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Ground-based synthetic aperture radars (GBSAR) are popular instruments widely used for the monitoring of infrastructures. One of the main problems of ground-based interferometric radars is the elevation ambiguity. Multiple-input multiple-output (MIMO) arrays could solve this problem. This work proposes a study on possible MIMO configurations to achieve elevation resolution in ground-based radar measurements. Specifically, two array configurations are compared: a random sparse array suitable for the compressive sensing technique, and a non-uniform array. The two solutions are compared by means of simulations and experimental tests. An ad hoc system has been developed to jointly test the two configurations, and results obtained in a controlled and real urban scenario are shown. It is found that both systems are able to solve elevation ambiguity. The non-uniform array seems to achieve good performance in a general scenario, while the CS processing can outperform the other only after optimization, depending on the specific scenario and application.

Since the 1950s, radar sensors have been widely used for the monitoring of the earth’s surface. The current radars for remote sensing can be divided into two main categories: Space/aerial-borne and ground-based systems. The unmanned aerial system (UAS) could bridge the gap between these two technologies. Indeed, UAS-borne radars can perform long scans (up to 100/200 m) in a brief time (a few minutes). From the 2010s, the interest in UAS-borne radars has increased in the research community, and it has led to the development of some commercial equipment and more than 150 papers. This review aims to present a study on the state-of-the-art of UAS-borne radars and to outline the future potential of this technology. In this work, the scientific literature was categorized in terms of application, purpose of the paper, radar technology, and type of UAS. In addition, a brief review of the main national UAS regulations is presented. The review on the technological state-of-the-art shows that there is currently no standard in terms of radar technology, and that the multi-helicopter could be the most used UAS in the near future. Moreover, the UAS-borne radar can be used for several remote sensing applications: From landmine detection to smart agriculture, and from archeological survey to research and rescue applications. Finally, the UAS-borne radar appears to be a mature technology, which is almost ready for industrialization. The main developmental limit may be found in the flight regulation, which does not allow for many operations and imposes strict limits on the payload weight.

The authors propose an interferometric multiple-input multiple-output radar specifically designed for monitoring/testing bridges. It makes use of compressive sensing and synthetic aperture radar techniques for providing coherent images of its field of view. The radar prototype has been tested in controlled environment and in operative conditions during the static test of a pedestrian bridge.

A monostatic/bistatic radar for retrieving the three-dimensional (3D) displacement vector is proposed for static and dynamic monitoring of bridges. The radar is particularly suitable for dynamic test, as it is able to detect the three components at high sampling frequency (132 Hz, in the reported case study). The monostatic/bistatic technique makes use of a multiple input multiple output (MIMO) interferometric radar equipped with two transponders. Each single transponder consists of an antenna and an amplifier and it is connected to the radar with a radiofrequency (RF) cable. A simulation has preliminarily estimated the expected accuracy in terms of positing error of transponders. The equipment and the method were experimentally tested in a controlled scenario using a vertical steel bar as target. Finally, the method was applied on a real case of interest: the dynamic monitoring of a bridge (“Ponte Nuovo sull'Arno” in Lastra a Signa, Firenze, Italy) excited by the vehicular traffic. The 3D displacement vector was detected in time and the modal frequencies of each component were estimated.

Although stroboscopic pulse (SP) ground penetrating radar (GPR) is the most popular and widespread equipment for subsoil investigation, continuous-wave (CW) radar has better performance in terms of noise, system dynamic range, and penetration depth, at the expense of greater complexity and cost of the components. The aim of this letter is a direct comparison between SP GPR and CW GPR through an extensive measurement campaign in five different locations representative of the different conditions where a GPR could operate.

