Bistatic Configuration Research Articles

Performance Analysis of Tomographic Methods Against Experimental Contactless Multistatic Ground Penetrating Radar

Ground-penetrating radar (GPR) technology for underground exploration consists of the transmission of an electromagnetic signal in the ground for sensing the presence of buried objects. While monostatic or bistatic configurations are usually adopted, a limited number of multistatic GPR systems have been proposed in the scientific literature. In this article, we investigate the recovery performance of a specific and unconventional contactless multistatic GPR system, designed at the Georgia Institute of Technology for the subsurface imaging of antitank and antipersonnel plastic mines. In particular, for the first time, tomographic approaches are tested against this experimental multistatic GPR system, while most GPR processing in the scientific literature processes multimonostatic experimental data sets. First, by mimicking the system at hand, an accurate theoretical as well as numerical analysis is performed in order to estimate the data information content and the performance achievable. Two different tomographic linear approaches are adopted, i.e., the linear sampling method and the Born approximation (BA) method, this latter enhanced by means of the compressive sensing (CS) theoretical framework. Then, the experimental data provided by the Georgia Institute of Technology are processed by means of a multifrequency CS- and BA-based method, thus generating very accurate 3D maps of the investigated underground scenario.

Non-Intrusive Measurements of Wave-Induced Flow over Dikes by Means of a Combined Ultrasound Doppler Velocimetry and Videography

The performance of non-intrusive instruments, such as acoustic profilers and cameras, to describe the wave-induced flow processes over maritime dike crest was investigated in experiments carried out at the University of Bologna. Direct and derived measurements from the acoustic probes deployed along the structure crest were discussed in relation to the observed backscatter rates. Image processing was implemented by means of clustering algorithm, in order to detect the free surface during overtopping events and characterize wave front propagation over the dike crest. UVP data were processed to indirectly derive flow depths and overtopping rates and compare them with the direct measurements in order to assess the measurement reliability and discuss their limits. Individual overtopping volume distribution as obtained by UVP data were estimated and compared with well-consolidated formulations, showing a good agreement. Finally, suggestions for an appropriate use of non-intrusive instruments to characterize a shallow, transient and aerated flow were provided, such as the control of the artificial seeding density, the use of a bi-static UVP configuration and adjustments to light exposure.

A 3-D Novel Fast Back-Projection Imaging Algorithm for Stratified Media Based on Near-Field Monostatic and Bistatic SAR

A fast back-projection (FBP) imaging algorithm is proposed to generate 3-D uniformly balanced images of an object embedded in a layered structure based on a combined near-field monostatic and bistatic synthetic aperture radar (SAR) system. The spectral layered Green's function is used in the FBP algorithm. Like conventional real-time imaging algorithms, the complexity of the imaging function is reduced via the stationary phase method, and the fast Fourier transform is implemented to accelerate the computation. Unlike conventional real-time imaging algorithms, the singularity problem associated with Green's function and antenna pattern is solved in this proposed algorithm by using a point spread function (PSF). For a real-time imaging system, besides fast image generation, an efficient data acquisition is needed. A combined monostatic and bistatic SAR configuration is used here to decrease the data collection time. The performance of this proposed FBP method is demonstrated by measurements of a stratified environment.

Bistatic Scattering From Anisotropic Rough Surfaces via a Closed-Form Two-Scale Model

Bistatic radars have been a topic of increasing interest in recent years, thanks to the introduction of new bistatic (and multistatic) configurations, including those based on the opportunistic exploitation of global navigation satellite systems (GNSSs). The research on bistatic electromagnetic scattering models plays an important role in the analysis of these systems, in their simulation, and in the prediction of their performance. The two-scale model (TSM) is a widely used approach for the computation of scattering from rough surfaces, since it is able to account for depolarization effects due to surface tilting. However, in its original formulation, it requires a computationally intensive numerical integration, in order to perform appropriate average over surface random slopes. To overcome this limitation, a closed-form polarimetric TSM (PTSM) was developed, which has been also recently extended to the case of anisotropic rough surfaces (A-PTSM), with a focus on the sea surface. The A-PTSM can be efficiently used to compute the backscattering from anisotropic rough surfaces and can support the development and analysis of monostatic radar missions. In order to extend its scope to the general case of bistatic and multistatic configurations, in this article, we extend the A-PTSM to the case of bistatic electromagnetic scattering, presenting the evaluation of all the elements of the bistatic polarimetric covariance matrix. Due to the relevance of circularly polarized signals in opportunistic GNSS reflectometry applications, both the linear and the circular polarization bases are considered. The behavior of the obtained elements is discussed, and simplified expressions of the elements of the covariance matrix are provided for the case of scattering within the incidence plane. Relevant numerical examples are provided and compared to those obtained by the more refined, but more computationally intensive, second-order small-slope approximation (SSA2) method. In the examples, we consider both a wind-driven sea surface and a tilled soil, and both L-band and X-band frequencies. However, the presented method can be used at all frequencies of interest for microwave remote sensing and for all observation geometries, except for near grazing incidence and/or scattering.

