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In 2023, the Chinese company PIESAT launched the multi-static X-band SAR constellation Hongtu-1 (HT1). HT1 consists of the active monostatic SAR sensor HT1-A and the three additional passive SAR receivers HT1-B, HT1-C and HT1-D. The passive sensors are arranged as a cartwheel in a circle around the active sensor. For our SAR interferometric investigation, we were able to use a multi-static HT1 recording. After a brief introduction of HT1, we describe the processing performed. Based on the phases of the six single-pass interferometric pairs, we calculated height differences relative to the Copernicus DEM. Larger deviations were observed mainly for mining areas and for forest areas. Thanks to the simultaneous acquisition of the interferometric pairs, the high spatial resolution and the good signal quality, the necessary processing was relatively easy to perform. Besides the interferometric phase, we also investigated possible applications of multi-static coherence. Forest can be recognized by its reduced single-pass coherence values. Based on our results, we expect that the multi-static HT1 coherence and its dependence on the interferometric baseline can be used to estimate parameters such as forest biomass. [ABSTRACT FROM AUTHOR]

Smart windows effectively respond to the ever‐changing climatic conditions, offering a smart solution for low‐carbon buildings. However, current smart windows derived from chromic materials often have inferior solar modulation ability, or showcase high haze that obstructs outdoor views. Here, instead of developing new chromic materials, a new bistatic window is proposed for ultra‐high solar modulation and luminous transmission. The new developed window can reduce the indoor surface temperature for ≈11 °C, and reduce the building space cooling and heating energy consumption by 30% to 40%, providing significant energy‐related advances over traditional smart windows. In detail, the bistatic window exhibits excellent solar modulation ability (ΔTsol = 61%), high visible transmittance in both bleached (Tlum,bleached = 91%) and colored (Tlum,colored = 56%) states, low haze (< 1%), rapid switching response (switching time < 1 min), high color rendering index (CRI > 80), and long‐cyclic stability after 1000 cycles. With the advantages of facile fabrication and scalability, it is foreseen the developed bistatic window holds promising prospect for the next‐generation low‐carbon buildings, paving a new way for future advancements in the fields of smart windows. [ABSTRACT FROM AUTHOR]

Extensive studies have been carried out on the characteristics of quantum radar cross section (QRCS) of targets. However, one crucial question related to multi-photon quantum radar cross section (M-QRCS) for targets in the atmospheric medium has not been explored yet. Understanding this question is vital for target detection and identification of quantum radar. This paper presents a universal method to solve M-QRCS in a homogeneous atmospheric medium (HAM-QRCS). The process is based on the photon wave function in a homogeneous atmospheric medium and the interaction mechanism of multi-photon and multiple atoms. It is suitable for analyzing the HAM-QRCS characteristics of targets of arbitrary shapes. The simulation results show that the molecules, particles, and other factors in the atmospheric medium cause the signal photons' energy to decrease and the propagation direction to change, leading to a decrease in the target return responses. However, in a specific angle range, as the photon number increases, the main lobe and first side lobe structures of the bistatic HAM-QRCS response are enhanced. These findings can be utilized to design target detection strategies and optimize stealth target structures of the quantum radar in the atmospheric medium. [ABSTRACT FROM AUTHOR]

Abstract Smart windows effectively respond to the ever‐changing climatic conditions, offering a smart solution for low‐carbon buildings. However, current smart windows derived from chromic materials often have inferior solar modulation ability, or showcase high haze that obstructs outdoor views. Here, instead of developing new chromic materials, a new bistatic window is proposed for ultra‐high solar modulation and luminous transmission. The new developed window can reduce the indoor surface temperature for ≈11 °C, and reduce the building space cooling and heating energy consumption by 30% to 40%, providing significant energy‐related advances over traditional smart windows. In detail, the bistatic window exhibits excellent solar modulation ability (ΔTsol = 61%), high visible transmittance in both bleached (Tlum,bleached = 91%) and colored (Tlum,colored = 56%) states, low haze (< 1%), rapid switching response (switching time < 1 min), high color rendering index (CRI > 80), and long‐cyclic stability after 1000 cycles. With the advantages of facile fabrication and scalability, it is foreseen the developed bistatic window holds promising prospect for the next‐generation low‐carbon buildings, paving a new way for future advancements in the fields of smart windows.

Positioning a Synthetic Aperture Radar (SAR) receiver at a significant distance from an illuminator in a Low Earth Orbit (LEO) enables wide-angle repeat-pass bistatic interferometry. It's crucial to define the positioning limits of the receiver with respect to the illuminator to ensure reasonable coherences, altitudes of ambiguity, and a suitable common wavenumber support. I extended the standard monostatic formula, which yields the change in the interferometric travel path due to the vertical baseline. In the wide-angle bistatic case, the range-azimuth coordinate system is no longer orthogonal and we must consider the full 2D vector nature of the LOS orthogonal components. The two vertical baselines and the horizontal baseline of the receiver are needed to parameterize the system. Their impact on the interferometric coherence is quantified and bistatic limits are seen to be more flexible than those in the monostatic case due to the combination of independent baselines. I estimated the coherence, and then extended the analysis to the wavenumber domain, to quantify the common spectral support due to the wavenumber shifts due to the three baselines. The analysis carried out geometrically allows for combining together the effects of the transmitter's and receiver's vertical baselines and thus representing the residual spectral fraction with two instead of three parameters. Finally, I examined the achievable spatial resolution of the position of the interferer obtainable from the delay of the arrival of the interference between the illuminator (supposed to be silent) and the receiver. [ABSTRACT FROM AUTHOR]