In recent years, interferometric radars have been extensively used as sensors for static and dynamic monitoring of bridges. Generally speaking, a radar can only detect the displacement component along its view direction. As the movement of a real bridge or a large structure can be rather complex, this limitation can be a significant drawback in engineering practice. In order toovercome this limitation, in this article, a multi-monostatic interferometric radar with radio link is proposed. This radar is able to detect a second component of displacement using a transponder. The transponder is connected to the radar through a radio link. The radio link allows the installation of the transponder far away from the radar, and even in the opposite direction. The equipment is based on a MIMO radar, two transceivers for the radio link, and a transponder. The transceivers and the transponder are essentially two antennas and an amplifier system. The equipment is experimentally tested in controlled scenarios and in the case study of Indiano Bridge, Florence, Italy.

This paper is focused on the issue of measuring both dielectric and magnetic characteristics of a penetrable material at radio frequency through a time-domain reflectometry probe. The goal is to prove that a time-domain reflectometry probe can grant the possibility to discriminate the dielectric permittivity from the magnetic permeability of the material. This is due to the fact that the time-domain reflectometry datum depends both on the propagation velocity of the electromagnetic waves in the probed medium and on the intrinsic impedance of the probe. However, the possibility to attain such a clear discrimination is bound by the condition that the reflection coefficient is measured (or calculated) along the probe at the air–soil interface in the frequency domain. Generally, time-domain reflectometry probes measure the total (incident plus reflected) field in the time domain, and subsequently, the datum is needed to meet the condition that such claimed purpose is not just the Fourier transform of a datum collected by means of a common time-domain reflectometry probe. Rather, either a devoted hardware should be implemented or a very accurate knowledge of the incident field should be guaranteed in order to separate the reflected wave from the incident one. In this preliminary work, our survey has been restricted to a theoretical investigation in the frequency domain. In particular, our focus is set on the lossless case, and the attention is devoted to the issue of possible multiple solutions to demonstrate that this obstacle can be overcome by making the frequency step narrower or, alternatively, by narrowing the length step of the probe. Simulation results based on a bifilar transmission line model are shown.

ArcSAR is a ground-based synthetic aperture radar (GBSAR) that has recently been receiving increasing interest in the scientific literature. While the conventional GBSAR exploits the movement of an antenna along a linear rail to synthesize a large aperture, an ArcSAR exploits the spatial diversity of the data acquired by an antenna fixed to a rotating arm. The great advantage of ArcSAR is its capability to synthesize images at 360° with a constant resolution in azimuth. In this paper, the authors propose and test a new focusing algorithm that does not require to operate in the far field and neither with narrow beam antennas; moreover, it is flexible enough to focus on any plane (not necessarily on the rotation plane) as well as in the whole 3-D space. Furthermore, the authors demonstrate theoretically and experimentally that ArcSAR images can be affected by a “defocusing effect” of the targets far from the rotation plane, which has to be taken into consideration when designing such radars.

In this paper a no-contact high-frequency large-bandwidth ground penetrating radar (GPR) scanner specially designed for investigating floors is presented. It operates at 10 GHz central frequency with 4 GHz bandwidth. Its mechanical positioner is able to scan a surface 0.3 m wide and 1.9 m large. The aim of this radar is to gather information about the shallow layers (coat, screed, embedded heating or water pipes) up to 0.5 m depth. The radar has been tested in laboratory and in a real case study: the second floor of the Bellariva sport Centre in Florence (Italy). This floor is under investigation because an evident long crack.

Real-aperture radar are commonly used for remote monitoring/testing of bridges. They are fast, accurate and reliable, but they cannot provide two-dimensional images of the bridge under test. Interferometric multiple-input multiple-output (MIMO) radar is an emerging technology that promises to give good-quality two-dimensional interferometric images of buildings and bridges. During the static test of the Vespucci bridge in the city of Florence, Italy, a X-band MIMO radar has been operated simultaneously with a real-aperture interferometric Ku-band radar. The experimental results obtained by the two different radar have been successfully compared.

