Resolution of inverse aperture synthesis in the near-field zone of the horn antenna radiation

Synthetic aperture and inverse synthetic aperture approach is rather powerful tool of improving resolution along transversal axis [1, 2]. The scalar reflectometer operating in frequency range of 38-52 GHz at the grid of frequencies allowed us to save information about complex reflectivity of objects due to application of Fourier-holography principle [3]. The experimental data at the grid of frequencies are transformed to time domain by means of digital spectral analysis. The combination of synthesis of time-domain signals for obtaining longitudinal radio profiles of objects and scanning in transversal direction produces 3D images with values of insert reflectivity along z-axis. A pyramidal horn with the length of 120 mm and aperture sizes of 46×46 mm is used as radiating and receiving antenna and the source of reference signal for Fourier-holography principle implementation. The resolution in longitudinal direction is specified by the frequency operating band. For our experimental facility the duration of synthesized pulse is equal to ~70 ps at the level of 3 dB. The further improvement of longitudinal resolution demands widening the operating frequency band. The resolution in transversal direction is specified by the antenna sizes. The further improvement of transversal resolution can be achieved by application of synthetic aperture or inverse synthetic aperture. The last approach is rather convenient for experimental implementation for facilities [3]. Traditionally for aperture synthesis one uses Fourier or Fresnel transform in dependence if the object is in far-field or intermediate zone. But the problem is stating more sophisticated if the object is situated in near-field zone. The purpose of this investigation is to determine experimentally the resolution of radar with inverse synthetic aperture in the near-field of the antenna. The objects for the investigation were the pairs of the copper strips with width of 10 mm under conditions of variation of di- tance between them. Inverse filtering with regularization of complex data of spatial distribution of received signal was implemented as the practical realization of inverse aperture synthesis if information about phase characteristics and distance is absent. All experimental results have been obtained using facilities [3].