The ability to capture and record high-resolution images over long distances is essential for a wide range of applications, including connected and autonomous vehicles, defense and security operations, as well as agriculture and mining industries. Here, we demonstrate a self-assembled bistatic long-range imaging LiDAR system. Importantly, to achieve high signal-to-noise ratio (SNR) data, we employed a comprehensive suite of denoising methods including temporal, spatial, spectral, and polarization filtering. With the aid of these denoising technologies, our system has been validated to possess the capability of imaging under various complex usage conditions. In terms of distance performance, the test results achieved ranges of over 4000 m during daylight with clear weather, 19,200 m at night, 6700 m during daylight with haze, and 2000 m during daylight with rain. Additionally, it offers an angular resolution of 0.01 mrad. These findings demonstrate the potential to offer comprehensive construction strategies and operational methodologies to individuals seeking long-range LiDAR data. [ABSTRACT FROM AUTHOR]

Synthetic Aperture Radar (SAR) has become, nowadays, one of the most important techniques in remote-sensing, and the increasing interest in Earth monitoring reinforces this trend. Next-generation SAR sensors will enhance the radar resolution capabilities by means of digital beamforming (DBF) techniques along with multistatic systems. In order to support the technological development of future spaceborne SAR missions, airborne sensors become an essential scope of research. Thereby, future airborne SAR systems demand enhanced DBF capabilities that involve phased array antennas with a high density of array elements. However, the antenna aperture size is significantly limited in airborne applications, which leads to low-profile and highly integrated antenna solutions, becoming a more challenging task for lower frequency operations such as L-band. In this work, a compact L-band dual-polarized multilayer phased array antenna with beam steering in elevation, developed for the next-generation German Aerospace Center (DLR) airborne SAR system, is presented. The proposed design makes use of truncated cavities to improve the array element isolation and provides 66% more antenna elements than the previous L-band phased array of the current DLR airborne SAR sensor with the same antenna aperture size. Measurements of a manufactured prototype show an antenna bandwidth of almost 20%, matching levels better than 17 dB, up to 15 dB gain, and cross-polarization suppression values higher than 35 dB. Thus, the proposed work will allow the application of advanced DBF techniques in the upcoming first pulsed bistatic L-band airborne SAR sensor.

Radar networks offer the possibility to overcome the limitations of single sensors. However, without coherent signal processing, the full potential of radar networks cannot be exploited. Therefore, the synchronization of independent radar sensors is crucial in order to mitigate performance degradation due to frequency deviations of uncoupled reference oscillators. Systematically derived synchronization criteria for coupling-induced errors such as sampling frequency offset (SFO), carrier frequency offset (CFO) and symbol timing offset (STO) of uncoupled orthogonal frequency-division multiplexing (OFDM) radar sensors enable to determine hardware specifications, such as the required reference oscillator frequency stability. This aids in the design of digital radar networks and ensures error-free signal processing in advance. The concept of deriving said criteria is applicable to other modulation types used for digital radar sensors. Additionally, signal processing-based error estimation and correction methods are presented. By exploiting standard radar images, namely range profiles and range-Doppler images, the proposed methods estimate and correct any occurring SFO, CFO and STO. Furthermore, by applying the signal processing synchronization methods, they allow for a more lenient radar network design. Measurements using a 77 GHz digital radar demonstrator verify the derived criteria and error estimation and correction methods.

Synthetic aperture radar (SAR) is an established technique for high-resolution radar imaging relying on the ego-motion of the radar. The long coherent integration time of the radar in conjunction with a linear or circular trajectory enables the illumination of targets under different observation angles leading to robustness against short-time interference signals. The use of a bistatic setup to simultaneously acquire multiple observation angles further increases the aforementioned benefits. In this work, a bistatic SAR system using an active repeater is proposed, in the following called Bistatic Repeater-SAR (BR-SAR). In BR-SAR the repeater acts as a second observer providing a bistatic path with low system cost and hardware effort. This article discusses the BR-SAR approach with all signal processing and hardware considerations necessary to build the bistatic radar system. This includes a description of the full system and signal chain with a focus on the signal separation and evaluation of both monostatic and bistatic signals and the subsequent SAR processing. The approach is verified via simulations and measurements, in which conventional SAR is compared to the proposed BR-SAR system. Hardware effects leading to a deteriorated system performance of the bistatic system are analyzed and countermeasures are discussed and implemented. Finally, the benefits of the bistatic system are demonstrated based on a realistic measurement scenario.

