Microwave imaging technique allows obtaining images of hidden objects in structures and media using microwaves. Usually in short-range microwave imaging systems, the back-scattered signal is used, when a combined transmit-receive antenna scans over a plane, forming a two-dimensional synthesized aperture, while the signal reflected from the object of observation is recorded, as a result of which a microwave hologram of the object is formed. The second option involves registering the forward-scattered signal, when the transmitting and receiving antennas are located on opposite sides of the object and scan synchronously. The purpose of this work is a theoretical and experimental comparison of these two sounding options, identifying the advantages and disadvantages of each option, taking into account the features that arise when solving various problems of microwave imaging. Keywords: microwave holography, mivrowave image, back-scattered signal, forward-scattered signal, range resolution.

Due to their high performance qualities, vivaldi antenna designs are employed in numerous communication applications, including ground penetrating radar. Due to its high gain and wide band properties, the Vivaldi antenna is one of the most commonly utilized antenna design for Ground Penetrating Radar (GPR) applications. However such designs often require for a huge design area or a smaller size with poorer performance metrics. More and more focus has recently been placed on Frequency Selective Surfaces (FSS) designs with unit cell widths considerably lower than the operation wavelength. The interactions between the unit cells are what give the structure the desired FSS properties. Here, a unique design with FSS loaded structures is suggested to have a high performance wide-band Vivaldi antenna for GPR application. The obtained performance results are benchmarked with results from literature. As a result the proposed antenna has a high gain performance (13.6 dBi) within the operation range of 0.6-4 GHz.

Abstract The ancient pyramids keep a lot of mysteries and attract the attention of historians, archaeologists and just tourists from all over the world. Their assignment, construction methods and especially the discovery of previously unknown voids and structures inside them require further study using new technical solutions. The most famous of these structures is the Great (Khufu’s) Pyramid on the Giza plateau in Egypt. Since the middle of the last century, several non-destructive technical methods have been proposed for examining the internal structure of the pyramid. Electromagnetic translucence in the radio frequency range and the use of muon sensors are among them. Although the use of muon sensors has supposedly revealed two previously unknown voids within the Pyramid, independent confirmation of their presence is required. The article discusses the fundamental possibility of using gravimetry to examine the internal structure of the Great Pyramid, and analyzes combining it with other modern technical means including unmanned aerial vehicle (UAV) for the implementation of this project. The paper proposes the simplest model that allows evaluating the required accuracy of a microgravimeter capable detecting the supposed voids in the Pyramid. The advantage of this approach, in addition to its simplicity, is the ability easily checking the presented in paper results. The main purpose of writing the article is to draw the attention of the scientific community to another method of non-destructive testing for the study of the internal structure of the Great Pyramid and discussion further needed steps in this direction.

Glass fiber Reinforced Polyurethane Foam (RPUF) has found wide application primarily in the aerospace and construction industry due to its outstanding properties. Unlike pure non-reinforced Polyurethane Foam (PUF), it has increased strength characteristics and at the same time retains unique heat and sound insulation properties. The paper considers the features of reinforced polyurethane foam examination in the Microwave (MW) range. It is shown that there is a significant difference in the recorded MW images compared to pure polyurethane foam. This is expressed primarily in the fact that the reinforcing fibers have a dielectric constant that is different from enclosing polyurethane foam, which leads to scattering and reflection of the incident electromagnetic wave on them. Experimental studies have shown that for RPUF, in contrast to PUF, there is an optimal wavelength range in which the contrast of defects against the background of glass fiber reflections is of the greatest value. An experimental comparison of two methods of examination back-scattering and forward-scattering methods was also carried out for RPUF. It is shown that the forward-scattering technology of measurements, if it can be implemented, has certain advantages since allows reducing the contrast of background reflections from the reinforcing fibers.

Many constructions built by ancient civilization hold many mysteries and attract the attention of historians, archaeologists and tourists from all over the world. The most famous and enigmatical among them is the Great (Khufu's-Cheops') Pyramid on the Giza plateau in Egypt. The assignment, construction methods and especially the inside structure of the Pyramid have been the cause of heated debate among historians and researchers since ancient times. Unfortunately, not only researchers are interested in ancient structures and excavation sites of archaeological values, but also robbers and illegal seekers of archaeological values. This led to the partial destruction of the Pyramid by the order of the Arab caliph Al-Ma'mun in the 9th century AD. From the middle of the last century, the point of view has prevailed that only non-destructive testing methods are acceptable when examining ancient structures. In the 1960s, a technology was proposed for transilluminating the pyramids of Giza by muons, which are generated by cosmic rays in the Earth's atmosphere. This method gave promising results. Other means were also proposed, which include radar, as well as gravitational and vibration technologies. However, despite numerous attempts to use them, no significant success or discovery has been achieved. A discussion of these methods, their applications and partial successes is the subject of this article.