On the Reflectivity Extraction Based on Partial Bistatic Near-Field Scattering From Microwave Blackbody

This work reports numerical investigations on the scattering from microwave blackbody, for developing a national standard facility on the emissivity determination in China. Based on the Kirchhoff’s law of thermal equilibrium, the standard facility follows a bistatic scattering measurement configuration and covers a wide frequency range. In general, the reflectivity is to be extracted based on partially collected scattering in the near-field region. The whole electromagnetic process, from the quasi-optical antenna illumination, to target scattering, and finally the aperture coupling reception, is modeled to be analyzed. It is shown in the simulation results that the Floquet scattering properties of the array-shaped blackbody can be obtained without fulfilling the far-field condition. Then, evaluations on the reflectivity extraction are performed at 54, 89, 118, and 183 GHz, addressing typical blackbody structures of coated cones and coated pyramids. The factors of referencing targets, curve integration, and the oblique incidence are discussed, as well as the case of geometry flawed blackbody. This work offers important reference and specific suggestions for achieving accurate and stable reflectivity determination in the bistatic near-field configuration.

An Improved Frequency-Domain Image Formation Algorithm for Mini-UAV-Based Forward-Looking Spotlight BiSAR Systems

Mini-unmanned aerial vehicle (UAV)-based bistatic forward-looking synthetic aperture radar (SAR) (mini-UAV-based BFSAR) is much more attractive than the monostatic one because of the flexibility of the system geometry selection as well as its simplicity of system operation, especially with the mini-UAV platform. However, the trajectory of the mini-UAV needs to be accurately modeled since it is very sensitive to the external environment, and the forward-looking configuration results in more severe spatial variance in image formation processing. In the paper, an improved frequency-domain imaging algorithm based on a very accurate slant range model is proposed for mini-UAV-based BFSAR with spotlight illumination. First, a more accurate slant range expression considering the motion characteristics of the UAV and bistatic spotlight configuration is re-derived. Second, a new range nonlinear chirp scaling (NLCS) operator was derived based on the accurate bistatic slant range model. Third, an improved azimuth NLCS operator in the Doppler frequency domain was established for the spotlight illumination of the transmitter and receiver in mini-UAV based BFSAR systems. Finally, the proposed algorithm is validated by both simulations and real datasets.

FMCW-InISAR imaging for high-speed target based on bistatic configuration

Interferometric inverse synthetic aperture radar (InISAR) imaging has been proved to be a powerful means for obtaining three-dimensional (3-D) space shape of noncooperative targets. Frequency modulated continuous wave (FMCW) InISAR (FMCW-InISAR) has unique advantages of low power, low cost, and small volume compared with traditional coherent pulsed InISAR. However, FMCW-InISAR imaging has two additional issues to consider, the one is the invalidation of the assumption of stopg the other is the isolation of the transmitting and receiving antennas, which is the inherent issue of the transmitter-receiver community radar systems. To solve these two problems, a bistatic FMCW-InISAR imaging algorithm for high-speed targets is proposed in this paper. For improving the isolation of the transmitting and receiving antennas, a bistatic configuration based FMCW-InISAR system is designed. According to the characteristics of bistatic, a bistatic equivalent motion model and corresponding signal model are established. Since the assumption of stop&go is invalid in the case of FMCW, indicating that the target cannot be viewed as motionless during a sweep repetition interval (SRI), a parametric estimation based quadratic phase factor (QPF) compensation method is investigated to eliminate the range walk caused by the radial motion of the target during the SRI. In addition, considering the far-field trait of the target and combining the traditional InISAR imaging process, a combined QPF compensation technique is proposed to reduce the computational burden of the algorithm. Finally, the effectiveness and the robustness of the proposed algorithm are evaluated by some simulations.

