V.V. Razevig, A.S. Bugaev, S.I. Ivashov, I.A. Vasiliev, A.V. Zhuravlev
A family of holographic subsurface radars, named RASCAN, has been produced during more than fifteen years. Until recently RASCAN radar had the direct-gain receiver that was registering amplitude of reflected signal only. Therefore, the obtained images were used without any processing. However, in the most cases it was possible to identify the shape of the subsurface objects on the images and to solve the assigned tasks.
In 2010 the new generation of RASCAN radars with quadrature receiver have been developed at Bauman Moscow State Technical University. This technique allows to record the amplitude and phase of reflected signal. The performance of such devices crucially depends on the reconstruction algorithms. The goal of this paper is to discuss the basics of various approaches such as single-frequency, two-frequency and multifrequency methods. All methods form focused images of subsurface objects from coherent-wave data gathered remotely over a 2D aperture.
Single-frequency reconstruction method is based on solving of conjugate wave equation with utilization of registered complex amplitude of the signal as boundary conditions. The focused image can be reconstructed at any plane that parallel to the scanning surface. This method is well suited for reconstruction of holograms of the objects with flat or almost flat illuminated (directed to the antenna) surface.
Two-frequency method uses the filter function to stress those parts of reconstructed image whose depth are equal to the distance between scanning surface and focusing plane. The filter function is built on the difference between phases of holograms recorded on two near frequencies. With this method it is possible to reconstruct the shape of more complicated objects having sizeable surface relief.
Multifrequency method implemented with 3D fast-Fourier-transform algorithm shows best results for any objects and for any its arrangement (even when one object is located above another one). Also it allows building the 3D images of subsurface objects. However, this method requires more complicated radar transmitting ultra wide band signal.
The comparison of the methods was carried out on the simulated and experiment holograms