Airborne Acoustic Surveillance Using an Autonomous Powered Parafoil Unmanned Aerial Vehicle, 14-9462Printer Friendly Version
Inclusive Dates: 02/28/04 - 05/31/04
Background - There has been increasing interest in the military, security, and law enforcement communities to collect acoustic data from airborne platforms, particularly from unmanned aerial vehicles (UAVs). An airborne platform offers the significant advantage over ground-based sensing of mobility: the ability to be deployed remotely and to cover large areas quickly and easily. This capability can allow the military to acquire remote battlefield intelligence from a safe standoff distance and offers a unique data collection tool for security specialists. Sounds originating on the ground are severely attenuated over ground paths by scatter from foliage and buildings and by upward refraction, limiting detection range for a ground-based sensor. Placing the sensor in the air can often improve detection range and localization accuracy significantly. Acoustic data also are unaffected by poor visibility conditions such as might be encountered with dense fog or nighttime operations. The types of data that can be obtained from an airborne platform include position information on emitters through acoustic direction finding and signature data that can be used to identify vehicles and machinery through spectral analysis. Typical applications include location of gunfire and vehicles and target identification.
Approach - Although an airborne platform appears ideal for remote acoustic data collection, it poses two difficult challenges to practical implementation: interference from engine noise and interference from wind noise. The amplitudes of these sounds can be many times greater than that of the desired signals and therefore pose a serious limit to sensitivity. The approach taken to solve these problems in this internal research project was to use large microphone arrays in conjunction with UAV platforms capable of quiet flight. Three experimental flight platforms were adapted for airborne acoustic use and evaluated. These platforms included an existing electric-powered, radio-controlled airplane used in previous airborne acoustic experiments and two larger platforms constructed during the project consisting of a large radio-controlled, gas-powered airplane and large autonomous powered parafoil UAV. The airborne arrays were constructed in three sizes to study the effects of array aperture and population density. All arrays contained 16 microphones, configured as two orthogonal 8-element arrays. The three array sizes studied were: a 3.5-foot array with 6-inch spacing mounted on the electric-powered RC glider, a 7-foot array with 12-inch spacing mounted on the large RC airplane, and a 14-foot array with 24-inch spacing mounted on the large autonomous powered parafoil. These arrays provided identical beam widths in successively lower frequency octaves. Photographs of the three array-equipped flight platforms used in the project are shown in the illustrations below.
Accomplishments - The project was completed at an accelerated pace to ready the hardware for flight at Sensor Week 2004, sponsored by Eglin Air Force Base in May 2004. The flight tests were successful and excellent airborne acoustic data were collected on a variety of military ground and air vehicles.