Passive aircraft detection and noncooperative helicopter identification using extremely low frequency (ELF) electric field sensors

A new type of passive electric field sensor concept that measures extremely low frequency (ELF) electric fields shows promise for detecting low-flying aircraft near, for example, airport runways or national borders. Because different types of aircraft (jets, airplanes, and helicopters) exhibit different signature characteristics, this type of sensor allows basic target classification. Additionally, helicopters generate ELF electric fields with strong spectral lines corresponding to the rotation of the main and tail rotors, so that the sensor can perform passive, noncooperative helicopter identification. Arrays of these sensors can be used to estimate aircraft speed, direction, and height above the ground. We developed detailed 3-D models of an electrically charged helicopter and ground-based sensors, and simulated both target fields and sensor responses. We designed and built prototype sensors, which we used to collect data for various aircraft in the field; the collected signatures compare favorably with the simulated data. Our investigations to date indicate that these sensors naturally complement both fixed radars and distributed acoustic sensors; such a sensor could also be used alone as a low-cost and rugged alternative. This paper outlines the ELF sensor concept, the models, and the sensor hardware we used, and compares simulated and collected signatures.