Bistatic-Range-Doppler-Aperture Wavenumber Algorithm for Forward-Looking Spotlight SAR With Stationary Transmitter and Maneuvering Receiver

Bistatic forward-looking spotlight synthetic aperture radar with stationary transmitter and maneuvering receiver (STMR-BFSSAR) is a promising sensor for various applications, such as the automatic navigation and landing of maneuvering vehicles. Because of the bistatic forward-looking configuration and the receiver’s maneuvers, conventional image formation algorithms suffer from high computational complexity or small size of a well-focused scene if applied to STMR-BFSSAR. In this article, we propose a wavenumber-domain algorithm for STMR-BFSSAR image formation, which is termed the bistatic-range-Doppler-aperture wavenumber algorithm (BDWA). First, a novel range model in bistatic-range and Doppler-aperture coordinate space instead of conventional Cartesian coordinate space is established by employing the elliptic polar coordinate system and the method of series reversion. The novel range model not only makes the echo’s samples to be regular along the direction of the bistatic-range wavenumber axis but also constructs a curved wavefront close to the true wavefront. Second, an operation termed wavenumber-domain gridding is conceived to regularize the echo’s samples along the Doppler-aperture wavenumber axis, which can be implemented by 1-D interpolation. The proposed algorithm significantly outperforms the conventional algorithms in terms of computational complexity and scene size limits. Both point and distributed targets are simulated for two STMR-BFSSAR systems with different parameters. The simulation results verify the validity and superiority of the proposed BDWA.

Cyclostationary-Based Vital Signs Detection Using Microwave Radar at 2.5 GHz.

Non-contact detection and estimation of vital signs such as respiratory and cardiac frequencies is a powerful tool for surveillance applications. In particular, the continuous wave bio-radar has been widely investigated to determine the physiological parameters in a non-contact manner. Since the RF-reflected signal from the human body is corrupted by noise and random body movements, traditional Fourier analysis fails to detect the heart and breathing frequencies. In this effort, cyclostationary analysis has been used to improve the radar performance for non-invasive measurement of respiratory rate and heart rate. However, the preliminary works focus only on one frequency and do not include the impact of attenuation and random movement of the body in the analysis. Hence in this paper, we evaluate the impact of distance and noise on the cyclic features of the reflected signal. Furthermore, we explore the assessment of second order cyclostationary signal processing performance by developing the cyclic mean, the conjugate cyclic autocorrelation and the cyclic cumulant. In addition, the analysis is carried out using a reduced number of samples to reduce the response time. Implementation of the cyclostationary technique using a bi-static radar configuration at 2.5 GHz is shown as an example to demonstrate the proposed approach.

Monostatic MIMO Backscatter Communications

Backscatter communications have two major antenna configurations: the bistatic configuration, in which the reader employs two different sets of antennas to transmit and receive, and the monostatic configuration, in which the reader employs one set of antennas for both transmitting and receiving. In this paper, we provide a comprehensive study on the MIMO techniques for the M × L monostatic channel. Particularly we study on the joint design of the query and the coding matrices. We show that the maximum achievable diversity order of the monostatic channel is the diversity order achieved by the block-lever unitary query and orthogonal space-time block code (BUTQ-OSTBC) design pair, which is ML 2 , exactly the half of the diversity order of the conventional MIMO channel. Then we show that uniform query, the simplest query approach, cannot achieve the maximum achievable diversity order in the monostatic channel. We generalize BUTQ-OSTBC to the general augmenting approach, and show that unitary matrix is optimal in terms of SER performance among all possible query matrices, when OSTBC is employed. The above results further indicate that in the backscatter channel, additional diversity can be obtained by varying the query signals over time slots within the channel coherent time, which is quite different from the results from the conventional MIMO channels. We verify our results by Monte Carlo simulations.

Geosynchronous Spaceborne-Airborne Bistatic Moving Target Indication System: Performance Analysis and Configuration Design

Geosynchronous spaceborne-airborne bistatic synthetic aperture radar (GEO SA-BSAR), consisting of GEO transmitter and airborne receiver, has stable coverage for a long time and benefits moving target detection. However, the performance of GEO SA-BSAR moving target indication (MTI) system varies widely between bistatic configurations. The traditional configuration design for GEO SA-BSAR system only considers the imaging performance, which may cause the poor MTI performance. In this paper, we propose a bistatic configuration design method to jointly optimize the MTI and SAR imaging performance for GEO SA-BSAR MTI system. The relationship between the MTI performance and bistatic configuration parameters is derived analytically and analyzed based on the maximum output signal to clutter and noise ratio (SCNR) criterion. Then, the MTI performance and SAR imaging performance are jointly considered to model the configuration design problem as a multi-objective optimization problem under the constrained condition. Finally, the optimal configuration for GEO SA-BSAR MTI system is given.