Multiple Input Multiple Output (MIMO) radar allows to reduce the number of antennas maintaining performances comparable to a phased array radar. The Compressive Sensing (CS) approach is a processing technique that promises to reduce further the number of antennas. The strict control of the unwanted phase shift and of the position on each antenna is a critical condition in any MIMO array for radar application. So an open question is if CS approach makes the MIMO more o less sensitive to possible phase errors and positioning error. The aim of this paper is just a compared analysis of the effects of phase/positioning error for conventional MIMO radar (with dense pattern of antennas) and CS-MIMO (with random pattern of antennas). The results of the simulations show that both MIMOs are rather sensible to phase error and positioning error along $y$ (the direction in front of the radar), but rather robust to positioning error along $x$ (the direction of the synthetic aperture) and $z$ practically. Furthermore, C-MIMO seems a bit more robust to phase error then dense MIMO.

Step-frequency continuous-wave (SFCW) modulation can have a role in the detection of small unmanned aerial vehicles (UAV) at short range (less than 1&ndash, 2 km). In this paper, the theory of SFCW range detection is reviewed, and a specific method for correcting the possible range shift due to the Doppler effect is devised. The proposed method was tested in a controlled experimental set-up, where a free-falling target (i.e., a corner reflector) was correctly detected by an SFCW radar. This method was finally applied in field for short-range detection of a small UAV.

Summary The city of Florence, Italy has a modern and continuously updated underground map archive. After the construction of a new parking area in a terrain of the University of Florence, the area has been mapped using a high performance Step Frequency Continuous Wave Ground Penetrating Radar. The images acquired by the GPR have been compared with the official cartography. We noted important features not reported in the map. This confirm the necessity of GPR survey also in city provided with modern and updated underground cartography.

Summary This paper presents the results of a Ground Penetrating Radar survey on a historical building taking part of the “Bellariva” sport Centre, designed and built in Florence (Italy) in 1957-1960 years by the Word-famous Italian engineer Pier Luigi Nervi. The building under survey hosts the locker rooms of the swimming pool and a bar at the first floor, a restaurant at the second where an evident crack was observed. A Ground Penetrating Radar survey was performed in order to determinate the internal structure of the floor, dimensions and disposition of steel bars, and to gather information about the connections between perimeter beams and balcony at the level of the restaurant. The survey confirmed what were reported on the original drawings of the building. The results of the survey will be used for defining a numerical model and to design possible retrofit interventions.

A small TEM horn antenna for ground penetrating radar (GPR) applications is proposed in this paper. TEM horn antennas are appropriate for GPR applications due to their wide bandwidth, high gain, and good efficiency. The operative band of a GPR antenna is a trade-off between the penetration depth and desirable resolution. At low frequency, radar has more penetration depth but the possible problem is the big size of antenna. So the aim of this study is to design a small size horn antenna with low cut-off frequency. The band is extended to the lower frequency with three major changes with respect to a standard horn: 1) using the TEM double-ridged waveguide with a cylindrically shaped cavity as transition section; 2) putting absorbing material in the back plate of the antenna; 3) using a semi-circle of radius 5 mm set on the top of the flare end section. The proposed antenna has a small size of $(311 \ \mathbf{mm} \times 265 \ \mathbf{mm} \times 385 \ \mathbf{mm})$ , and the simulated results demonstrated good impedance matching from 100 MHz to 2.5 GHz under a return lass $(\mathbf{S}_{11} \leq-10\ \mathbf{dB})$ , and good radiation pattern. The designed TEM horn antenna is modeled in free space and in contact with the ground. The capability of the antenna for detecting buried objects is confirmed with simulation.

The aim of this paper is a direct comparison between: 1) a double-ridged horn antenna; 2) the same doubleridged horn filled with sawdust; 3) a bow-tie antenna operating in (approximately) the same band (0.5 GHz 1.5 GHz). The three antennas have been simulated and experimentally tested. The best performances in terms of capability to detect a target behind a 40 cm wall have been obtained with the horn antenna filled with sawdust.

In this paper an improved free-space technique for measuring the complex dielectric constant of materials in grains or powder (like the sawdust) is proposed. While the known free-space techniques rely on strong assumptions that are quite unrealistic in real experimental conditions, the proposed improved free-space technique is able to take under control these assumptions using a suitable calibration curve.

