Earth Observation Satellites
  
   _See also the list below, containing pointers to detailed information
   and online imagery._
   
   Earth Observation imagery takes a number of forms, of which the most
   traditional are optical and near-infrared radiation, from about 0.4
   (blue) to 2.0 (IR) micrometers. Examples include Landsat, Spot and
   NOAA. These generally use tracking instrunents, the basic principles
   of which are briefly described in Part 2 of this FAQ _(someone point
   me to a proper intro on the net - SURELY there must be one)!_.
   
    Colour
    
   After basic processing, imagery from these satellites may appear as
   photographs. With certain visual imagery - eg SPOT - it is even
   possible to display images in more-or-less their natural colour. In
   practice, images for display are generally manipulated to appear
   visually pleasing and to show interesting detail, and appear in _false
   colour_. Visible and non-visible (IR) bands may be freely mixed in
   false colour images. There are no firm rules about this, but by
   convention clouds are shown as white, and vegetation red or green,
   depending on the context.
   
    Resolution
    
   Resolution is determined primarily by instrument design, and generally
   involves various compromises:
    1. High spatial resolution implies imaging a small area. For an image
       of 1000 pixels square, at 20m resolution the area viewed is
       20x20Km, but at 1Km resolution this increases to 1000x1000Km
       (actually rather more, due to the variation in viewing angle over
       a large area). The latter is therefore intrinsically suited to
       large-scale studies.
    2. High spatial resolution also implies a high sampling frequency,
       which may limit the sensitivity of the sensor.
       
    Types of Imagery
    
   Apart from visual and near-infrared, other bands of the spectrum
   commonly used include thermal infrared (heat) and microwave (radar).
   Each of these has its own applications.
   
    3-dimensional Imagery
    
   We see the world in three dimensions by virtue of having two eyes,
   viewing the world at slightly different angles. It is possible to
   emulate this and produce 3-dimensional (stereo) satellite imagery, by
   superimposing images of the same ground area, viewed from different
   angles (and at different times). A limited number of satellites have
   this capability.

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