Polymer composites, for example, carbon or aramid fibre-reinforced plastics, are widely used in aviation technology, aircraft engines for the manufacture of load-bearing elements. The main advantage of composite materials based on synthetic reinforcing fibers is a high specific strength, the ability to control the strength and rigidity of a structure over a wide range, achieving its high weight perfection. A significant disadvantage of traditional layered composites is the low resistance to transversal impact. Low-velocity impacts are commonly occurred in the operation of aircraft structures: tool drops during maintenance, hail, gravel from the runway, bird strikes, etc. Intra- and interlaminar damages (delamination, fibre fracture) that occur during impacts significantly affect the residual strength of the composite element and can lead to its sudden failure during normal operation. In this regard, an important part of health monitoring of composite structures is the identification of impact defects for their further repair or the decision to replace the entire element. In this regard, it is of considerable interest to determine the location of a possible defect in a semi-automatic mode using remote control methods, in particular, radar and radio waves. The paper presents the results of state control for a sample of an aramid plate with defects, obtained microwave holograms in the frequency range of 15–24 GHz, and reconstructed microwave images in three-dimensional planes. The proposed method of non-destructive testing can be successfully used to determine damage in composite structures reinforced with glass and organic fibers, etc.

Abstract The ancient pyramids keep a lot of mysteries and attract the attention of historians, archaeologists and just tourists from all over the world. Their assignment, construction methods and especially the discovery of previously unknown voids and structures inside them require further study using new technical solutions. The most famous of these structures is the Great (Khufu’s) Pyramid on the Giza plateau in Egypt. Since the middle of the last century, several non-destructive technical methods have been proposed for examining the internal structure of the pyramid. Electromagnetic translucence in the radio frequency range and the use of muon sensors are among them. Although the use of muon sensors has supposedly revealed two previously unknown voids within the Pyramid, independent confirmation of their presence is required. The article discusses the fundamental possibility of using gravimetry to examine the internal structure of the Great Pyramid, and analyzes combining it with other modern technical means including unmanned aerial vehicle (UAV) for the implementation of this project. The paper proposes the simplest model that allows evaluating the required accuracy of a microgravimeter capable detecting the supposed voids in the Pyramid. The advantage of this approach, in addition to its simplicity, is the ability easily checking the presented in paper results. The main purpose of writing the article is to draw the attention of the scientific community to another method of non-destructive testing for the study of the internal structure of the Great Pyramid and discussion further needed steps in this direction.

The study, preservation and restoration of the cultural heritage objects of mankind are not only of great cultural importance but also have a significant economic component because cultural values of past centuries attract tourists from all over the world. The use of modern technical and scientific achievements in the field of non-destructive testing makes it possible to obtain new knowledge about cultural objects regarding their origin and dating, as well as to contribute to their better restoration and preservation. An important component of their use is additional opportunities to identify high quality fakes of original cultural objects that have historical significance. The capabilities of various non-destructive testing (NDT) methods used to examine cultural objects are characterized by their penetration depth, resolution, and sensitivity to material properties. Thus, in many cases, it is necessary to perform multi-sensor non-destructive testing and creating large data sets that require an efficient evaluation. This article considers an example of using microwave (MW) holographic sensors for the examining of an old Orthodox Russian Icon dated of the late 19th century. The paper describes the technology of microwave holography, which has recently been applied to the examination of art works. Unlike the well-studied X-ray method, MW holography makes it possible to examine objects with one-sided access. Its other advantages are the relative cheapness of the equipment and the safety of use due to the low level of radiation. The article describes a MW holograms reconstruction algorithm, as well as a method for improving the quality of obtained MW images. The data collected at MW research of the Icon are compared with the results of X-ray examination and confirmed by subsequent opening and visual examination performed by professional restorers.

This article describes a prototype of microwave personal screening system, which uses the motion of the target to obtain synthetic aperture. A linear multiple-input and multiple-output (MIMO) antenna array, developed for this purpose, consists of 16 transmit and four receive vertically distributed antennas and uses a stepped frequency modulated signal in the range from 12 to 18 GHz. Position of the target relative to the antenna array is obtained by a depth sensor. The processes of designing the antenna array, aligning the antennas individually, calibrating antenna-feeder paths in the signal channels, and finding the transformation between the reference frames of the depth sensor and the antenna array are given. A method of calculating the radar image by the back projection method with the application of the fast Fourier transform is described in detail. The results of experiments with moving objects and hidden items are presented with their radar images.

