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Remote Sensing

Remote , sensing, distance, space, electromagnetic, photographic,radiation,Satellites,Spacecrafts

Introduction

A picture is worth a thousand words. Have you ever wondered why?
Picture concisely convey information about positions, sizes, and interrelationships between objects-by their nature, they portray spatial information that we can recognize as objects. These objects in turn tell a story that can convey a different kind of meaning. Human beings are good at deriving information from such images, so we experience little difficulty in interpreting even scenes that are visually complex because of our innate visual and mental abilities.
If you have heard the term "remote sensing" before you may have asked, "what does it mean?" It's a rather simple, familiar activity that we all do as a matter of daily life, but that gets complicated when we increase the scale. As you view the screen of your computer monitor, you are actively engaged in remote sensing.
A physical quantity (light) emanates from that screen, which is a source of radiation. The radiated light passes over a distance, and thus is "remote" to some extent, until it encounters and is captured by a sensor (your eyes). Each eye sends a signal to a processor (your brain), which records the data and interprets this into information.
Specialized knowledge is important because remotely sensed images have qualities that differ from those we encounter in everyday experience:
  • Image Presentation

  • Unfamiliar scales and resolutions

  • Overhead views from aircraft or satellites

  • Use of several region of the electromagnetic spectrum.

Remote Sensing refers to the group of techniques of colleting information about an object & its surroundings from a distance without being in contact with it. This gives rise to some form of imagery which is further processed & interpreted to produce useful data for application in Agriculture, Archaeology, Forestry, Geography, Geology, Planning & other fields.

The prime objective of Remote Sensing is to extract environmental and natural resources data related to our earth. Information about the object concerned is conveyed to the observer through electromagnetic energy, which is the information carrier & thus provides a communication link.

Definition

“Remote sensing” is the term currently used by a number of scientists for the study of remote objects (earth, lunar, and planetary surfaces and atmosphere, stellar and galactic phenomena, etc.) from great distances. Broadly defined…remote sensing denotes the joint effects of employing modern sensors, data-processing equipment, information theory and processing methodology, communications theory and devices, space and airborne vehicles, and large-systems theory and practice for the purpose of carrying out aerial or space surveys of the earth’s surface. (National Academy of science 1970)

  • Remote Sensing is a technology for sampling electromagnetic radiation to acquire and interpret non-immediate geospatial data from which to extract information about features, objects, and classes on the Earth's land surface, oceans, and atmosphere (and, where applicable, on the exteriors of other bodies in the solar system, or, in the broadest framework, celestial bodies such as stars and galaxies).

  • [/list]

    Major Remote Sensing Instruments

    In order to detect electromagnetic energy, which exists in a wide range of wavelengths & frequencies, different varieties of remote sensing instruments are applied.
    • Photographic Cameras

    • Photographic camera is designed to detect energy in the visible and near infrared portions of the electromagnetic spectrum. This had been made possible with the use of suitable white-black or color films sensitized to these spectral regions.
      An important component of the photographic camera is a lens system, which can cause images of points to be displaced on the photograph if lens aberrations are present. Another important component of photographic remote sensing systems is photographic film, which consists of a base and a thin layer of chemical emulsion on top.

    • Electro-optical Decoders

    • Electro-optical Decoders are transducers that transform electromagnetic radiation into electrons or electrical signals from a scene viewed. The detector elements behave like the grains of photographic film & produce an analogue electrical record of each point of the scene scanned according to the nature of incoming electromagnetic radiation.
      Electro optical decoders are subject to unpredictable fluctuations in their electrical output known as noise, which is analogous to granularity in the photographic film.

    • Vidicon Television Camera

    • The vidicon camera is a form of image tube, which can be regarded as the electronic counterpart of the photographic cameras, because its principle application is sensing in the visible and near infrared portions of the electromagnetic spectrum. It makes use of an optical system (camera lens) to focus an optical image onto a photosensitive surface of an electron gun which then converts the image into the electronic form, either as charge pattern on storage device or in non storing mode, traveling electrons.

    • Thermal infrared scanner

    • It is a good example of the optical mechanical line scanners developed for generating imagery outside the spectral region of the photographic film. The characteristics of the line scanner is the multifaceted rotating mirror inclined at 45 degree to the rotation axis, which scans the ground along the lines perpendicular to the flight direction. The radiation emitted or reflected from the ground is projected by the rotating mirror onto the detector, which senses the intensity of the radiation in specific part of electromagnetic spectrum & converts it into electrical signals. These signals are used to modulate the intensity of the signal line cathode ray tube to expose an image line on the photographic film, which is made to advance.
      Thus, as the sensor platform moves forward, successive new scan lines of the ground surface are swept. Together continuous scan lines give a real time image of the ground surface sensed.

