Extrapolation of near-field RCS measurements to the far zone

An algorithmic procedure for extrapolating near-field radar cross-section (RCS) measurements to the far zone has been derived, coded, and experimentally validated. The deviation of the extrapolation algorithm uses an optical model to estimate the surface currents induced on the scattering body by the incident field, and a specially weighted version of the Fourier transform to calculate the near-field scattering amplitudes associated with such surface currents. The extrapolation entails three steps. First, near-field measurements of the scattered electric and/or magnetic field are used to infer the monostatic vector potential. Next, the inverse Fourier transform of the inferred vector potential is multiplied by a special weighting function to estimate an equivalent obliquity factor. Finally, the far-field scattering pattern is estimated by taking the Fourier transform of the reweighted obliquity factor. This extrapolation procedure has been validated using anechoic-chamber data taken on a right-circular aluminium cylinder 25 λ high and 2.5 λ in radius at near-field range of 19% of 2 D²/λ where D is the nominal target diameter and λ the radiation wavelength. The extrapolated RCS pattern for this target was compared with an analytical estimate of its far-zone pattern and good amplitude and phase agreement was observed over a 20° cone of scattering angles