Abstract: A radar system which uses signals from an opportunity transmitter to detect and track radar targets instead of transmitting signal itself can not be discovered by the reconnaissance receiver thus, avoiding the attack of anti-radiation missile. So has very powerful survivability and will be important detection tool for future air defense information systems. In this paper the characteristics and studied examples of passive radar system based on frequency modulated broadcast signal (FMBS) are described. The spectrum of the FMBS, radar deployment, range and velocity ambiguity, constant false alarm processing, flight path map, and detection range are described and analyzed.