It has been shown that bistatic radars using radio telescopes as receivers can be used to increase the sensitivity of orbital debris measurements over traditional terrestrial monostatic radar. A method to calculate the lateral surface area of a bistatic radar is needed to evaluate the efficacy of a proposed bistatic radar configuration for orbital debris measurements. For over three decades, models of the orbital debris (OD) environment in low Earth orbit (LEO) have been developed to assess the risk posed by OD to spacecraft. While terrestrial radar measures debris 3 mm and larger and in situ measurements provide data for debris smaller than 1 mm, no good data sources exist for debris between 1 mm and 3 mm in size. This results in large variations between competing OD models. It also happens to be the size regime which poses the highest mission-ending risk to spacecraft. It is, therefore, of interest to investigate potential new data sources for this under-sampled size regime. There are many radars and radio telescopes that could be combined to create sensitive bistatic radars that could potentially bridge the size gap. In addition to sensitivity, it is necessary to predict the expected count rate of a candidate sensor to evaluate its performance. NASA's Orbital Debris Engineering Model (ORDEM) can be used to predict the flux of debris passing through the line of sight of a radar or telescope. This flux is related to a count rate through the calculation of the lateral surface area of the sensor. While this can be done easily for monostatic radars, a method for calculating the lateral surface area of a bistatic radar is needed. This new method of calculation has been developed and is described. The new method maps the radar beam overlap in 3D space, calculating the area of the complex surface formed by the gain product of the two antennas. Comparisons of the monostatic and new bistatic lateral surface area calculation methods for the monostatic case are presented. Results of a sample lateral surface area calculation for a bistatic radar observation configuration currently employed by NASA are shown. Finally, a guide for total error as a function of baseline and target altitude is established. [ABSTRACT FROM AUTHOR]

The ability to capture and record high-resolution images over long distances is essential for a wide range of applications, including connected and autonomous vehicles, defense and security operations, as well as agriculture and mining industries. Here, we demonstrate a self-assembled bistatic long-range imaging LiDAR system. Importantly, to achieve high signal-to-noise ratio (SNR) data, we employed a comprehensive suite of denoising methods including temporal, spatial, spectral, and polarization filtering. With the aid of these denoising technologies, our system has been validated to possess the capability of imaging under various complex usage conditions. In terms of distance performance, the test results achieved ranges of over 4000 m during daylight with clear weather, 19,200 m at night, 6700 m during daylight with haze, and 2000 m during daylight with rain. Additionally, it offers an angular resolution of 0.01 mrad. These findings demonstrate the potential to offer comprehensive construction strategies and operational methodologies to individuals seeking long-range LiDAR data.

Positioning a Synthetic Aperture Radar (SAR) receiver at a significant distance from an illuminator in a Low Earth Orbit (LEO) enables wide-angle repeat-pass bistatic interferometry. It’s crucial to define the positioning limits of the receiver with respect to the illuminator to ensure reasonable coherences, altitudes of ambiguity, and a suitable common wavenumber support. I extended the standard monostatic formula, which yields the change in the interferometric travel path due to the vertical baseline. In the wide-angle bistatic case, the range-azimuth coordinate system is no longer orthogonal and we must consider the full 2D vector nature of the LOS orthogonal components. The two vertical baselines and the horizontal baseline of the receiver are needed to parameterize the system. Their impact on the interferometric coherence is quantified and bistatic limits are seen to be more flexible than those in the monostatic case due to the combination of independent baselines. I estimated the coherence, and then extended the analysis to the wavenumber domain, to quantify the common spectral support due to the wavenumber shifts due to the three baselines. The analysis carried out geometrically allows for combining together the effects of the transmitter’s and receiver’s vertical baselines and thus representing the residual spectral fraction with two instead of three parameters. Finally, I examined the achievable spatial resolution of the position of the interferer obtainable from the delay of the arrival of the interference between the illuminator (supposed to be silent) and the receiver.

