This paper presents a new analytical approximation for determining the refraction point at the air-soil interface in groundpenetrating radar (GPR) imaging. In contrast to the previously widely used approximations by Johansson-Mast and Zhou-Huang-Su, the proposed formula is derived from a local linearization of the exact wave propagation geometry, achieving significantly higher accuracy. Numerical evaluations indicate that the associated travel-time error consistently remains below 0.1 ns, making the approach suitable for direct integration into high-resolution back-projection imaging algorithms.

Calorimetry is the standard measurement method for monitoring the hydration of cementitious materials and retrieving their hydration degree. However, it is destructive and impractical for in situ measurements. Electromagnetic measurements are sensitive to the changes in the porosity and water content of materials, making dielectric techniques good candidates for characterizing the hydration of cementitious materials. Besides, they can be implemented in a noninvasive way by means of microwave reflectometry sensors. In this article, the hydration of a cement mortar is monitored using both calorimetry and microwave reflectometry at 800 MHz, over a 140 h period covering its hydration process. The measured calorimetry and dielectric data prove to be consistent with each other as well as with the phenomena at stake along hydration. Besides, the combination of this data is carried out to obtain the variations of the mortar dielectric permittivity as a function of the hydration degree. On the other hand, we develop a dielectric model for monitoring the mortar hydration degree. The latter, which is based on mixing equations, accounts for the age-dependent mortar dielectric properties due to the hydration process and porosity decrease experienced by cementitious materials, especially at an early age. This model is the first mechanistic approach to describe the dielectric properties of mortar during hydration. Thus, in order to retrieve the hydration degree, the used model implements mean-field homogenization based on the dielectric features of the mortar constituents. The combination of calorimetry and dielectric experimental data is compared to the dielectric model of hydration, showing good agreement.

The interpretation of dielectric measurements in cement-based materials, as well as multiscale modeling strategies, requires the knowledge of the intrinsic permittivity of their various constituent phases. Calcium silicate hydrates (C-S-H) is the major hydrated phase in concrete (when clinker is the main cement compound), but to date, its frequency-dependent complex dielectric response remains unknown. Direct experimental measurements of C-S-H intrinsic dielectric behavior are challenging due to the scales to be probed and the difficulties in isolating this component in cement systems. Molecular simulations arise as a helpful tool to provide reliable estimates of properties bottom-up. This study adopts a multiscale approach to estimate the complex dielectric response of C-S-H over a frequency range of [0; 100 GHz]. We perform molecular dynamics simulations to compute the frequency-dependent dielectric response of water in C-S-H using theoretical framework of Statistical Physics, which enables us to associate the microscopic decay in water polarization correlations to the dielectric response. Several configurations are considered by varying the interlayer distance, covering the range of pore sizes associated with interlayer pores and gel pores in C-S-H. The dielectric response is anisotropic and pore size dependent, as expected in layered materials. The results at the molecular scale are then used as inputs in a homogenization model to estimate the dielectric permittivity of C-S-H gel, which we compare with estimations obtained from inverse analysis based on Micromechanics. Our results are a valuable input for multiscale modeling of non-destructive testing and evaluation in cement-based materials.

As a porous hydrophilic phase, C–S–H is expected to exhibit a frequency-dependent dielectric response. This C–S–H property remains unknown, despite the importance of dielectric assessment for non-destructive evaluation of concrete. This study adopts a multiscale approach to estimate the complex dielectric response of C–S–H over the 0 to 1000 GHz frequency range. We perform molecular dynamics simulations to compute the frequency-dependent dielectric response of water in C–S–H from the polarization correlation formula. We consider pore sizes covering the range associated with interlayer and gel pores in C–S–H. The dielectric response obtained is anisotropic and pore size dependent, as expected in layered materials. The results at the molecular scale are then used as inputs in a homogenization model to estimate the dielectric permittivity of C–S–H gel, which we compare with estimations obtained from inverse analysis based on experiments. Our results are a valuable input for multiscale modeling of non-destructive testing and evaluation in cement-based materials.

In this paper, we propose a microwave electromagnetic approach with a view to characterize the moisture in bio-sourced concretes. A rectangular waveguide cell is used for measuring the transmission and reflection of microwaves through hemp concrete samples and the inversion of a suitable electromagnetic propagation model is implemented to derive the dielectric permittivity of the samples from the measurements. Hemp concrete samples featuring relative humidities ranging from 50% to 98% are manufactured. Their dielectric spectrum is characterized over the 1.72-2.61 GHz frequency band currently used in wireless local area networks.

