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Traditional radar sends directional pulses of electromagnetic energy and detects the presence, position and motion of an object (such as an aircraft) by analyzing the portion of the energy reflected from the object back to the radar station. Imaging radar attempts to form a picture of the object as well, by mapping the electromagnetic scattering coefficient onto a two-dimensional plane. Objects with a higher coefficient are assigned a higher optical reflective index, creating an optical image.
Several techniques have evolved to do this. Generally they take advantage of the Doppler shift caused by the rotation or other motion of the object and by the changing view of the object brought about by the relative motion between the object and the back-scatter that is perceived by radar of the object (a plane) flying over the earth. Through recent improvements of the techniques, this can be precisely calculated. Imaging radar has been used to map the Earth, other planets, asteroids, other celestial objects and to categorize targets for military systems.
See also 
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