Microwave imaging technique allows obtaining images of hidden objects in structures and media using microwaves. Usually in short-range microwave imaging systems, the back-scattered signal is used, when a combined transmit-receive antenna scans over a plane, forming a two-dimensional synthesized aperture, while the signal reflected from the object of observation is recorded, as a result of which a microwave hologram of the object is formed. The second option involves registering the forward-scattered signal, when the transmitting and receiving antennas are located on opposite sides of the object and scan synchronously. The purpose of this work is a theoretical and experimental comparison of these two sounding options, identifying the advantages and disadvantages of each option, taking into account the features that arise when solving various problems of microwave imaging.

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.

In this paper the possibility of radio frequency range using to survey the Great (Khufu’s) Pyramid in Egypt is considered. This task is especially urgent in light of the reports that have appeared on the alleged detection of previously unknown voids in the pyramid body by using muon sensors. Given that this method is indirect, in the absence of the possibility of drilling or making passes, independent confirmation is required based on other non-destructive testing methods. The results of mathematical modelling will help determine the further direction of research and expand the areas of applicability of radio holography when examining objects of the cultural heritage of mankind.

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.

The study of ancient Egyptian monuments attracts the attention of experts from around the world. A recent event that confirms this is the discovery, using muon sensors, of previously unknown cavities in the Great Pyramid of Giza (or Khufu's Pyramid). Since it is unfeasible to directly confirm this discovery by drilling, another independent non-destructive method is necessary to confirm this discovery and provide accurate determination of the locations and shapes of the cavities. Following a literature review of the different methods used in evaluating cultural objects, this paper analyses a possible framework for simulation of a holographic radar for detecting openings or other unknown structures of interest to archaeologists/Egyptologists and the public.

In modern microwave personnel screening systems, a 2-D synthetic aperture is used to obtain a radar image with resolution limited by the diffraction of electromagnetic waves. Therefore, the antenna system of the screening system has large dimensions, a large number of switched channels, and a high cost, which limits a wider use of these systems, for example, at ground transport hubs or administrative buildings. In this article, we consider a different method of forming synthetic aperture, in which it is formed due to the relative movement of a subject near a stationary antenna system. Using the described experimental setup and imitation experiment with a mannequin, it was shown that the proposed method can significantly reduce the dimensions of a prospective screening system and the number of channels in the antenna system and at the same time obtain detailed radar images of hidden objects.

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.

Holographic subsurface radar (HSR) is not currently in widespread usage. This is due to a historical perspective in the ground-penetrating radar (GPR) community that the high attenuation of electromagnetic waves in most media of interest and the inability to apply time-varying gain to the continuous-wave (CW) HSR signal preclude sufficient effective penetration depth. While it is true that the fundamental physics of HSR, with its use of a CW signal, does not allow amplification of later (i.e., deeper) arrivals in lossy media (as is possible with impulse subsurface radar (ISR)), HSR has distinct advantages. The most important of these is the ability to do shallow subsurface imaging with a resolution that is not possible with ISR. In addition, the design of an HSR system is simpler than for ISR due to the relatively low-tech transmitting and receiving antennae. This paper provides a review of the main principles of HSR through an optical analogy and describes possible algorithms for radar hologram reconstruction. We also present a review of the history of development of systems and applications of the RASCAN type, which is possibly the only commercially available holographic subsurface radar. Among the subsurface imaging and remote sensing applications considered are humanitarian demining, construction inspection, nondestructive testing of dielectric aerospace materials, surveys of historic architecture and artworks, paleontology, and security screening. Each application is illustrated with relevant data acquired in laboratory and/or field experiments.

This paper explores the possibility of using a synthetic aperture radar to detect surface defects of rails and measure parameters of rail junctions. Experimental data were obtained with a setup consisting of a two-coordinate electromechanical scanner and a radar emitting continuous stepped-frequency signal in the range of 22.2 - 26.2 GHz. As an object of study, fragments of narrow-gauge rails were used, in which surface defects of various sizes and depths were created. A phase method for radar signal processing based on the backward propagation of its wavefront was developed, with which radar images of rails with defects were obtained. Experimental studies have shown that the developed rail surface imaging method allows detecting the presence and measuring characteristics of cracks on the tread, cleavages of the railhead, the width of the joint gap and the magnitude of the vertical step at the rails joint. High accuracy and sensitivity of the radar method, confirmed with the contact measurements matching, allow using it for fast noncontact diagnostics of the rails condition.