A new type of radar system of ship-based passive Synthetic Impulse and Aperture Radar(SIAR)is presented on the basis of multiple frequency-carrying signals transmitted with multiple antennas to the whole directions,while its transmitting beams are formed by digital signal processing in receivers.The system shall not only make up a radar system of bi-station or multi-station through transmitting signals from the bank,and receiving signals on ships,but also be equivalent to a passive position locater at the sink.The basic constitutions and operational principles are presented.There are many distinguished features in the new radar system in comparison with conventional radars.Its performance is analyzed and simulative results are given.Finally,some key problems to be solved are discussed in the radar.