Signal decorrelation represents a challenging issue for interferometric application of Ground-Based Synthetic Aperture Radar (GBSAR) systems. The behavior of the signal autocorrelation has been widely studied in literature for satellite SAR, for which it decays exponentially in time. Recently, a double exponential decorrelation model has been proposed to account for GBSAR systems. Although be confirmed by experiments, this behavior is far from been completely understood. This work aims to deepen this subject, through experimental studies performed on both natural and controlled scenarios, in order to shed light and get insights on the nature of the fast decorrelation. To this aim, GBSAR data acquired in different frequency bands have been analyzed, together with data acquired with a fast interferometric radar not subject to movements along a rail. From the analyzed data, we can argue that the fast exponential decorrelation is an intrinsic characteristic of interferometric radar measurements.

The performance of Ground-Based Synthetic Aperture Radar (GBSAR) systems significantly varies with the operative frequency band. For instance the signal coherence, which provides important information on the radar signal quality, drastically changes with the frequency band. Aim of this study is to deepen the relation between the coherence of GBSAR signals and atmospheric parameters, in different frequency bands, on environmental scenarios. In this work we show results obtained from the analysis of GBSAR data in three different frequency bands, S, C, and Ku, acquired throughout a one-year experimental campaign performed on a vegetated tailings dam. The extensive data set acquired allowed us to extract useful information on the trend of coherence, as the signal frequency and the environmental parameters varied.

Ground-based synthetic aperture radar (GBSAR) systems are widely used for monitoring slopes, especially in quarries and open pits. Unfortunately, the movement of the machinery (cranes, trucks, shovels, and excavators) in the radar field of view often prevents to correctly focus large circular sections of the radar images, producing a characteristic artifact, often named “blurring.” Therefore, the aim of this article is to propose both an acquisition modality and an image processing technique able to mitigate the blurring due to moving clutter. The proposed acquisition modality (named “free-running” or “on-the-fly”) allows to filter out the high-frequency clutter, which is often noticeable in GBSAR images, but it is not effective with low-frequency clutter caused by slow movement of machinery like heavy trucks or large cranes. For this reason, the authors propose even a suitable processing method that can be used in combination with high-frequency sampling. The application of these two methods has been proved to be very effective in realistic quarry scenario.

Ground penetrating radar (GPR) systems are equipment able to acquire underground images scanning the surface of the soil/pavement under investigation. Usually GPR records its own position along the scan line using a mechanical odometer, i.e., a rolling wheel in contact with the ground. Unfortunately, this simple and cheap solution can be not effective on uneven terrains. In this article, a completely different solution is proposed for retrieving the radar position along the scan. An additional couple of transmitting/receiving (TX/RX) antennas detects the distance of one or two corner reflectors (CRs). As the signal backscattered by the CR appears in GPR trace (with a suitable delay for separating air and ground signals), the position data are co-registered in the radar trace itself and no external synchronization is necessary. The technique has been successfully tested both for detecting the position of the radar along a line (1-D case) and on a surface (2-D case).

When a ground-based interferometric radar is monitoring a slope on the other side of a lake (or a see loch), water level changes can give apparent displacements on the slope because of possible multipath effects. It this paper a polarimetric method is proposed for suppressing (or at least mitigating) these apparent displacements. The method has been developed on the basis of a suitable theoretical model that takes into account the particular measurement geometry in this kind of application. Finally, the method has been tested in an experimental facility especially built with this purpose.

Interferometric radars are widely used for static and dynamic displacement monitoring of large structures as bridges, culverts, wind turbine towers, chimneys, masonry towers, stays cables, buildings, and monuments. The most of these radars operates in Ku-band (17 GHz). Nevertheless, a higher operative frequency could allow to design smaller and lighter equipment. The aim of this paper is to verify if a commercial 77 GHz radar designed for automotive applications could be used as interferometric radar for static and dynamic monitoring of large structures. The preliminary results obtained in controlled conditions are very promising.

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.

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.

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.

Ground-based synthetic aperture radar (GBSAR) systems are popular remote sensing instruments for detecting ground changes of slopes, and small displacements of large structures as bridges, dams, and construction works. These radars are able to provide maps of displacement along range direction only. In this letter, we propose to use a transponder for operating a conventional linear GBSAR as a bistatic radar with the aim to acquire two different components of the displacement of the targets in the field of view.

