Ambient noise field edge effects in product array processing

Abstract

Product array processing, also called split beam correlation, is often the favored process for both detection and tracking because it has higher spatial resolution than sum and square processing and because it has been thought to yield a near zero mean output for either independent or isotropic noise inputs. Numerical evaluation, however, shows that when steered near endfire a product line array embedded in isotropic noise will yield a substantially nonzero mean, e.g., 15% correlation. This effect is studied in terms of both interelement correlation and directional response patterns for both split arrays and element pairs. It is shown to be a consequence of the discontinuity in the isotropic noise field when considered in the total interelement delay space that the array is capable of being steered to. It is thus seen to apply to product arrays of arbitrary configuration whenever the product steering delays are nearly equal to the maximum possible delays in the medium, e.g., near endfire for any element pairs. This effect is also shown to apply to ambient noise fields that are highly nonisotropic. Finally, it is shown that this effect can be greatly reduced by the addition of noise at each of the array elements, the noise being correlated in such a manner as to extend the isotropic noise field further into the delay space covered by the array.