RECURSIVE DIGITAL-FILTER REALIZATION AND INVERSE FILTERING FOR AN IONOSPHERIC MODEL

This paper presents a recursive filter realization for an ionospheric model. This realization is in the form of a cascade of identical second-order allpass filters, and is determined by only three parameters; two coefficients of an allpass section, and the number of sections. The values of these parameters are optimized by a nonlinear optimization algorithm called the "downhill simplex method", so that the resulting time delay function closely approximates that of the ionospheric model. Two inverse filtering methods (nonrecursive and recursive) based on the realization are developed and evaluated.