When using multichannel synthetic aperture radar (MSAR) to perform moving target detection, the coherence between the received signals is closely linked to the detection capacities of the system. In airborne MSAR context, because of the platform movement, the antenna patterns, as well as the phase shifts between the channels, evolve in different ways during the integration time. In this paper, these dissimilarities are shown to cause a degradation of coherence, and hence of detection capacities. A method to enhance the coherence is then developed. This method is based on the estimation of the steering vector. It is applied on two sets of real data, and the obtained results validate the method.

When using STAP on (range, Doppler) high resolution SAR images to detect moving targets [1], strong scatterers (corner reflectors, double bounces) tend to be detected as moving targets. In this paper, a method to discriminate strong scatterers from an actual moving target is presented. It uses the fact that on a SAR image, a moving target appears at a wrong position (shifted in azimuth). A consequence of this azimuth shift is that the covariance matrix of the moving target is not proportional to the clutter covariance matrix, as it would be the case for the covariance matrix of a strong scatterer. Hence, by testing the proportionality between the detected targets covariance matrix and the clutter covariance matrix, it is possible to discriminate the moving targets from still targets. The dependency of the test on the target parameters (SNR, velocity) is studied through simulations. The test is then applied on real data.

In this paper, the problem of estimating complex-valued proportional covariance matrices is addressed. The obtained estimate generalizes the fixed point estimate to scaled packets of data, and is hence called the generalized fixed point estimate (GFPE). The statistical properties (bias, consistency, and asymptotical distributions) of the estimate are presented, and verified through simulations. As an example of application, a radar detection problem is considered. The GFPE is used in the well-known adaptive normalized matched filter (ANMF), and the obtained empirical probability of false alarms threshold curve is compared to the theoretical one. The ANMF is also used to compare the GFPE to the fixed point estimate.

We report a method relying on a dielectric spectroscopy laboratory experimental setup and on a suitable electromagnetic model for the broadband characterization of compacted partly saturated bentonite under controlled boundary conditions. This work is a step in the development of a non destructive method with a view to the monitoring of the hydromechanical properties of clay materials such as bentonites used in civil engineering works such as nuclear waste repository closures.

In the context of ground surveillance, moving targets detection and localization are key issues. Recently, SAR/GMTI algorithms involving several antennas have been proposed. However, the antennas may have different antenna patterns, and they point to a direction with slightly different angles. These difference evolve dissimilarly as the aircraft moves along the azimuth direction, causing a loss of coherence between the obtained images. This loss of coherence causes a loss in detection capacities. In this paper, we propose a method of recalibrating the antennas with respect to one another. This method does not require any a priori knowledge on the antennas, and enables an improvement of the detection capacities.

Broadband electromagnetic frequency or time domain sensor techniques present high potential for quantitative water content monitoring in porous media. Prior to in situ application, the impact of the relationship between the broadband electromagnetic properties of the porous material (clay-rock) and the water content on the frequency or time domain sensor response is required. For this purpose, dielectric properties of intact clay rock samples experimental determined in the frequency range from 1 MHz to 10 GHz were used as input data in 3-D numerical frequency domain finite element field calculations to model the one port broadband frequency or time domain transfer function for a three rods based sensor embedded in the clay-rock. The sensor response in terms of the reflection factor was analyzed in time domain with classical travel time analysis in combination with an empirical model according to Topp equation, as well as the theoretical Lichtenecker and Rother model (LRM) to estimate the volumetric water content. The mixture equation considering the appropriate porosity of the investigated material provide a practical and efficient approach for water content estimation based on classical travel time analysis with the onset-method. The inflection method is not recommended for water content estimation in electrical dispersive and absorptive material. Moreover, the results clearly indicate that effects due to coupling of the sensor to the material cannot be neglected. Coupling problems caused by an air gap lead to dramatic effects on water content estimation, even for submillimeter gaps. Thus, the quantitative determination of the in situ water content requires careful sensor installation in order to reach a perfect probe clay rock coupling.

This paper proposes an algorithm for estimating the fractal dimension of real sea bistatic synthetic aperture radar data. The algorithm is based on the use of the fractal dimension estimated by the box counting method to detect the sub-data which contain the targets. Based on this approach, the performance of the fractal detector proposed in this paper was demonstrated by the evaluation of the probability of detection by means of Monte Carlo simulation and was verified that we can use the fractal dimension to distinguish between targets and clutter, even for a small signal-to-noise ratio.