Building upon the concepts of polarization conversion and the mechanism of destructive interference, a single-layered ultra-broadband metasurface for radar cross-section (RCS) or backscattering reduction is proposed in this work. A diffusion-assisted checkerboard metasurface with re-tailored unit cells at the center leads to a significant improvement in the stealth characteristics. For this, a low profile and less complex unit cell is proposed, comprising a long metallic strip along the diagonal and two thin horizontal stubs at the edges. This unique arrangement exhibits more than 90% polarization conversion efficiency from 13.2 to 38.2 GHz. Modifying the geometry of central elements in a conventional checkerboard metasurface achieves a minimum of 15 dB RCS reduction from 10 to 38 GHz. Also, as we ascend the frequency spectrum, the beams are scattered in unintended directions with reduced signal strength. Notably, there is no beam in the boresight direction, as opposed to the conventional chessboard configurations. For off-normal incidences, the metasurface exhibits good angular stability, and a minimum of 10 dB backscattering reduction is maintained up to an incidence angle variation of 60°. The final optimized fabricated prototype exhibits measurement results that agree with the simulation results for normal and wide-angle incidences. ➢ This work proposes a single-layered ultra-broadband metasurface for radar cross-section (RCS) or backscattering reduction based on polarization conversion and the mechanism of destructive interference. ➢ To realize this, a low-profile reflective-type unit cell with efficient cross-polarization conversion capabilities is synthesized, which comprises of a long diagonal conductive strip with attached stubs at the edges on the top layer of the substrate and a metallic ground plane at the bottom. More than 90% cross-polarization conversion efficiency is obtained over the entire broad bandwidth. ➢ Conventional chessboard metasurfaces of different configurations (formulated based on unit cell arrangement in sub-arrays) are labeled as MS#1, MS#2, MS#3, MS#4, MS#5 are studied, and their monostatic and bistatic patterns are evaluated. ➢ It is observed that increasing the number of unit cells grouped in an array can improve the RCS performance only to a limited extent. Alternatively, a diffusion-assisted checkerboard metasurface with novel rearrangement of unit cells at the center leads to a significant improvement in the stealth characteristics is demonstrated. ➢ When an X/Y-polarized wave impinges on the MS#6, more than 15 dB reduction is observed in the complete polarization conversion bandwidth, i.e., from 13.2 to 38.2 GHz, and an out-of-band RCS reduction of more than 10 dB is achieved from 10 to 38 GHz. Also, it is reported that a maximum of 22 dB, 32 dB, and 36 dB RCS reduction is realized at 14 GHz, 24.5 GHz, and 38 GHz, respectively. The novelty in the proposed design lies in the substantial enhancement of RCS reduction with increasing frequency, as opposed to conventional chessboard configurations. The MS also exhibits good angular stability of 60°. ➢ To provide further details on the backscattering reduction and validate the monostatic and bistatic results, electric field distribution and 3D scattering patterns are analyzed for the proposed MS#6 and conventional MS#4. ➢ The fields emitted from MS#6 are effectively nullified in the boresight direction and scattered with reduced magnitude in unintended directions. Another important observation is that, as we ascend the frequency spectrum, in addition to the scattered beams, another main beam emerges from the MS#4 along the normal direction with significant strength, which is effectively mitigated in the proposed MS#6. ➢ A chessboard MS with the proposed MS#6 is constructed comprising of 24x24 elements and analyzed its performance with the conventional MS and PEC surface of same size. It is observed that the proposed metasurface offers comparable backscattering performance even at larger dimensions. ➢ A sample of the final optimized MS#6 comprising of 12 × 12 unit cells with an overall size of 72 × 72 mm2 is fabricated using PCB technology and measured transmission coefficients of the proposed metasurface and a metal sheet of equivalent size. A reduction of more than 10 dB is observed across the entire frequency range of interest for the normal and the oblique incidences. ➢ The key advantage of our proposed structure is, it is a single-layer design that is easy to fabricate, operates across a wide bandwidth, reduces backscatter reflection in the normal and oblique directions, and does not necessitate the use of any computational algorithms to design metasurfaces with increased dimensions. [ABSTRACT FROM AUTHOR]

The 3D time-of-flight (ToF) cameras have recently received a lot of attention due to their wide range of applications. Despite remarkable advancements in ToF imaging, state-of-the-art ToF cameras are still afflicted by the power hungriness of their illumination sources. To tackle this problem, we exploited existing lighting infrastructure, which ensures the ubiquitous presence of modulated light sources in indoor spaces that serve as opportunity illuminators. We explored the bistatic geometry for passive imaging using the pulse-based ToF approach. Our work is inspired by the recently introduced visible light communication (VLC) or light-fidelity (Li-Fi) infrastructure. VLC allows the infrastructure to provide indoor simultaneous illumination, communication, and sensing (SICS). To this end, we designed a bistatic geometry for the purpose of attaining passive 3D imaging. Such capabilities are achieved by exploiting the pulse shape of the autocorrelation function of real optical signals generated by VLC/Li-Fi modules (e.g., OpenVLC and LiFiMAX). We demonstrated passive imaging by means of matched filtering. In this work, we also studied different sampling strategies in the time-shift domain, including uniform, random, and sparse rulers, which is another step forward toward preserving high depth accuracy with a minimal number of measurements. The proposed methodology achieved successful depth reconstruction with negligible root-mean-square error (RMSE) for the low signal-to-noise ratio (SNR) of the measurements. Parametric models, such as Gaussian and sum of sines, are used to characterize the cross correlation functions and allow for robust parametric depth retrieval from a few measurements. Moreover, we attained a 20-mm worst case error for a target at 25 cm. The experiment proved that the bistatic passive depth reconstruction is feasible. [ABSTRACT FROM AUTHOR]

Objective: This study demonstrates high volume rate bistatic 3-D vascular strain imaging, to overcome well-known challenges caused by the anisotropic resolution and contrast inherent to ultrasound imaging.&#xD;Approach. Using two synchronized 32x32 element matrix arrays (3.5 MHz), coherent 3-D ultrasound images of ex vivo porcine aortas were acquired at 90 Hz during pulsation in a mock circulation loop. The image data of interleaved transmissions were coherently compounded on one densely sampled Cartesian grid to estimate frame-to-frame displacements using 3-D block matching. The radial displacement components were projected onto mesh nodes of the aortic wall, after which local circumferential and radial strain estimates were calculated with a 3-D least squares strain estimator.&#xD;Main results. The additional reflection content and high-resolution phase information along the axis of the second transducer added more distinctive features for block matching, resulting in an increased coverage of high correlation values and more accurate lateral displacements. Compared to single array results, the mean motion tracking error for one inflation cycle was reduced by a factor 5 to 8 and circumferential elastographic signal-to-noise ratio (SNRe) increased by 5-10 dB. Radial strain remains difficult to estimate at the transmit frequency used at these imaging depths, but may benefit from more research into strain regularization and sub-pixel interpolation techniques.&#xD;Significance. These results suggest that multi-aperture ultrasound acquisition sequences can advance the field of vascular strain imaging and elastography by addressing challenges related to estimating local-scale deformation on an acquisition level. Future research into 3-D aberration correction and probe localization techniques is important to extend the method's applicability towards in vivo use and for a wider range of applications., (Creative Commons Attribution license.)

