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.

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