A Numerical Study of Roughness Scale Effects on Ocean Radar Scattering Using the Second-Order SSA and the Moment Method

The roughness scale effects on ocean radar scattering are studied using the second-order small slope approximation (SSA-II) and the method of moments (MoM). The KHCC03 spectrum is employed to represent 2-D and 1-D sea surfaces in the above two scattering methods, respectively. Criteria of full spectrum truncation are proposed for the numerical simulations of ocean scattering. Numerical results are illustrated in fully bistatic configuration at L- and C-bands. It is found that scattering at higher frequency is relatively more sensitive to the small-scale roughness but less sensitive to the large-scale roughness. At L- and C-bands, short waves with wavenumber larger than 316 rad/m have little effect on ocean scattering. The large-scale waves put more impacts on scattering in the forward directions, especially for large incidence angles. Other than the specular direction, the effects of large-scale roughness on ocean scattering are in general smaller at VV-pol than HH-pol. The bistatic scattering at cross polarizations is less sensitive to the roughness scale as compared to the copolarizations. For numerical simulations of ocean scattering with incidence angle less than 60°, using small surface profiles with size about 1/6 of those accounting for full spectrum yields results with errors less than 2 dB. Results also indicate that the incoherent parts dominate the scattered power from ocean surfaces with large-scale roughness.

Bi-static high resolution imaging for ship targets with sparse apertures and its application in InISAR

ABSTRACTIn this letter, we focus on the high-resolution imaging of ship targets under sparse aperture condition and its application in three-dimensional (3D) imaging. A bi-static high-resolution algorithm dealing with the sparse echo condition is proposed, which consists of the following processing steps: (1) A side-lobe apodization–multiple orthogonal least-square method is proposed to recover the complete signal and reduce the influence of side-lobe at the same time, which enhances the precision of phase extraction in interferometric inverse synthetic aperture radar (InISAR) imaging; (2) The bi-static configuration is used to accentuate the relative motion between the target and radar, such that the requirement for long coherence processing time can be avoided and (3) The 3D InISAR technique is employed to obtain the 3D images of the ship targets. Experiments with simulated and real data are utilized to demonstrate the effectiveness of the proposed method.

Single pixel scanning based millimeter wave imaging

A food processing line is comparatively vulnerable to contaminants, whether through inattentive workers and employees or from the processing machines made largely out of steel or from raw materials that are contaminated themselves. Any of these risks can affect quality resulting in poor hygiene or even can put customer health in danger. Millimeter-wave imaging plays a significant role in many fields such as security inspection and medical diagnostics. Moreover, millimeter wave instruments are useful in detecting objects behind and inside visibly opaque barriers such as, concrete walls and plastic boxes. We describe a millimeter wave imaging system that uses a one-dimensional detector in combination with a single pixel mask in order to acquire two-dimensional images out of buried objects inside a bread. Our system can be used to detect metals and salty substances for bread quality control and defect detection. The system uses 60 GHz center frequency to image the target in transmission mode which utilizes a heterodyne sub-harmonic receiver placed in a bi-static configuration

Bistatic and Stereoscopic Configurations for HF Radar

Most HF radars operate in a monostatic or quasi-monostatic configuration. The collocation of transmit and receive facilities simplifies testing and maintenance, reduces demands on communications networks, and enables the use of established and relatively straightforward signal processing and data interpretation techniques. Radars of this type are well-suited to missions such as current mapping, waveheight measurement, and the detection of ships and aircraft. The high scientific, defense, and economic value of the radar products is evident from the fact that hundreds of HF radars are presently in operation, the great majority of them relying on the surface wave mode of propagation, though some systems employ line-of-sight or skywave modalities. Yet, notwithstanding the versatility and proven capabilities of monostatic HF radars, there are some types of observations for which the monostatic geometry renders them less effective. In these cases, one must turn to more general radar configurations, including those that employ a multiplicity of propagation modalities to achieve the desired illumination, scattering selectivity, and echo reception. In this paper, we survey some of the considerations that arise with bistatic HF radar configurations, explore some of the missions for which they are optimal, and describe some practical techniques that can guide their design and deployment.