A time-domain novel beamforming algorithm for ultra-wideband (UWB) microwave imaging is furnished in this paper. A monostatic and bistatic antenna setup is utilized to transmit and receive custom UWB pulses. This setup is employed to image various metallic and non–metallic threats attached to a human body. The imaging results demonstrate that the proposed beamforming algorithm allows effective reconstruction of different targets for both monostatic and bistatic cases. An ultra-wideband high gain Antipodal Vivaldi Antenna (AVA) is used as the transducer for the proposed imaging setup. The designed transducer demonstrates measured impedance bandwidth from 660 MHz to 15 GHz (182%) with a peak gain of 15 dBi at 8 GHz. The quality and accuracy of the proposed beamforming algorithm is tested outside the anechoic chamber just to make it more convincing in a real-world dynamic scenario. The result shows that different concealed materials can be detected accurately with the beamforming algorithm. To the best of the authors’ knowledge, the proposed algorithm can effectively add up the target responses to obtain a clear and distinct image of the metallic and non-metallic targets.

A novel interdigital capacitance (IDC) frequency selective surface (FSS) based linear‐cross reflective polarizer for K‐ and Ka‐band applications is demonstrated in this work. The suggested device includes an IDC‐loaded dipole printed on a thin FR‐4 grounded dielectric substrate. From 17.02 to 37.30 GHz, it shows linear‐cross conversion with a minimum 90% polarization conversion ratio (PCR) of 20.28 GHz bandwidth (74.7% fractional bandwidth [FBW]) for a y/x polarized incident electromagnetic (EM) wave. Along with linear‐cross conversion, this polarizer also performs linear‐circular conversions with axial ratio ≤ 3 dB in two frequency bands (Ku band‐right circular, 15.13–16.15 GHz; Ka‐band‐left circular, 38.52–39.78 GHz) with 6.52%, and 3.14% FBW, respectively. The design performance is stable up to 45° for both transverse electric (TE) and transverse magnetic (TM) modes. The polarizer unitcell has structural dimensions 0.177 × 0.177 × 0.061λL3, where λL corresponds to the free‐space wavelength of the lowest working frequency. The polariser's transmission line equivalent circuit model is extracted, and a good match is observed with the EM solver response. The broadband behavior of FSS and the ground plane is better understood by looking at surface current patterns at resonant frequencies. The proposed design's radar cross‐section (RCS) analysis is compared to perfect electric conductor for the bistatic condition. The design novelty lies in its compact architectural volume of the unitcell 1.911(λL3/1000), low‐profile, broadband PCR response with superior angle stability, dual‐polarization conversion with circular rotation switching makes it a good option for real‐time applications. [ABSTRACT FROM AUTHOR]

This paper presents an optimal design method for the acoustic stealth shape for a bottom object with relatively lower echo strength (ES), based on the physical acoustics method (PAM) and genetic algorithm (GA). Specifically, the performance of the PAM was evaluated with acoustic scattering from a Manta-like object, using two-dimensional (2D) axisymmetric calculation method. In the optimization method, GA, the object shape represented by the Bernstein polynomial, grid topology acquired by using the MATLAB-COMSOL module and the scattering calculation are combined into a process. The optimization objective function is given as the weighting function of the ES of the bottom object with different grazing angles and frequencies. Finally, the two optimal shapes of the bottom object under different conditions are given, in which the ES and the angle detection rate after optimization are greatly reduced. This optimal method provides guidance for the lower ES shape design of bottom targets. [ABSTRACT FROM AUTHOR]

Exploring polarimetric diversity of synthetic aperture radar (SAR) data is directly applicable to conventional monostatic cases. For this, the most used convention is the backscatter alignment. While establishing important advantages for the monostatic case (possibility to have equal values on the cross-polarimetric channels), it has been proven to introduce some difficulties for the bistatic case. This appears in relation to the so-called conjugate similarity operation, when (mathematically) asymmetric scattering matrices occur. In this article, we propose the detailed algorithm which provides a solution to the conjugate similarity operation, in the case of general scattering matrices. The proposed algorithm is based on the real representation (RR) matrix transformation. Furthermore, we investigate the characterization of canonical bistatic scatterers (three elementary targets). Raw bistatic polarimetric signals are obtained using simulations with a computationally electromagnetic (EM) software, capable of complete EM analysis. The eigenvalue classification illustrates the potential of additional information brought using the proposed RR scattering matrix (RRSM). The presence of complex eigenvalues is investigated in relation to the bistatic angle and one nonreciprocity parameter. [ABSTRACT FROM AUTHOR]

