000 meter.
Besides these platforms the photographs taken on Gemini and Apollo missions and images generated by spacecraft such as Nimbus provide views of the earth’s surface from altitudes.
The image made available to user community by National Aeronautics and Space Administration (NASA). Earth resources program has given the geologists an opportunity to view the earth at different scales and in different perspectives.
These images enable the geologist to see regional structures and regional geological features. NASA has employed variety of sensors for producing images form space platforms in aircrafts; the main sensor employed is a 10 inches aerial camera.
The sensors employed in other space platform are:
i. Multi-band cameras,
ii. Multi-band sensors,
iii. Infrared sensors,
iv. Radar, microwave and thermal radiometers, and
v. Spectrometers and scatter meters.
The data from all these sensors contribute significantly to the remote sensing data pertaining to the earth.
Space photographs can be interpreted by visual examination just like air photos, but new techniques are applied to extract more information available in multi-band photographs and multi-band images.
Multi-band photography, which records, reflected radiations in different spectral bands has been found to be useful for geological interpretations, for example, near infrared images or photographs show bodies of water and water-saturated soil distinctly.
They are made out by their dark tone. Blue and green band images are of great value to sedimantologists who are interested in mapping suspended sediment in water. Iron rich rocks are clearly seen in infrared images.
Glaciologist and geomorphologists studying active glaciers find that melting glaciers can be distinguished best in the near infrared range because of the presence of a film of eater. The physiography and structure of a glacier than in other bands. Hydrologists have determined snow lines clearly on green bands.
Red band images show heavily vegetated areas in dark tones because this band is absorbed by chlorophyll. The amount of red light reflected or absorbed depends upon the type, quality and vigour of vegetation. Rocks marked by characteristic vegetation can be mapped by red band images.
Combination of either positive or negative images of different bands is possible by the aid of device called I2S and by such a combination more data can be obtained. Besides these a computer can extract a wealth positive of data recorded on magnetic tape. A computer can do even map printing from the data furnished by magnetic tapes. Thus land sat photography and images supply miscellaneous information about the terrain.
The US geological survey in cooperation with NASA has developed a program aimed at collecting facts about the natural resources of the earth from earth orbiting satellites, which carry sophisticated remote sensing instruments. This project is called EROS. This project is dedicated to the tasks of assembling regional and continental data on the needed minerals, water supplies, water pollution, agricultural crops, forests and human habitation.
From these space vehicles it will be possible in future to determine rates of reservoir sedimentation.
Movements of glaciers and sea encroachment it will also be possible to check the effluents of major rivers, to monitor the levels of lakes and reservoir and to access the growth of deltas. Space photos that are available enables to construct maps showing land use and vegetation.
Other remote sensors may yield measurement of ground surface temperature and the moisture content of ground useful in a variety of engineering and agricultural problems. Even population counts and measurements of daily fluctuations in the traffic flow of people and vehicle appears to be within the capabilities of improved sensors.