5 CONCLUSION The objectives of this study were to perform an acoustic analysis

5 conclusion the objectives of this study were to

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5. CONCLUSION The objectives of this study were to perform an acoustic analysis of a small commercial drone (DJI Phantom IV) in order to perform acoustic localization measurements. Two methods have been proposed to ensure the localization process. The first one is based on the so called Delay-&-Sum beamforming in the time domain. Preprocessing are used to isolate desired harmonics of the signals before the beamforming step. The second method is based on the estimation of the TDOA using the GCC function. The acoustic characterization experiment of the drone consisted in (i) identifying the acoustic signature emitted by the drone with and without the propellers, (ii) evaluating the intermodulation of the sound emitted by the propellers and (iii) measuring the sound
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0 48.0192 1 Start 2 0.158 3 48.0191 Height (m) End 4 N E O S 0.1578 5 Latitude (°) 48.019 6 Longitude (°) 0.1576 48.0189 0.1574 48.0188 0.1572 Path followed Array center Projection 10 1 10 2 10 3 Frequency (Hz) -180 -160 -140 -120 -100 -80 -60 -40 -20 0 Amplitude (dB) Original Signal Array Signal Framework Filtered Signal Figure 10: Trajectory followed by the drone for the "realistic" measurements (left) and the associated PSD of the reference signal for the TFDSB step (right) with 4 harmonics selected. The projection (black) allows to better estimate the height of the drone. 0 5 10 15 20 25 30 35 40 45 50 Time (s) -150 -100 -50 0 50 100 150 200 Azimuth (°) GPS position Goniometry TFDBS 0 5 10 15 20 25 30 35 40 45 50 Time (s) -10 0 10 20 30 40 50 60 70 Elevation (°) GPS position Goniometry TFDBS Figure 11: Results of the localization estimation of the drone using the TFDSB algorithm (magenta) with 4 harmonics selected and the acoustic goniometry method (red) for the outdoor measurement conditions in azimuth (left) and elevation (right). level at the periphery and underneath the drone. The acoustic energy of the sound emitted by the drone is mainly of aerodynamic origin with harmonics of great amplitude at frequencies multiple of the double of the passage of a blade. The intermodulation of the di ff erent sounds from the propellers has been investigated. Low frequency harmonics are slightly shifted while high harmonics have more modulation. Finally, the peripheral and underneath sound pressure levels of the drone were measured. The SPL is invariant around the drone but the sound level is maximum (twice as much) at about 34 under the drone. Experimental measurements were made in the anechoic chamber and outdoors. The microphone array was designed using data from acoustic characterization measurements. Some di ffi culties related to the temporal synchronization between the array data and the UAV data were encountered. The low accuracy of the GPS for the outdoor case did not allow to evaluate the actual performance of the results obtained by acoustic measurements. Nevertheless, the analysis of the data gives good results compared to those expected both in anechoic chamber and outside. In the case of the TFDSB algorithm, some estimates are inconsistent.
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6. ACKNOWLEDGEMENTS This work is funded by the Direction Générale de l’Armement (DGA).
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