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.

A novel ground based multi bistatic interferometric radar sensor has been realized in order to measure the three dimensional displacement vector of civil structures as buildings, bridges and towers. Data processing techniques were developed to determine the position of each target and to calculate three dimensional displacement starting from mono dimensional displacement measured from three different positions in space. Experimental tests were finally carried out in controlled environment in order to validate the proposed technique and to assess the accuracy of displacement's measurement.

Ground Penetrating Radars are a unique and well known tool for soil investigation. In order to improve its penetration depth and/or its resolution limits, in the past several radar designs, based on the so called Continuous Wave-Stepped Frequency technique have been proposed. However, the resulting design was often a quite complex equipment. The radar system prototype proposed in this paper, while exhibits state of the art performances with a contained hardware size, it explores a new horizon applicable only to the CW-SF radar technique: the radar reconfigurability (GPR-R).

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.

This work reports on the results obtained with a high speed ground based radar interferometer applied to the monitoring of the explosive activity of Stromboli volcano, Italy. The sensor illuminated a few craters below the summit distinguishing among them according to their distances from the sensor. The sampling rate allowed tracking the craters' movements even while they were erupting providing information about both the area affected from the deformation and its extent. Radar data were compared with seismic signals from the permanent monitoring network. The comparison demonstrated that the radar interferometer is a powerful device for retrieving ground deformation before and after an explosive event. In particular, eruptions were clearly identified from the observation of peculiar characteristics of the amplitude, phase and coherence data.

In this paper, we report on an experimental activity aimed at investigating the potential of two terrestrial remote-sensing techniques, namely, ground-based SAR (GB SAR) interferometry and terrestrial laser scanning, in order to retrieve snow-depth (SD) measurements in mountainous regions. Terrestrial laser scanning is a more consolidated technique based on the measurement of the optical (near infrared) reflectivity, and it is affected by the surface of the snow layer: a temporal data sequence allows us to estimate the absolute SD variation. Recent use of SAR interferometry to evaluate snow-mass characteristics is based on relating the measured interferometric phase shift to a change in the snow mass. Interferometric GB SAR measurements and terrestrial laser scanner scans were collected together with pointwise conventional measurements of physical snow parameters during the winters of 2005/2006 and 2006/2007. The experiment was carried out in the Wattener Lizum, a high Alpine area at about 2000-m elevation north of the main ridge of the Austrian Alps in Tyrol. Notwithstanding the difficulty of providing both lengthy data record in dry snow conditions and detailed knowledge of the observed snow characteristics, the obtained results confirmed the presence of a clearly measurable interferometric phase variation in relation to the growing height of the snow layer. A comparison of the SD maps obtained through the two techniques shows differences partly due to the different nature of the two observations.

Belvedere Glacier, east face of the Mount Rosa, has experienced drastic changes in flow regime and morphology in the last years. Within the activities of the European project GALAHAD a Ground Based SAR (GB-SAR) interferometer was employed for the first time on this glacier. In the present work it is reported how the sensor capability of generating digital elevation models (DEMs) was exploited in order to retrieve information about the occurred surface changes. In particular, in this case study, the DEM obtained by the radar on summer 2007 was compared with two previous DEMs from other sources: one relative to summer 2005 and the other to summer 1995. Significant differences in the ice surface height have appeared from the comparison reflecting both the extraordinary surge type movement occurred in 2003-2004 and afterwards the overall reduction of the ice mass.

The ORFEUS (Optimised Radar for Finding Every Utility in the Street) project addresses the requirement for advanced technologies for locating, maintaining and rehabilitating buried infrastructures and, in particular, it fulfils the requirement for locating buried assets, including the use of trenchless techniques for deploying pipes and cables in an urban environment. The aim of this project is to develop two radars. The first operates on the surface for planning deployment activities; the second (a bore-head GPR) will be installed in the drilling head of horizontal directional drilling (HDD) equipment so that they may avoid collision and possible damage to existing cables and other infrastructure.