In this paper the authors propose a novel Ground-based Synthetic Aperture Radar (GBSAR) system able to acquire both monostatic and bistatic images. Monostatic images are acquired in a few minutes but they provide only 2D information about the targets in its field of view. Bistatic images require several hours, but they give 3D information. This 3D imaging capability can be of interest when the radar equipment is used to image complex scenarios, where the altitude (z-axis) is not an unambiguous function of the (x,y) position. The letter reports the working principle and an experimental test of the equipment.

In this paper a no-contact ground penetrating radar for investigating painted walls is presented. It operates at 10 GHz central frequency with 4 GHz bandwidth. Its mechanical positioner is able to scan a surface 1.4 m wide and 1.9 m high. This equipment has been specifically designed for investigating the painted walls of the Tutankhamon tomb in the Kings' valley in Egypt. The aim of this radar survey is to gather information about the shallow layers (the plaster and the existence of possible plaster voids affecting the future stability of paintings) up to 0.5 m depth.

Ground-based synthetic aperture radar (GBSAR) systems are popular remote sensing instruments for detecting the ground changes of landslides, glaciers, and open pits as well as for detecting small displacements of large structures, such as bridges and dams. Recently (2017), a novel mono/bistatic GBSAR configuration was proposed to acquire two different components of displacement of the targets in the field of view. This bistatic configuration relies on a transponder that consists&mdash, in its basic implementation&mdash, of just two antennas and an amplifier. The aim of this article was to design and experimentally test an improved transponder with cross-polarized antennas and frequency shifter that is able to prevent possible oscillations even at very high gain, as required in long-range applications. The transponder was successfully field-tested, and its measured gain was 91 dB gain.

GB-InSARs (Ground-Based Interferometric Synthetic Aperture Radar) have been demonstrated able to detect changes and even the target elevation when they are provided with a mechanical system for rising the radar head. In this paper a novel acquisition mode is proposed that uses a GB-InSAR with a transponder for obtaining the target elevation, without the need to move the radar head.

No-contact Ground Penetrating Radars (GPRs) are popular microwave sensors for investigating soils or masonry/stone walls. In this paper the authors evaluated the compressive sensing (CS) as possible technique for speeding up the acquisition time of this kind of application. In effect the CS approach could reduce the number of acquisition points, and then the measurement time by using only a random pattern of the antennas positions. The authors found that the data reconstruction loses quality even with a reduction of 25 % of the number of acquisitions, but the features of the targets still visible. With a reduction of 50 % the SNR decrease sensibly and most of the targets are not detectable. Such a time reduction results rather marginal in most practical cases.

Since the number of flying small unmanned aerial vehicles (UAV) is expected to increase dramatically in the next years, their radar detection has become a priority. In this paper Continuous Wave Step Frequency (CWSF) is evaluated as possible modulation for radar sensors aimed to detect small quadcopters at short range.

ArcSAR is a Synthetic Aperture Radar which recently has been receiving increasing interest in scientific literature. It operates exploiting the spatial diversity of the data acquired by an antenna fixed to a rotating arm. Its great advantage is its capability to synthesise images at 360° with constant resolution in azimuth. The ArcSAR, in addition to operate as interferometric differential radar, can detect the elevation of the targets, i.e. it is potentially able to generate digital elevation models of the surrounding field of view.

In this paper Inverse Synthetic Aperture Radar (ISAR) technique has been applied for obtaining 2D and 3D radar images of a small quadcopter. The aim is to prove the capability of radar technology for 3D imaging small drone targets.

Many different antennas are proposed and tested for GPR, but bow-tie or horn antennas are doubtless the most used. Their simple and robust design makes them effective in most applications. The aim of this paper is a direct comparison between a horn and a bow-tie antenna, used in the same operative conditions, i.e. in the same band, with the same transceiver and on the same ground. Although this is a preliminary study based on a very limited set of experimental data, horn antenna appears to have advantages with respect to bow-tie antenna in GPR applications.

