Microwave imaging technique allows obtaining images of hidden objects in structures and media using microwaves. This technique has various applications such as: nondestructive testing, medical imaging, concealed weapon detection, through-the-wall imaging, etc. Obtaining radar images in these applications is based on processing phase and amplitude of the reflected signal recorded over an aperture (a microwave hologram). To design and evaluate the effectiveness of modern radars, to test the developed reconstruction algorithms, microwave holograms of various objects obtained under different conditions are required. Obtaining microwave holograms by experimental methods is associated with measuring the scattering fields of real objects. Such experiments are rather laborious and expensive. Therefore, the problem of modeling the processes of scattering of electromagnetic waves by the objects of study is very important. Since the implementation of rigorous methods for solving the scattering problem is associated with large computational costs, it is sometimes advisable to use various simplifications and assumptions to analyze scattering fields on objects of complex spatial configuration. One of these methods is the physical optics method, which is a very common method for calculating fields scattered from objects of various shapes. The physical meaning of the approximation of the physical optics approach is that the field on the surface of the scattering object is taken to be equal to the field in the absence of the object. In other words, multiple reflections of an electromagnetic wave between different parts of the object are not taken into account. The aim of this work is to study the influence of the effects of re-reflection of an electromagnetic wave and to reveal the dependence of the accuracy of the calculation of the scattered field, performed by the physical optics method, on the shape of scattering objects. A comparison of microwave holograms obtained by the physical optics method with the results of calculations using the computational electromagnetic software product FEKO is carried out. It was found that for objects consisting of separate elements, spatially separated in a plane parallel to the registering plane, rereflections have the strongest effect on the recorded microwave hologram of an object if a distance between the elements equal to about 65% of the wavelength. In general, for such objects, the degree of influence of multiple reflections is small, and they can be ignored when modeling microwave holograms. For objects consisting of separate elements spatially spaced in a direction perpendicular to the registering plane, the influence of multiple reflections is approximately five times greater than for the previous case. The greatest effect is observed when the distance between the elements is equal to 30% of the wavelength. Under such conditions, ignoring re-reflections when modeling microwave holograms can lead to incorrect results when reconstructing them. When modeling scattering by spatially extended solid objects, the degree of influence of re-reflections depends on the shape of the object: if the side of the object oriented to the registering plane of the microwave hologram is convex, then the influence is minimal, and if it is concave, re-reflections must be taken into account.

Microwave imaging is a technique for evaluation of hidden or embedded objects in an optically opaque structure (or media) using electromagnetic waves in microwave regime. The result of the study is a microwave image of the internal structure of the investigated object, which is built by reconstructing the electromagnetic field scattered by the object (microwave hologram), recorded using some radar system at some aperture. Along with the widespread flat aperture, a cylindrical aperture is often used in personnel screening systems, microwave system for automated body measurement for apparel fitting, and medical tomographic scanners. Cylindrical geometry requires special holograms reconstruction methods. The work is dedicated to comparison of three hologram reconstruction methods: №1 – back projection, №2 – back propagation and №3 – Gauss–Newton, and identifying the advantages and disadvantages of each method. All methods were adopted to cylindrical geometry, software implemented using Python programming language and compared. Comparison was performed by reconstruction of microwave holograms of the same objects. Microwave holograms for comparison were calculated in accordance with the principles of physical optics for point scatterers and using the computational electromagnetics software product FEKO for solid objects. Comparison criteria were: speed of calculations, quality of obtained microwave images, required random access memory (RAM) of the computer. Based on the results of numerical experiments, the following conclusions can be made. For both point and solid objects, all methods have showed a similar quality of the obtained microwave images, the difference turned out to be minimal both in visual and numerical estimation. The advantage of method №1 is the simplicity of its software implementation. In addition, using the first method, you can easily do reconstruction for any area (line, surface, volume), the position of which can be arbitrary in relation to the positions of the samples of the radar signal. Method №2 is the fastest method. With the parameters considered in the article, it is two orders of magnitude faster than method №1, and its performance can be easily increased by parallelizing calculations for different radii. Among the shortcomings, one can note the complexity of its software implementation and the dependence of the position and size of the reconstructed area on the location and number of samples of the radar signal. A significant drawback of method №3 is its high requirements to the RAM of the computer, as well as low speed of calculations. When processing microwave holograms with a large number of samples, calculations may require more memory than is installed in the computer, and the calculation time will increase many times due to the continuous exchange of data with the hard disk, or it will be impossible to do the calculations at all.

Microwave imaging technique allows obtaining images of hidden objects in structures and media using microwaves. This technique has various applications such as: nondestructive testing, medical imaging, concealed weapon detection, through-the-wall imaging, etc. Obtaining radar images in these applications is based on processing phase and amplitude of the reflected signal recorded over an aperture (a microwave hologram). Recently, systems began to appear in which the radar part is supplemented by an RGB-D sensor, which allows to obtain new capabilities. For example, there is the a microwave screening system architecture in which inverse synthetic aperture is formed by the natural motion of the subject in the vicinity of a stationary linear antenna array. The microwave system is complemented with an synchronous RGB-D video sensor which captures the trajectory of the moving subject in 3D and allows coherent processing of the radar signal. Another system detects objects buried under irregular surface and uses RGB-D sensor for capturing the surface relief for suppressing reflection of the sounding signal from the surface. Calibration between a radar and an RGB-D sensor is an essential process for microwave and optical data fusion. This article presents a novel approach for calibration, using a planar calibration target which is made of radiotransparent material (such a foam plastic sheet) with square marker and six small metal balls embedded in target surface and representing point objects. The proposed method exploits 3D-3D correspondences between coordinates of point objects in two coordinate systems associated to the sensor and to the radar. One points set is extracted from optical data, using marked corners of the target as a base points. Second points set is obtained from microwave data as local maxima of 3D volume of data reconstructed from one-frequency microwave hologram. Computer modeling were performed using Autodesk 3ds Max software by which models of all components of the system were built and the optical image from the sensor was modeled. Test experiments were carried out using a measurement system composed of the following components: a compact vector network analyzer (VNA), two mechanical scanners with stepper motors, one transmitting and one receiving horn antennas, mounted on the VNA, an RGB-D sensor, a microcontroller board, and a computer. The high accuracy of the method is confirmed both by computer modeling and physical experiment. The accuracy of determination of relative position between the radar and the sensor is about one fifth of the signal wavelength used.