Medium-range localisation of aircraft via triangulation

In the framework of acoustic and seismic monitoring of airports for verifying disarmament or peace-keeping agreements, the propagation of sound emitted by aircraft close to the ground was investigated. The sound of taking-off, landing, and overflying aircraft was used to locate them by means of two 3-dimensional microphone arrays consisting of 4 microphones each and placed at a mutual distance of 150 m at 146 m to the runway centre. The base-length of the microphone arrays was 1.6 and 3.4 m. The varying propagation time of the sound from the source to the different sensors was determined by a cross correlation of the signals with a time resolution of 0.18 and 0.36 ms. Possible ambiguities due to periodic behaviour of the signals could be excluded: the dominant frequencies of the propeller-driven aircraft (<80 Hz) correspond to wavelengths larger than 4 m. In the case of jet aircraft, the broad-band noise leads to unambiguous cross correlation maxima (except for ground reflections). Normalised vectors pointing to the sound source were calculated from the propagation time differences of the sensors of a single array. Using these normalised vectors of both arrays, the location of the sound source in three dimensions was determined via triangulation. This was done for constant time intervals, yielding the whole trajectory of the aircraft. The experimental set-up used here allowed a localisation accuracy of moving sound sources at distances of around 100–200 m in the order of 10 m.