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Satellite Imagery FAQ - 3/5
Section - Imaging Instruments

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  Imaging Instruments
  
    How do Remote Sensing Instruments work?
    
   If you put a camera into orbit and point it at the Earth, you will get
   images. If it is a digital camera, you will get digital images.
   
   Of course, this simplistic view is not the whole story.
   
   Digital images comprise two-dimensional arrays of pixels. Each pixel
   is a sensor's measurement of the albedo (brightness) of some point or
   small area of the Earth's surface (or atmosphere, in the case of
   clouds). Hence a two-dimensional array of sensors will yield a
   two-dimensional image. However, this design philosophy presents
   practical problems: a useful image size of 1000x1000 pixels requires
   an array of one million sensors, along with the corresponding
   circuitry and power supply, in an environment far from repair and
   maintenence!
   
   Such devices (charge coupled deices) do exist, and are essentially
   similar to analogue film cameras. However, the more usual approach for
   Earth Observation is the use of tracking instruments:
   
    Tracking Instruments
    
    1. A tracking instrument may use a one-dimensional array of sensors -
       one thousand rather than one million - perpendicular to the
       direction of the satellite's motion. Such instruments, commonly
       known as pushbroom sensors, instantaneously view a line. A
       two-dimensional image is generated by the satellite's movement, as
       each line is offset from its predecessor. If the sampling
       frequency is equal to the satellite's velocity divided by the
       sensor's field of view, lines scanned will be contiguous and
       non-overlapping (although this is of course not an essential
       property).
       _btw, would the above be better expressed in some ASCII
       representation of mathematical notation?_
    2. Another approach is to use just a single sensor. It is now not
       sufficient to use the satellite's motion to generate an image:
       cross-track scanning must also be synthesised. This is
       accomplished by means of a rotating mirror, imaging a line
       perpendicular to the satellite motion. These are known as scanning
       instruments. This is somewhat analagous to the synthesis of
       television pictures by CRT, although the rotating mirror is a
       mechanical (as opposed to electromagnetic) device.
       As the sensor now requires a large number of samples per line, the
       sampling frequency necessary for unbroken coverage is
       proportionally increased, to the extent that it becomes a design
       constraint. A typical Earth Observation satellite moves at about
       6.5 Km/sec, so a 100m footprint requires 65 lines per second, and
       higher resolution imagery proportionally more. This in turn
       implies a sampling rate of 65,000 per second for a 1000-pixel
       swath. This may be alleviated by scanning several lines
       simultaneously.
       Either design of scanning instrument may have colour vision (ie be
       sensitive to more wavelength of light) by using multiple sensors
       in parallel, each responding to one of the wavelengths required.
       
    List of Imaging Spectrometers
    
   http://www.geo.unizh.ch/~schaep/research/apex/is_list.html

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Top Document: Satellite Imagery FAQ - 3/5
Previous Document: Do I need geocoded imagery?
Next Document: What is a Sounding Instrument?

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Last Update March 27 2014 @ 02:12 PM