Detecting and locating moving targets are key issues for radar ground surveillance. Because of its attractive resolution, the use of SAR imaging for this purpose is of prime interest. However, SAR was designed to image non-moving targets, and thus cannot be used as it stands. Some research has been carried to develop simultaneous SAR/STAP processing, because of STAP capacity to detect moving targets. In this paper, a new algorithm for detecting moving targets and for estimating their true location and their velocity is presented. Results obtained on real data are also presented.

In this paper, the problem of proportional covariance matrices estimation for random Gaussian complex vectors is investigated. The maximum likelihood estimates of the matrix and the scale factors are derived, and their statistical performances are studied, through bias, consistency and asymptotic distribution. It is also shown that the problem treated here generalizes the covariance estimation problem for Spherically Invariant Random Vector (SIRV). An iterative estimation algorithm is proposed. A simulation based on a detection problem is presented. The results suggest that the asymptotic distribution obtained is a really good approximation, even for a small number of data.

The use of electromagnetic sensors such as Time Domain Reflectometry (TDR) probes has gained increasing importance for long term monitoring of the water content in radioactive waste repositories. TDR probes are sensitive to changes in electromagnetic properties of the surrounding material, a clay rock in our case. Prior to the in situ application, it is mandatory to have an accurate relationship between the electromagnetic properties of the intact host clay rock and the water content. For this purpose, the dielectric properties of intact clay rock samples were systematically studied at frequencies from 1 MHz to 10 GHz with network analyzer technique in combination with coaxial transmission line cells. Samples were conditioned to achieve a water saturation range from 16 % to nearly saturation. The relaxation behavior was quantified based on a generalized fractional relaxation model under consideration of an apparent direct current conductivity assuming three relaxation processes: a high-frequency water process and two interfacial processes which are related to interactions between the aqueous pore solution and mineral particles (adsorbed/hydrated water relaxation, counter ion relaxation and Maxwell-Wagner effects). In a second step, these data are introduced in 3-D numerical frequency domain finite element field calculations to model the one port broadband frequency or time domain transfer function for a three rode based TDR-probe embedded in the clay rock. The results are analyzed with classical travel time analysis (onset/inflection) which under/overestimates the value of the permittivity compared to effective permittivity at 1 GHz. Indeed, apparent permittivity contains not only the water-content contribution but also effects due to water-mineral interaction processes. The results demonstrate the capabilities of a combined TD/FD analysis procedure for the monitoring of physical and chemical properties of materials with high frequency electromagnetic sensor techniques.

This paper examined the feasibility of performing bistatic Radar Cross Section (RCS) measurements in the Boris Vian anechoic chamber. To test this anechoic chamber, a small PEC circular cylinder is selected as a target. The capability to measure the bistatic RCS of a target versus frequency and versus target azimuth angle is established. Accurate bistatic RCS measurements are obtained using a Vector Network Analyzer and using background subtraction, bistatic calibration, and software range gating. A method to establish the chamber directivity is proposed. The accuracy of the measurement is estimated and a new value of the RCS sensibility is given as a function of the value of the chamber directivity.

The ultimate simplified point scatterer model contains no object's geometry informations, so a more complex prediction model containing geometrical informations is needed. The Physical Optics (PO) approximation has been widely used and considered as a good approximation of the far field electromagnetic scattering. Here, a forward model of bistatic scattering by PEC infinite elliptic cylinder based on the Physical Optics approximation is presented. The aim is to develope a simple scattering model for 2D targets illuminated by a monochromatic wave bistatic Radar. The reduced model is obtained by applying the stationary phase method to the PO integrals in both TE and TM wave cases. A parameter estimation procedure is also presented in order to examinate the efficiency of the presented model.

This paper focuses on the computation of the generalized ambiguity function (GAF) of a multiple antennas multiple frequencies radar system (MAMF). This study provides some insights into the definition of resolution parameters of a MAMF radar system. It turns out that the range and azimuth resolutions are not the most suitable criteria to specify the MAMF radar resolution. Therefore a new set of resolution parameters is introduced like the resolution ellipse which expresses the resolution anywhere in the image plane or δ→max, (δ→min) which expresses the highest (lowest) bound of the spatial radar resolution. To point out the pertinence of our study, we illustrate it with a MAMF radar system built around GPS satellites. The effect of the radar system geometry on resolution is investigated. For several scenarios, the GAF and its numerical form, the point spread function (PSF), are computed and their results are compared.