A New Algorithm about Transforming from Near-field to Far-field of Radar Target Scattering

In order to meet the approximate plane-wave irradiating condition, adequate large field or compact range system is needed for RCS measurement of large aircraft targets. However, an outside testing field site or a compact range system is very expensive, so a new RCS extrapolation method named Hankel algorithm based on near-distance testing has been presented. According to the fact that objective reflectivity spatial distribution is unchanged with measurement state in ISAR imaging, here the multi-scattering center model is built. Then, the height of stealth target is always short, and the far-field condition can easily be satisfied on vertical plane, so the incident spherical wave is equivalent to cylindrical wave. Once more, the relationship between far-field and near-field is deduced by regarding image of the target as secondary radiating source. Finally, the RCS of target can be obtained with the scattering information of near-field extrapolated at full angular aspect. Taking derivative of the near-field data with respect to frequency in algorithm, the error can be reduced in simplifying formula, and just meet the requirement of stepped-frequency system. The results of simulation and experiments show that the algorithm is precise as well as effective.