Synthetic Aperture Radar
  
    What is SAR?
    
   Synthetic Aperture Radar. An active microwave instrument, producing
   high-resolution imagery of the Earth's surface in all weather.
   
   There is a good introduction to imaging radar by Tony Freeman of JPL
   at http://southport.jpl.nasa.gov/desc/imagingradarv3.html
   
   _Should we have an embedded intro for the benefit of non-WWW readers?
   I can ask to include the above, or try and solicit an equally expert
   intro from someone here_
   
    What are the main SAR platforms?
    
   Several past, present and future Earth Observation Satellites. Also
   the Shuttle Imaging Radar missions. See the table for a full list.
     * ERS-1/ERS-2
     * JERS-1
     * Shuttle Imaging Radar SIR-C/X-SAR
     * Almaz
     * RadarSat
       
   the future...
     * ENVISAT (I'm not even making a link until I've something REAL to
       put there)!
     * _OK, what have I forgotten about (or never heard of)?_
       
    What distinguishes SAR from hi-res optical imagery?
    
   Two main properties distinguish SAR from optical imagery:
     * The SAR is an active instrument. That is to say, it generates its
       own illumination of the scene to be viewed, in the manner of a
       camera with flash. The satellite's illumination is coherent: i.e.
       all the light in any flash is exactly in phase, in the manner of a
       laser, so it does not simply disperse over the distance between
       the satellite and the Earth's surface. A SAR instrument can
       measure both intensity and phase of the reflected light, resulting
       not only in a high sensitivity to texture, but also in some
       three-dimensional capabilities. Experiments with the technique of
       _Interferometry_ (measuring phase differences in exactly aligned
       images of the same ground area) have shown that SAR can accurately
       model relief, and appears able also to detect small changes over
       time.
       Some consequences of being an active instrument (and using
       coherent light) are:
          + Works equally day or night
          + Polarised - can be used to gain additional information (esp.
            when different polarisations are available on the same
            platform - as on the most recent Shuttle missions).
          + Needs a lot more power than passive sensors, and can
            therefore only operate intermittently.
          + Suffers from speckle, an artifact of interference patterns in
            coherent light, sensitive to texture.
     * SAR is _Radar_ - i.e. it uses microwave frequency radiation.
       _(note that in consequence, references to "light" above should
       more strictly read "microwave radiation")._ Microwave radiation
       penetrates cloud and haze, so SAR views the Earth's surface (land
       and sea) in all weather. For general purpose Remote Sensing, this
       is probably _the_ major advantage of SAR.
       An example of its use is the ESA/Eurimage "Earthwatch" programme,
       producing imagery of natural and other disasters when weather
       conditions prevent other forms of surveillence. Earthwatch imagery
       is available at http://gds.esrin.esa.it/CSacquisitions
       
    What are SAR images good for ?
    
     * Sensitive to texture: good for vegetation studies.
     * Ocean waves, winds, currents.
     * Seismic Activity
     * Moisture content
       
   A list of SAR applications is available at
   http://southport.jpl.nasa.gov/science/SAR_REFS.html
   
    What is the meaning of colour in a SAR image?
    
   Of course, all SAR image colour is false colour: the notion of true
   colour is meaningless in the context of invisible microwave radiation.
   
   Most SAR images are monochrome. However, multiple images of the same
   scene taken at different times may be superimposed, to generate
   false-colour multitemporal images. Colour in these images signifies
   changes in the scene, which may arise due to a whole host of factors,
   such as moisture content or crop growth on land, or wind and wave
   conditions at sea. SAR is particularly well-suited to this technique,
   due to the absence of cloud cover.
   
   The shuttle SAR's images are the nearest to 'natural' colour, in the
   sense that they are viewing three different wavelengths, which can be
   mapped to RGB for pseudo-naturalistic display purposes (essentially
   the same as false colour in optical/IR imagery).
   
   
   _Need a proper multitemporal image entry_
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  Radar Altimetry
  
   Technique used extensively to map the oceans. There are introductions
   at http://www.satobsys.co.uk/ and http://dutlru8.lr.tudelft.nl/altim/.
   The latter includes the _Altimetry Atlas_, computed from GEOSAT, ERS-1
   and TOPEX-Poseidon altimetry data.
   
   An interactive browser offering sea surface height maps is available
   at http://www.ccar.colorado.edu/~hendricj/topexssh.html
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