Estimation of radio refractivity structure using radar clutter

Describes the estimation of low-altitude atmospheric refractivity from observations of radar sea clutter. Both surface and evaporation ducts are considered. The intended use of the technique is to provide near-real-time estimation of ducting effects for naval forces, which is important for radar performance prediction. For surface duct inversions, the authors use the Simulated Annealing/Genetic Algorithm (SAGA) general purpose inversion code. SAGA is configured to use an embedded parabolic equation electromagnetic propagation model, a four-parameter model for atmospheric refractivity, and a linear least-squares objective function. The mismatch between (a) the optimal replica field and the observed clutter and (b) the inferred refractivity profile and the range-dependent refractivity structure obtained by in situ measurements, is discussed. The inference of evaporation duct heights is simpler than the inference of surface duct parameters and has already been presented in the open literature. The material presented is an update on the performance of the algorithm based on at-sea testing.