Quantum radar cross section (QRCS) has attracted extensive attention due to its unique characteristics of enhancing the visibility of sidelobe targets compared with classical radar cross section. However, the previous studies were mainly with the aim of monostatic or bistatic quantum radar scattering of single-photon incident. In this paper, based on M. J. Brandsema's method, the characteristics of bistatic quantum radar cross section (BIQRCS) with multiple photons illumination for the typical ship structure, a rectangular plate, are studied in detail. Furthermore, we verify the resulting equation by comparing with numerical simulations. Subsequently, the effects of the incident angles, the number and wavelength of signal photons, and the target size on the BIQRCS are analyzed separately and compared with the case of monostatic. Simulation results show that appropriate selection of the number and wavelength of signal photons can enable the bistatic quantum radar to obtain more target information at certain angles, and it is found that the bistatic quantum radar has more advantages in stealth target detection. [ABSTRACT FROM AUTHOR]

This article presents a new scheme called 2-D mixed compressive sensing back-projection (CS-BP-2D), for synthetic aperture radar (SAR) imaging on a geocoded grid, in a single measurement vector frame. The back-projection linear operator is derived in matrix form and a patched-based approach is proposed for reducing the dimensions of the dictionary. Spatial compressibility of the radar image is exploited by constructing the sparsity basis using the back-projection focusing framework and fast solving the reconstruction problem through the orthogonal matching pursuit algorithm. An artifact reduction filter inspired by the synthetic point spread function is used in postprocessing. The results are validated for simulated and real-world SAR data. Sentinel-1 C-band raw data in both monostatic and space-borne transmitter/stationary receiver bistatic configurations are tested. We show that CS-BP-2D can focus both monostatic and bistatic SAR images, using fewer measurements than the classical approach, while preserving the amplitude, the phase, and the position of the targets. Furthermore, the SAR image quality is enhanced and also the storage burden is reduced by storing only the recovered complex-valued points and their corresponding locations.

Bistatic radar signal processing often assumes a constant velocity for target detection. The bistatic geometry alters, however, the effectively measurable velocity in relation to the target position and motion even if the target has a constant ground speed. Hence, the bistatic geometry induces instantaneous bistatic target acceleration. This is especially the case if a straight-line target trajectory cuts across several closely spaced velocity isolines. The letter tries to quantify this geometry-induced instantaneous bistatic acceleration, in general, for straight-line target trajectories. Thereby, position-dependent acceleration values are derived for a practical use case. [ABSTRACT FROM AUTHOR]

In this letter, the reflection elements based on artificial dielectric layers (ADLs) are proposed for broadband wide-angle bistatic radar cross section (RCS) reduction. ADLs are introduced to improve the interference cancellation bandwidth of the reflection elements, and avoid the dilemma of mixing multiple dielectric mediums. The reflection elements achieve an approximate 180° phase difference over the frequency range of 4.8–17.5 GHz (113.9%) under normal incidence, and the bandwidth under TE 40° incidence and TM 55° is 113.9% and 100%, respectively. To validate this design, a coding scatterer surface is simulated and fabricated. Measured results show that the proposed coding scatterer surface achieves a 10 dB bistatic RCS reduction from 5 to 17.5 GHz under normal incidence for both polarizations. Bistatic RCS reduction under oblique incidence is analyzed and presented as well. [ABSTRACT FROM AUTHOR]

The high-resolution bistatic lidar developed at the Physikalisch-Technische Bundesanstalt (PTB) aims to overcome the limitations of conventional monostatic lidar technology, which is widely used for wind velocity measurements in wind energy and meteorology applications. Due to the large measurement volume of a combined optical transmitter and receiver tilting in multiple directions, monostatic lidar generally has poor spatial and temporal resolution. It also exhibits large measurement uncertainty when operated in inhomogeneous flow; for instance, over complex terrain. In contrast, PTB's bistatic lidar uses three dedicated receivers arranged around a central transmitter, resulting in an exceptionally small measurement volume. The coherent detection and modulation schemes used allow the detection of backscattered, Doppler shifted light down to the scale of single aerosols, realising the simultaneous measurement of all three wind velocity components. This paper outlines the design details and theory of operation of PTB's bistatic lidar and provides an overview of selected comparative measurements. The results of these measurements show that the measurement uncertainty of PTB's bistatic lidar is well within the measurement uncertainty of traditional cup anemometers while being fully independent of its site and traceable to the SI units. This allows its use as a transfer standard for the calibration of other remote sensing devices. Overall, PTB's bistatic lidar shows great potential to improve the capability and accuracy of wind velocity measurements, such as for the investigation of highly dynamic flow processes upstream and in the wake of wind turbines. [ABSTRACT FROM AUTHOR]