Interferometric radar is popular equipment able to perform remote dynamic testing of large structures (towers, bridges, and buildings) by detecting the displacement in time in the line of s...

Measurement of molten glass level in glass furnaces is a key issue for the control of the production process. Nowadays several different technologies shall be used for this purpose, with their advantages and drawbacks. In this paper a novel level sensor is proposed, it is based on radar interferometry technique, and it is designed to provide high accuracy measurements, high reliability and a very low maintenance.

The paper presents a synergic and multidisciplinary approach where laser scanner survey, radar interferometric monitoring and finite element (FE) numerical modelling are used for expeditious and no-contact dynamic identification of monumental masonry towers. The methodology is applied to a real case of great historical interest: the “Torre del Mangia” (Mangia's tower) in Siena (Italy). The tower geometry was acquired through Terrestrial Laser Scanning (TLS) techniques. The tower oscillations were detected using an interferometric radar in “Piazza del Campo”, the square facing the Mangia's Tower, along three alignments, and movement of the structure at several heights were recorded. A FE model, built on the basis of the geometry acquired through the TLS, was used to interpret and verify the physical meaning of the experimental results. Through the discussion of the case study, the paper shows that the proposed approach can be considered as an effective and expeditious method for assessing the dynamic behavior of monumental buildings (and to plan interventions) on territorial scale.

Ground-based/terrestrial radar interferometry (GBRI) is a scientific topic of increasing interest in recent years. This article is a bibliographic review, as much complete as possible, of the scientific papers/articles published in the last 20 years, since the pioneering works in the nineties. Some statistics are reported here about the number of publications in the years, popularity of applications, operative modalities, operative bands. The aim of this review is also to identify directions and perspectives. In the opinion of authors, this type of radar systems will move forward faster modulations, wider view angle, MIMO (Multiple Input Multiple Output) systems and radar with capability to detect the vector of displacement and not only a single component.

Interferometric radars are sensors able to remotely detect the dynamic displacements of large structures such as bridges and towers, but they have to be positioned on stable ground. There are several applications for which this limitation is critical, so in this paper the authors propose and evaluate in field a possible solution for overcoming this requirement. The basic premise is to install an accelerometer inside the radar head to detect its own movement. This device with suitable data process chain, in the best conditions, is able to mitigate the effects of movements of the radar head above 30 dB. Lower values have been obtained in operative conditions.

In this issue, we host ? for the second time ? a paper on the Italian-American physicist, Enrico Fermi. The f rst paper (IEEE Antennas and Propagation Magazine, 53, 3, June 2011, pp. 226?230) was focused on a commemoration of Guglielmo Marconi by Enrico Fermi. The present paper spans a period of the life of Fermi that is not so well known, which he spent at the University of Florence as an Assistant Professor. He also taught at the School of Engineering, to which the three authors of this paper belong. This period is relevant for electronic engineers. In those Florentine years, Enrico Fermi developed and published the Fermi-Dirac statistics. These are funda mental in semiconductor physics, and therefore in modern electronics.

Microwave displacement sensors are becoming increasingly popular as vital sign detectors and structural vibration detectors. Until now there is not any method known for calibrating them that does not require accessing the electronic board by inserting and probing test signals or does not require cumbersome equipment that is able to provide large linear displacements. Hence, proposed is a calibration method for I-Q correction that makes use only of an external mechanical oscillator (a metallic bar fixed at an edge). As a test device, a low-quality X-band transceiver with a strong imbalance has been used. The calibrated sensor is compared successfully with a high-performance seismic accelerometer.

Interferometric radar is a rather popular equipment able to remotely perform dynamic testing of large structures (towers, bridges, buildings …) by detecting the relative displacement in the line of sight between the radar and geometric discontinuities in the engineering structure under test. This technique has been demonstrated effective when the radar is positioned at ground on a stable position. Unfortunately there are practical cases for which a suitable stable position is not available. This is the case of the towers of San Gimignano (Italy) that are embedded in the narrow streets of the old town. A solution to monitor the towers is to install the radar on the top of a tower, which is located in a central position of the town. The tower is not a stable arrangement for the radar and in this paper the authors propose and test the use of an accelerometer fixed to the radar head for detecting and removing its own movement.

