Multi-rate Adaptive Nulling of Moving Interferers

**Henry Cox
ORINCON Corporation
4350 N. Fairfax Drive
Arlington, VA 22203
Email: hcox@east.orincon.com**

**Abstract**** **
A fundamental problem in
using adaptive beamforming on large underwater acoustic arrays is that the noise due to
multiple moving surface ships is non-stationary. As the number of elements increases, the
number of snapshots required for covariance matrix estimation increases. As the array size
increases, the travel time across the array increases, thereby increasing the interval
between snapshots, and the motion of sources measured in beamwidths increases, thereby
decreasing the time available for snapshots. The net result is a severe sample support
problem in which significant motion occurs before sufficient snapshots can be obtained for
matrix estimation. A new approach to this problem is multirate adaptive beamforming. This
approach exploits the fact that the nearby interferers appear to be both the loudest and
the most dynamic. The approach involves using a relatively few snapshots to estimate the
subspace of the K loudest interferers. These strongest interferers are tracked in this
interference subspace. In addition, this information is used to determine a dynamically
changing orthogonal space in which the K strongest interferers have been nulled. Slow
adaptation is carried out in this orthogonal space looking for weak signals. The theory of
this approach is explained, including sample support requirements and effects of motion on
eigenvalues. The algorithm is illustrated with simulation. Results from application to
sea-test data are presented.