Effects of noise in clutter-locking m.t.i. systems

In clutter-locking m.t.i. systems, the presence of noise leads not only to a direct contribution to the residue at the canceller output but it also perturbs the clutter-locking mechanism and so reduces the cancellation of the clutter. The clutter locking is usually achieved by introducing a differential phase shift between the delayed and direct channels to cancel that induced by the mean clutter velocity. This cancellation may be achieved either by measuring the induced phase shift and feeding forward the mean value to the differential phase shifter or by measuring the residual phase shift after the differential phase shifter and feeding back the amplified output, thereby driving the mean value to zero. The limitations of the feed-forward method compared with the feedback one are identified, and the losses for the latter are derived. It is shown that the effect of the noise perturbation can be considerably greater than the noise residue for single-delay cancellers but the effect is of less importance for higher-order ones.