A Fully Integrated SiGe Radar Sensor for Aerosol Flow Rate Measurements

This letter introduces a bistatic $W$ -band continuous wave radar sensor based on a silicon-germanium (SiGe) transceiver chip with respect to its ability to measure aerosol streams with particles in the micrometer range. The introduced homodyne SiGe sensor relies on a phase-locked loop (PLL)-stabilized oscillator embedded into a monolithic microwave integrated circuit (MMIC), while up until now particle stream monitoring radars are built as waveguide systems. After the radar hardware, including the bistatic antenna configuration, is examined, the physical relationships between the particle stream and the hardware parameters are given. Finally, two different experiments with coarse metallic steel particles and fine coffee powder streams are presented and analyzed.

Measuring the Directional Ocean Spectrum from Simulated Bistatic HF Radar Data

HF radars are becoming important components of coastal operational monitoring systems particularly for currents and mostly using monostatic radar systems where the transmit and receive antennas are colocated. A bistatic configuration, where the transmit antenna is separated from the receive antennas, offers some advantages and has been used for current measurement. Currents are measured using the Doppler shift from ocean waves which are Bragg-matched to the radio signal. Obtaining a wave measurement is more complicated. In this paper, the theoretical basis for bistatic wave measurement with a phased-array HF radar is reviewed and clarified. Simulations of monostatic and bistatic radar data have been made using wave models and wave spectral data. The Seaview monostatic inversion method for waves, currents and winds has been modified to allow for a bistatic configuration and has been applied to the simulated data for two receive sites. Comparisons of current and wave parameters and of wave spectra are presented. The results are encouraging, although the monostatic results are more accurate. Large bistatic angles seem to reduce the accuracy of the derived oceanographic measurements, although directional spectra match well over most of the frequency range.

Impact of Onset Ambiguity on SEM Signature and Reduction Approach by Scattering and Polarization Diversification

The paper assesses how multiple-static scattering mitigates the effect of late-time onset on the robustness of the extracted resonance modes in the context of radar target classification. The assessment exploits the mode distribution vs onset shift to verify the sensitivity of the mode’s extraction to the selected onset, especially higher-order, to onset. However, within some bistatic directions, the modes have enhanced energies with lesser specular energy, making the modes estimate less sensitive to onset shifts. Also, the mode distribution per bistatic and polarization configuration has demonstrated different onset windows of accurate and consistent mode extraction. Notably, the distribution of the mode energy distribution reveals that classification performance degrades with poorly selected onset.

Enhancing the SEM Signature via the Optimum Onset With a Bistatic and Cross-Polarization Radar Configuration

This paper assesses the feasibility of finding a time bin of optimum onset to improve the robustness of the resonance modes of a midsized aircraft target in the context of radar target identification subject to bistatic and polarization diversities. The approach utilizes the frequency data (noncoherent) to determine a reference onset and a reference resonant frequency set of interest and then employs the time data (coherent) to determine an optimum onset that leads to a minimum error between the reference and extractable mode frequencies. Both sets of data are subject to a qualitative assessment to investigate the impact of the bistatic and polarization radar configurations on the optimum onset to improve the mode extraction. The results show that an optimum onset, compared to the reference onset, is more immune to noise perturbation and has a better discriminative ability for same-class targets.

A Time-Domain Multigrid Solver With Higher-Order Born Approximation for Full-Wave Radar Tomography of a Complex-Shaped Target

This article introduces and evaluates numerically a multigrid solver for non-linear tomographic radar imaging. Our goal is to enable the fast and robust inversion of sparse time-domain data with a mathematical full-wave approach utilizing a higher-order Born approximation (BA). Full-wave inversion is computationally expensive, hence techniques to speed up the numerical procedures are needed. To model the wave propagation effectively, we use the finite element time-domain (FETD) method, which is equipped with a multigrid scheme to enable the rapid evaluation of the higher-order BA. As a potential application, we consider the tomography of small solar system bodies (SSSBs) and asteroid interiors particular, the latter of which can contain internal details observable by radar, e.g., layers, voids and cracks. In the numerical experiments, we investigated monostatic, bistatic and multistatic measurement configurations. The results obtained suggest that, with a relevant noise level, the tomographic reconstruction quality can be improved by applying the higher-order BA in comparison to the first-order case, which we interpret as a linearization of the inverse problem. Our open multigrid-FETD solver for Matlab (The Mathworks Inc.) is available online. It applies Matlabs features for graphics computing unit acceleration to enhance computational performance.