At the Defcon 19 (2011) conference Michael Scarito demonstrated how to construct your own radar system. The project is based on a design by Dr. Greg Charvat from MIT and uses synthetic aperture techniques to generate a two- or three-dimensional image. (You can read Dr. Charvat’s research paper here.)
The hardware operates in the 2.4 GHz ISM band (shared with WiFi.)
For a further detailed explanation of this technology, see the dissertation of Manh Hung V. Le, Dimitris Saragas and Nathan Webb from the Worcester Polytechnic Institute here.
A free MIT course on this topic is available through their Opencourseware presentation “Build a Small Radar System Capable of Sensing Range, Doppler, and Synthetic Aperture Radar Imaging.”