The potential of a continuous-wave microwave transceiver as a tool for detecting breathing and heartbeat of people buried in snow has been experimentally evaluated. The breathing has been clearly detected through a 1.8-m-thick snow barrier as well as through the 1.2-m-thick roof of an igloo dugout to simulate the experimental conditions of a human being trapped under an avalanche.

In this paper, the algorithms developed for satellite synthetic aperture radar (SAR) interferometry were adapted to the ground-based SAR (GB-SAR) configuration and used for detecting the displacements of an alpine landslide which have occurred over many years. Indeed GB-SAR interferometry is based on the same principles as satellite SAR techniques but benefits from the GB-SAR's versatility and capability of gathering many images per day. In monitoring applications of landslides moving only few centimeters per year, as the case here reported, the GB-SAR sensor is installed at repeated intervals several months apart over the observation period. Although the revisiting time is very similar to the satellite one, for each survey, lasting two or three days, more than ten images are available. They are analyzed separately and in combination with images from other surveys for coherent pixel selection. Interferograms are formed by cross-combining images from different surveys. Finally, the evolution of the deformation across the surveys is retrieved in a least square sense without any assumptions on its regularity. The used GB-SAR technique is described in detail in this paper, and the results obtained with regard to a landslide in the Italian Alps that has been monitored over a period of about three years are discussed.

This paper reports on some results obtained during an experimental campaign carried out for monitoring a snow covered alpine mountainside and based on the use of a ground based interferometer working at C band. The effect of the interaction between radar signal and a dry snow cover on interferometric phase has been investigated on the bases of a simple model, representing the snow cover as a homogeneous lossless dielectric layer over the soil. Measured differential phases have been converted into snow depth estimations and compared to ground truth data. Coherence behaviour has been also investigated to evaluate the effectiveness of the proposed technique. As previously documented from other researchers, the potential of the technique has been demonstrated but limits to its applicability were found to be analysed thoroughly.

The Belvedere Glacier, east face of Monte Rosa, has been investigated for many decades because of a long history of outburst threatening the village of Macugnaga and, in the latest years, a surge-type evolution, responsible for the formation of a large depression at the foot of the east wall of Monte Rosa, which hosted a supraglacial lake named “Lago Effimero” in summer 2002. In this contribution, a Ground Based SAR (GB-SAR) interferometer was employed for determining the ice-flow velocity of the illuminated part of the Belvedere Glacier in order to enhance the understanding of ice flow mechanics and hopefully reduce the glacier hazard. The deformation field and the velocities measured by the GB-SAR sensor have been validated with data by more conventional ground-based measurements.

Spaceborne differential synthetic aperture radar (SAR) interferometry has been proven to be a powerful tool in monitoring environmental phenomena and, in particular, in observing glaciers and retrieving information about their surface topography and dynamics. In the last decade, the use of this technique has been successfully extended from space to ground-based observations as a tool for monitoring, on a smaller scale, single landslides, unstable slopes, and more recently, areas covered by snow but not yet glaciers. In this letter, the results of an experimental activity carried out to evaluate the potential of ground-based microwave interferometry to estimate the velocity of an unstable area belonging to a glacier is reported. This experiment demonstrated the possibility of remotely monitoring surface displacements of the monitored glacier up to a distance of about 3 km even if, due to the lack of ground truths on the observed area, the data interpretation must be carefully worked out.

In this letter, a ground-based synthetic aperture radar (SAR) interferometer was used to generate digital elevation maps (DEMs) of the illuminated area. With respect to other ground-based data processing techniques, here, the effect of the propagation through the atmosphere is considered. An algorithm similar to multipass satellite SAR techniques was developed in accordance with the phase model used in the ground-based interferometry. Many images taken from different viewing angles were collected and combined to form different interferograms at a test site in Austria. Results from this technique have been compared with an existing geographic model of the test area.