Signals of opportunity (SoOp) reflectometry (SoOp-R) is a maturing field for geophysical remote sensing as evidenced by the growing number of airborne and spaceborne experiments. As this approach receives more attention, it is worth analyzing SoOp-R's capabilities to retrieve subsurface soil moisture (SM) by leveraging communication and navigation satellite transmitters. In this research, the Cramer-Rao lower bound (CRLB) is used to identify the effects of variable SoOp-R parameters on the best achievable estimation error for root-zone soil moisture (RZSM). This study investigates the use of multiple frequency, polarization, and incidence angle measurement configurations on a two-layered dielectric profile. The results also detail the effects of variable SM conditions on the capability of SoOp-R systems to predict subsurface SM. The most prevalent observation is the importance of using at least two frequencies to limit uncertainties from subsurface SM estimates. If at least two frequencies are used, the CRLB of a profile is retrievable within the root-zone depending on the surface SM content as well as the number of independent measurements of the profile. For a depth of 30 cm, it is observed that a CRLB corresponding to 4% RZSM estimation accuracy is achievable with as few as two dual-frequency-based SoOp-R measurements. For this depth, increasing number of measurements provided by polarization and incidence angle allow for sensing of increasingly wet SM profile structures. This study, overall, details a methodology by which SoOp-R receiver system can be designed to achieve a desired CRLB using a tradeoff study between the available measurements and SM profile.

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.

Conventional SAR resolution equations cannot be applied to ultrawideband ultrawidebeam systems, particularly, bistatic ultrawideband ultrawidebeam systems. This is because the dependence of system frequency response in the range and across range is not considered. In this case, area resolution, i.e., area of the resolution cell, will be an alternative. This article introduces a new area resolution formula for bistatic ultrawideband ultrawidebeam SAR systems based on the relationship between radar signal frequency, range, and cross-range wavenumbers for bistatic synthetic aperture radar. [ABSTRACT FROM AUTHOR]

For the geosynchronous-low earth orbit (GEO-LEO) bi-static multi-channel high-resolution and wide-swath synthetic aperture radar (HRWS SAR) system, a novel azimuth Doppler signal reconstruction algorithm is developed based on an equivalent baseline approximating in the azimuth Doppler chirp Fourier transform domain. The main difficulty for the GEO-LEO bistatic multi-channel HRWS SAR system is that the baseline is variant with the scene in the conventional Doppler Fourier transform domain during azimuth signal reconstruction. The developed approach first employs the linear range cell migration correction and chirp Fourier transform to obtain the coarse-focused ambiguity SAR image. More importantly, the baseline component can be approximated a uniform baseline length for the whole image scene and the ambiguous Doppler spectrum can be reconstructed using the available Doppler signal reconstruction approach. Finally, the experimental results for a simulation GEO-LEO bi-static multi-channel HRWS SAR system are employed to validate the theoretical investigations and the proposed approach.

As the spaceborne mission dedicated to bistatic SAR interferometry (InSAR), the TerraSAR-X/TanDEM-X satellite constellation can acquire highly coherent data pairs with flexible baseline configurations and minimal atmospheric disturbances, which are well suited for high-accuracy DEM generation. However, hindered by the intrinsic contradiction between sensitivity of height measurement and reliability of phase unwrapping in terms of normal baseline length, as well as geometric distortions of layover and shadow, it is still difficult to generate high-quality DEM in mountainous areas using single-baseline InSAR data. Aiming at these two problems, we developed a cascaded multi-baseline InSAR processing framework for DEM generation from multiple bistatic InSAR observations with variable normal baselines and different imaging geometries. By starting from phase unwrapping and phase-to-height conversion for the interferogram with shortest normal baseline, this approach adopted a strategy of DEM updating assisted by a reference elevation dataset. The updated DEM is then used as the reference for processing interferograms with longer baselines successively. In this iterative way, the quality of the resultant InSAR DEM can be improved gradually. In addition, fusion between multiple geocoded InSAR DEMs derived from different orbits is carried out to further refine the results by filling data voids and reducing random height noise. To demonstrate the effectiveness of the proposed method, we apply it to four pairs of stripmap-mode TerraSAR-X/TanDEM-X bistatic data covering the Mount Song area in central China. The vertical accuracies of the generated DEMs are evaluated using an aerial photogrammetric DEM, ICESat/GLAS measurements, and the standard TanDEM-X global DEM product. The experimental results show that the absolute height error of the generated multi-baseline InSAR DEM at 5 m posting is less than 10 m in typical mountainous areas and can meet the American HRTI-3 standard, which means the generated DEM has similar height accuracy and higher spatial resolution in comparison with the 12 m TanDEM-X global DEM products. [ABSTRACT FROM AUTHOR]

Radar images of a 1-D perfectly conducting rough surface are compared for monostatic and for the equivalent bistatic imaging configurations. The results show that, while first-order “single” scattering images are largely similar; higher-order scattering effects can show differing signatures for these two imaging modalities. [ABSTRACT FROM AUTHOR]

