An Approach for Extrapolating Far Field Radar Cross-Section from Near Field Measurement

Measurement of the RCS(Radar Cross Section) of large targets at high frequencies often must be done at distances less than the standard far field criterion. In the near field, a target´s RCS pattern will vary with distance and may differ dramatically from the same pattern measured in the far field. Such differences, which become especially significant at microwave frequencies, are more serious when measuring RCS patterns than antenna-gain patterns, because both the illuminating and scattered waves are subject to wave front curvature. It is therefore necessary to extrapolate the resulting near field measurements into the far Field. An algorithmic procedure for extrapolating near-field radar cross section (RCS) measurements to the far Field has been evolved, and experimentally validated. The derivation of the extrapolation algorithm uses multi-scatters model about the scattering body. The extrapolation algorithm entails three steps. First, use FFT(Fast Fourier Transform) pairs in frequency and range domain to compensate the roundtrip propagation loss in near-field measurements, Next, use a focused function to correct the wave front curvature because of the spherical wave illumination, Finally, the far-field image and RCS is estimated by taking the Fourier transform and Stolt interpolation of the data in wave number domain.