LAVORO SVILUPPATO NELL'AMBITO DEL PROGETTO EUROPEO GALAHAD (Advanced Remote Monitoring Techniques for Glaciers, Avalanches and Landslides Hazard Mitigation)

Remote dynamic monitoring of bridges by a high-speed interferometric radar is proposed. The equipment is a continuous-wave step-frequency radar with very fast frequency hopping that is capable of sampling the structure at a rate high enough for transient analysis of motion through phase comparison of successively acquired images. An experimental test carried out on a highway bridge forced by vehicular traffic is presented

The alpine village of Lamosano, Belluno, Italy, located near a wide and active landslide, has been seriously threatened by ground instabilities since 1960. In this paper, the results obtained by two different remote-monitoring techniques, synthetic aperature radar interferometry and three-dimensional laser-scanner imaging, planned for Lamosano hazard assessment, are presented. Both techniques compare images taken at different times to map and classify changes that occurred on the imaged scenario. The radar and laser data are gathered at the same dates with about ten-month temporal separation. The displacements measured separately by each of the two techniques highlight a similar sliding motion on the Lamosano village area, providing a good validation to each other and contributing to the definition of the village instability hazard.

The authors propose the use of a high-speed interferometric radar for remotely measuring both transient displacements and steady-state vibrations of architectural heritage structures in order to test their stability conditions. Demonstrative results of application of the technique to a prominent cultural heritage artwork, the tower of Giotto in Florence, Italy, are reported.

Ground-based synthetic aperture radar (GB-SAR) interferometry has already been recognized as a powerful tool, complementary or alternative to spaceborne SAR interferometry, for terrain monitoring, and for detecting structural changes in buildings. It has been noted that, in spite of the very short range, compared with the satellite configuration, in GB-SAR measurement the disturbances due to atmospheric effects cannot be neglected either. The analysis of the interferometric phases of very coherent points, called permanent scatterers (PSs), allows the evaluation of the atmospheric disturbance and the possibility of removing it. In this paper, the PS analysis is carried out both on a test site facility and on a real campaign (Citrin Valley, Italy) that provided data with a temporal baseline of about ten months.

The application of ground-based radar interferometry for landslide monitoring is analyzed: a case study based on an experimental campaign carried out in Italy during 2002 is discussed. Interferometric data obtained from coherent synthetic aperture radar (SAR) images acquired by means of C-band ground-based equipment are analyzed. The campaign was aimed at retrieving potential terrain movements of a small landslide observed hundreds of meters away. Critical aspects related to spatial and temporal decorrelation are discussed: the use of optical photogrammetry as a technique for evaluating mechanical stability and correcting geometric distortion is presented. Results also confirmed that the application of ground-based radar interferometry can be attractive and effective if the acquired SAR images maintain an adequate coherence on different dates.

Joint time-frequency analysis of penetrating radar response has been tested using simulated and experimental data as a processing tool for detecting stratified features inside a masonry structure. The radar response has been simulated exploiting the formalism of the propagators inside a finite number of dielectric layers. Experimental testing has been carried out by varying the thickness of the hollow space between two walls.

A Continuous Wave Step Frequency Radar system, working at high frequencies, is suggested as a tool for intra-wall inspection. A processing technique based on Joint Time Frequency Analysis (JTFA) is applied to simulated and experimental data.

In this paper, the authors describe an innovative survey radar technique, based on microwave holographic images, for dynamic testing of large structures providing both vibration amplitude pattern and frequency. Theoretical background is provided and experimental results obtained during a dynamic test on a concrete and masonry building are reported.

3D optical scanning technologies for measuring tridimensional shapes of sculptures have emerged in the last few years as the most promising methods for digital analysis and preservation of cultural heritage. This paper presents the work leading to the generation of the digital 3D model of Donatello's Maddalena, kept in the museum of the Opera del Duomo in Florence. The acquisitions have been taken with a system based on fringe projection. The metric reliability of Maddalena's tridimensional model, obtained through iterative alignments of single 3D acquisitions, has been also verified through a well established technique such as photogrammetry. A set of point-to-point distances evaluated with both methods has been compared and discussed in order to estimate the overall accuracy of the final model.

The authors propose a ground-based interferometric synthetic aperture radar (SAR) technique for terrain mapping. It is based on a coherent continuous-wave step-frequency (CW-SF) radar moved along a linear horizontal rail. It works synthesising microwave holographic images taken from different points of view to obtain elevation maps by phase comparison. The focusing algorithm for imaging synthetic holograms and digital elevation models (DEM) is able to correct topographic distortion and phase wrap through an iterative multi-baseline procedure.