Sandra Collier

 

Atmospheric acoustic arrays are currently being used for target detection, identification, and direction finding - applications that have been well established in underwater acoustics. Generally, these systems perform direction finding by determining the wavefront angle of arrival (AOA) from the phase differences across the array.  The scattering of sound waves from random fluctuations in the propagation environment, whether it be atmospheric or oceanic, can create random distortions in the propagating wavefronts, which are perceived as fluctuations in the apparent bearing angles and source strength. (These phenomena are analogous to the scintillation and quivering of optical images, which may be observed above a roadway on a sunny afternoon.) The error in estimating the wavefront's AOA will increase as the propagation distance increases and/or the intensity of the turbulence increases.  It is well known that the net effects of these distortions can have a substantial impact on direction-finding in both the atmosphere and the ocean.  However, many of the current atmospheric beamformers only account for the medium effects through use of an effective sound speed.  Here we develop a maximum-likelihood estimator (MLE) for the AOA that directly accounts for the effects of scattering from atmospheric turbulence.  The MLE utilizes a statistical model for the received signal that is based on the theory of wave propagation in a random medium and realistic atmospheric turbulence models.  This statistical model has been previously used to predict the performance of atmospheric acoustic arrays - correctly predicting the trends with respect to meteorological conditions, frequency, and range when compared to experiment results. Simulated data sets are used to test the MLE.  It is found that the physically correct MLE outperforms conventional beamformers that were developed for a homogeneous medium.

 

 

 

Dr. Sandra Collier is a senior research physicist at the U.S. Army Research Laboratory (ARL) in Adelphi, MD.   She has worked in atmospheric acoustics with ARL as an employee since 2001, and as an American Society for Engineering Education Post-Doctoral Fellow from 2000-2001.  Her research interests are in sound propagation in the atmosphere, over a porous ground, and in other complex environments. She works on the development of theoretical and numerical models to describe the effects of these propagation media on acoustic waves and on determining how these propagation media affect the performance of battlefield acoustic sensors.  She also currently serves as the Computational and Information Sciences Directorate's directorate representative for the National Research Council and Oak-Ridge Associated Universities post-doctoral programs.  She received a Ph.D. in Physics from New Mexico State University in 1998, a M.S. in Physics from the University of Houston in 1993, and a B.S. in Mathematics and Physics from the University of Houston in 1991.