Detection and tracking of broadband airborne sources: An ‘‘acoustic fence’’ approach

An ‘‘acoustic fence’’ concept is discussed as an advance detection system for approaching subsonic airborne vehicles. Each ‘‘post’’ of the acoustic fence is an array of three closely spaced microphones at right angles to each other on the ground. The posts themselves are spaced approximately 1 kilometer apart. It is shown that the azimuthal and elevation angles for an airborne vehicle can be accurately determined from simple cross correlation of the broadband time series measured at a single array (i.e., at one post). With two arrays, the vehicle track can be determined by finding the stationary points of an iterative nonlinear equation. The equation is of the form τp=f(τe), where τe and τp are, respectively, the estimated and predicted acoustic travel-time differences for two adjacent arrays. The stationary points, i.e., where τp=τe, allow the vehicle track to be computed directly from triangulation. Examples are discussed using both synthetic data and actual field measurements.