Innovative Passive Electromagnetic Sensors for Submarines

New submarine missions will require submarines operating in littoral waters to monitor non-radiating aircraft in their operational areas while remaining electromagnetically silent themselves. Conventional passive sensors such as ESM and IR will be ineffective against silent aircraft, and conventional radar sensors will compromise the submarine's own security. Consequently, new passive sensors are required that can detect and track such aircraft. ISL proposes that submarine sensors detect and track aircraft by observing the effects of the aircraft on the existing ambient electromagnetic signal environment. The electromagnetic environment is rich with signals emitted from commercial radio stations, television, radar, and satellites. These electromagnetic signals are scattered by the aircraft and can be received at the submarine using existing antennae. The scattered signals are delayed and frequency (doppler) shifted to an extent that depends on the aircraft position and velocity. Time and frequency shifts can be measured relative to the unscattered direct signal also received on the same antenna. By monitoring several signals, the position and velocity of the aircraft can be estimated without radiation from the submarine or aircraft.