    • Multispectral Scanners

    • Multispectral scanners can sense 24 separate spectral channels simultaneously; built such a large amount of spectral data normally requires the use of high speed digital computer to help in analysis.
      A good example of multispectral scanner system employed in the Landsats 1,2,3 and 4, which is designed to provide images of the earth simultaneously in four spectral bands.

    • Microwave Imagers

    • Microwave imagers make use of the antennae to collect radiation information in the microwave wavelength from the ground & are generally classified into passive and active types:

      Passive Microwave imagers
      Passive microwave sensing collects thermal mission from the earth’s surface in the microwave spectrum. The commonly employed spectral bands are separated into the microwave regions; each of which is given a letter designation.
      The distinct advantage of microwave sensors is their capability to penetrate clouds, although water vapour and oxygen can still hamper them. These sensors can detect emitted, reflected and transmitted radiation within the 1 mm to 300 mm wavelengths. The strength of passive microwave radiation depends on the temperature and dielectric properties of the material rather than the surface roughness.

      Active microwave imagers
      Active microwave remote sensing involves the sending out of pulse of microwave energy to a target from the sensor and then measuring the reflected signal. This method of sensing is commonly known as Radar (Radio Detection and Ranging), which was rapidly developed during Second World War for military application. It was designed to measure distances and determining location of objects. Major advantage of Radar is its all-weather, day & night operation ability which make its application particularly valuable in areas with perennial loud covers where conventional aerial photography cannot be applied.

    Remote Sensing Platforms for Data Collection
    • Aircraft

    • Aircraft of various types provide exceptionally convenient and operationally flexible platforms for remote sensing, carrying payloads from a few tens of kilograms to many tonnes. With a suitable choice of vehicle a range of altitudes can be covered from a few tens of meters, were atmospheric propagation effects are negligible, to many thousands of meters, above most of the earth’s atmosphere. The choice of flying altitude will obviously have an impact on the scale, spatial coverage and spatial resolution of the data collected.
      The main disadvantage of aircraft as platforms for remote sensing when compared with spacecraft are:
      · A typical airborne observing mission has duration of only a few hours, as compared with few years for a space borne mission. This means that it is much more difficult to provide continuity of data for example, for ten year monitoring program.
      · Since airborne observations are acquired from much lower altitudes than space borne observations, the spatial coverage of the data is smaller and airborne observations are obviously unsuitable for studying large areas.
      · Since aircraft necessarily operate within the earth’s atmosphere, and the atmosphere is in motion, neither the position nor the motion of the aircraft may be exactly what was intended.

    • Spacecrafts

    • Spacecrafts are suitable sensor platforms because they have overcome the difficulties of the ceiling limits 7-operation duration. The use of spacecraft orbiting regularly around the earth from the heights of several hundreds of kilometers makes regular surveillance of the earth with suitable remote sensing devices possible. The term spacecraft refers to rockets, artificial satellites or manned space vehicles.

      Rockets
      Rockets are used as platforms for photographic system, but in most cases the photography acquired was only used for recording the orientation of the rocket in flight. The photographs are fairly clear & cloud free because the rockets were usually fired in the period of fine weather. But the coverage of this rocket photographs was very restricted because the rocket has to be fired vertically upward and it had to land not far from its launching site. Also the rocket went up & down so fast that the camera had at most, only 8 minutes to take photographs above the earth.

      Earth Satellites
      The earth satellite is an artificial object in speed, which revolves around the earth following a specific orbit. Unlike an orbit, a satellite can stay aloft for a much longer period of time, thus permitting constant surveillance of the earth. Earth satellites can be conveniently classified, according to their orbital characteristics, into 3 types:
      · Those with equatorial geosynchronous orbits
      · Those with near polar sunsynschronous orbit and
      · Those with general orbits.

      Landset, SPOT and other systems establish the technical and commercial value of the land observation satellite concept in the 1970s and 1980s. But they also revealed the high costs in the technical challenges of designing and operating a system intended to provide general-purpose data for a broad community of users who may have diverse requirements. So the need to operate smaller, special purpose earth observation satellites called micro satellites focused on the requirements of specific groups of users, thereby decreasing the costs of operation.

      Manned Spacecraft
      With manned spacecraft, more direct human control is possible in obtaining imagery of the earth from space. The space shuttle is much more flexible spacecraft and provides an ideal orbital platform for collecting remote sensing data & for testing advanced sensor systems because of its ability to carry the instruments into space and return them to the ground for recalibrating and refurbishing.



    If You want more information on Remote Sensing Click here
    Published: 2006-10-07
    Author: Neelam Doshi

    About the author or the publisher
    I am in the field of Computer Science. I am Working as Faculty member in a Post-graduate College. if you want to know something about M-commerce you can visit my site. Any other query you can mail me at neelam21582@yahoo.com
    www.geocities.com/neelam21582

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