Top Document: Satellite Imagery FAQ - 2/5 Previous Document: Earth Observation Satellites (for geosciences, etc) Next Document: List of some Earth Observation Satellites See reader questions & answers on this topic! - Help others by sharing your knowledge 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_ _________________________________________________________________ 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 _________________________________________________________________ User Contributions:Comment about this article, ask questions, or add new information about this topic:Top Document: Satellite Imagery FAQ - 2/5 Previous Document: Earth Observation Satellites (for geosciences, etc) Next Document: List of some Earth Observation Satellites Part1 - Part2 - Part3 - Part4 - Part5 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: satfaq@pobox.com
Last Update March 27 2014 @ 02:12 PM
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