The problem of the measure of both the dielectric permittivity and the magnetic permeability of a material is dealt with making use of a TDR probe. Simulation results are shown making use of a transmission line model in frequency domain.

Most of Ground Penetrating Radars (GPR) are pulsed system, nevertheless there are a number of applications for which Continuous Wave Step Frequency (CWSF) radar can be advantageous. Generally speaking, CWSF systems are more complex then pulse radar, but they can manage larger bandwidth, have larger time efficiency and better control of the band. Furthermore, power, integration time and step of each single frequency can be controlled. In this paper the authors, that work in the CWSF field since several years, are aimed to give some guidelines and a practical example of CWSF designing.

The “Fortezza da Basso” (Lower Fortress) is one of the most significant monument in Florence, Italy. Currently is a prestigious facility for exhibitions and international events. The municipal government intends to built new pavilions, but the Superintendency office of monuments and cultural heritage required a critical critical historic and architectonic study including a GPR survey. In this paper the authors report the results of a GPR measurement campaign carried out with this aim. In particular at “Cavaniglia” bastion the buried structures of the ruined tower have been mapped and a possible arched structure, previously not known, has been detected in the underground.

A microwave interferometer has been employed to remotely detect the oscillations induced by vehicular traffic on the dome of the Baptistery of S. Giovanni in Firenze (Italy). The measurement has been carried out to assess the structure oscillations reduction after the ordinance issued by the Major of Florence forbidding all kind of vehicular traffic in the square around the Baptistery starting from October 25, 2009. A simplified preliminary finite element model of the structure, using average literature mechanical parameters, has been used to determine the best instrument installation position and the structure area to be surveyed. Subsequently, the dome oscillations have been recorded for two consecutive weeks starting one week before the traffic block. A spectral analysis on the acquired data has been then performed and a more complex FEM model has been built to interpret the physical meaning of the acquired data. The material mechanical parameters have been finely tuned to reproduce the lowest spectral component found which was very close to the main mode oscillation frequency predicted by the simplified model. The obtained values of mechanical parameters are reasonable for the monument materials and slightly different from average. Measurement results indicate that the traffic block caused a reduction of the daytime maximum oscillations power of around 50÷60% and a decrease by approximately 33% has been observed in the peak component particle velocity. The residual measured oscillations are attributable to human activities around the square.

The Leaning Tower of Pisa, one of the world-famous architectural marvels of Italian heritage, needs continuous surveying to assess its stability. In this letter, remote-sensing equipment recently developed by the authors, based on the principle of microwave radar interferometry, has been experimented to measure the frequency response of the Tower without requiring any contact with its structure. Wind and human traffic were used as natural excitation sources, allowing the natural frequencies of the first vibration mode of the Tower to be measured in the north-south and in the west-east directions. Modal shapes of the Tower vibrations were also obtained from data acquired by the radar.

Interferometric radar has been recently proposed as a measurement instrument for static and dynamic testing of large structures, such as bridges, towers, dams. At the state of the art, the equipment has been demonstrated able to follow the displacements of a bridge provided that vertical bending movements are predominant with respect to torsional ones. In this paper, the authors propose and experimentally test an advancement of the interferometric technique able to overcome the previous assumption and to retrieve the effective displacement of each single section of a bridge in case of both vertical bending and torsional movements.

Radar interferometry, capable of remote detection of displacements and deformations of a structure, has been successfully experimented in a number of case studies. Radar interferometry offers high-speed range imaging and range displacement measurement capability, but it lacks in the identification of the different kinds of modes usually excited in a dynamic test. In this paper an approach to the measurement capable of identifying bending and torsional oscillation modes is described. The basic idea rely on the application of suitable loads to the structure under test in order to cause controlled changes of its frequency response. The results of such a dynamic test, performed on a canopy, are reported.