The effects of spatial averaging on coherence and resolution

Averaging of the spatial correlation function over the spatial variable-spatial averaging- is performed on the estimated spatial correlation matrix prior to beamforming calculations. Generally speaking, this preprocessing operation reduces the intersignal coherence in the spatial correlation matrix and the variance of the spatial correlation function estimate. Two types of spatial averaging are considered: subaperture averaging and redundancy averaging. Subaperture averaging is shown to significantly reduce coherence only when the source propagation directions are widely spaced while variance reduction is accompanied by a reduction in apparent aperture. In contrast, the reduction of coherence resulting from redundancy averaging is relatively insensitive to source bearing separations. While the aperture is not affected, redundancy averaging may induce bias in the estimated source bearings