  Recursive, causal and non-causal, multidimensional digital filters, with
infinite impulse responses and maximally flat magnitude and delay responses in
the low-frequency region, are designed to negate correlated clutter and
interference in the background and to accumulate power due to dim targets in
the foreground of a surveillance sensor. Expressions relating mean
impulse-response duration, frequency selectivity and group delay, to low-order
linear-difference-equation coefficients are derived using discrete Laguerre
polynomials and discounted least-squares regression, then verified through
simulation.