Super Dual Auroral Radar Network (SuperDARN) radars operate in a coordinated but monostatic configuration whereby high‐frequency (HF) signals scattered from ionospheric density irregularities or from the surface of the Earth return to the transmitting radar where Doppler parameters are then acquired. A bistatic arrangement has been developed for SuperDARN radars in which HF signals transmitted from one radar are received and analyzed by another radar that is separated by a large distance (>1,000 km). This new capability was developed and tested on radars located in Oregon and Kansas. Numerous 3‐day bistatic campaigns have been conducted over a period extending from September 2019 through March 2020. During these campaigns three distinct bistatic propagation modes have been identified including a direct mode in which signals are transmitted and received through the radar side lobes. Direct mode signals propagate along the great‐circle arc connecting the two bistatic radar sites, reflecting from the ionosphere at both E region and F region altitudes. Two additional modes are observed in which HF signals transmitted from one radar scatter from either ionospheric density irregularities or from the surface of the Earth before propagating to the bistatic receiving radar. Ray tracing simulations performed for examples of each mode show good agreement with observations and confirm our understanding of these three bistatic propagation modes. Bistatic campaigns continue to be scheduled in order to improve technical aspects of this new capability, to further explore the physical processes involved in the propagation and scattering of HF bistatic signals and to expand the coverage of ionospheric effects that is possible with SuperDARN. Key Points: Bistatic capability has been developed and demonstrated using two midlatitude SuperDARN radarsThree distinct propagation modes were commonly observed during bistatic campaignsGood agreement between observations and ray tracing using the IRI model is obtained [ABSTRACT FROM AUTHOR]

In 2007, synthetic aperture radar (SAR) imaging was modified by TerraSAR-X (TSX); this second-generation SAR satellite offers up to 1 m of spatial resolution. The satellite is capable of interferometric SAR (InSAR), which permits the generation of digital surface models (DSMs). TSX was designed for monostatic InSAR imaging, but the potential for acquired DSMs is limited due to the disadvantages of repeat-pass InSAR geometry under the influence of temporal and related spectral decorrelations. In 2010, TanDEM-X (TDX) was launched to operate in a synchronized helix orbit with TSX to achieve high-quality DSMs covering almost the entire World in the basis of bistatic SAR imaging. In this paper, we focused on the DSM potential of TDX InSAR pairs in inclined topographies, and open, built-up, and forest land cover classes. A 5-m gridded TDX DSM was generated by interferometric processing and comprehensively analyzed by using model-to-model comparison approaches utilizing a reference airborne laser scanning (ALS) DSM. Absolute and relative vertical accuracies were estimated separately for all land cover classes using standard deviation and normalized median absolute deviation considering the influence of terrain tilt. Coherence with the reference DSM was determined by vertical profiles and frequency distribution graphics of height differences. Inaccurate regions were visualized by the generation of height error maps (HEMs). During the analysis, the qualities of the generated 5-m TSX DSM and a freely available 1-arcsecond (~ 30 m) Shuttle Radar Topography Mission (SRTM) DSM were validated and compared with TDX. [ABSTRACT FROM AUTHOR]

Closed-form expressions for the bistatic quantum radar scattered by the circular and triangular plates with the single-photon incidence are derived in the field of the Fourier relationship between the surface atom distribution and the bistatic quantum radar cross section (BIQRCS). The proposed expressions are highly efficient useful tools to analytically estimate the BIQRCS of the typical simple plates in the condition of high frequency approximation. And the formulas have been preliminarily verified by the numerical solution for the typical incident angles. In addition, the analysis of the quantum effect of sidelobes of BIQRCS in scattering, which can be seen as a kind of quantum interference, is also presented. We expect that this achievement will apply to the modeling of bistatic quantum radar scattering. [ABSTRACT FROM AUTHOR]

High frame rate ultrasound (US) imaging techniques in 3D are promising tools for capturing abdominal aortic aneurysms (AAAs) over time, however, with the limited number of channel-to-element connections current footprints are small, which limits the field of view. Moreover, the maximal steering angle of the ultrasound beams in transmit and the maximal receptance angle in receive are insufficient for capturing the curvy shape of the AAA. Therefore, an approach is needed towards large arrays. In this study, high frame rate bistatic 3D US data (17 Hz) were acquired with two synchronized matrix arrays positioned at different locations (multi-aperture imaging) using a translation stage to simulate what a larger array with limited channel-to-element connections can potentially achieve. Acquisitions were performed along an AAA shaped phantom with different probe tilting angles (0 up to ± 30°). The performance of different multi-aperture configurations was quantified using the generalized contrast-to-noise ratio of the wall and lumen (gCNR). Furthermore, a parametric model of the multi-aperture system was used to estimate in which AAA wall regions the contrast is expected to be high. This was evaluated for AAAs with increasing diameters and curvature. With an eight-aperture 0° probe angle configuration a 69 % increase in field of view was measured in the longitudinal direction compared to the field of view of a single aperture configuration. When increasing the number of apertures from two to eight, the gCNR improved for the upper wall and lower wall by 35 % and 13 % (monostatic) and by 36 % and 13 % (bistatic). Contrast improvements up to 22 % (upper wall) and 12 % (lower wall) are achieved with tilted probe configurations compared to non-tilted configurations. Moreover, with bistatic imaging with tilted probe configurations gCNR improvements up to 4 % (upper wall) and 7 % (lower wall) are achieved compared to monostatic imaging. Furthermore, imaging with a larger inter-probe distance improved the gCNR for a ± 15° probe angle configuration. The gCNR has an expected pattern over time, where the contrast is lower when there is more wall motion (systole) and higher when motion is reduced (diastole). Furthermore, a higher frame rate (45 Hz) yields a lower gCNR, because fewer compound angles are used. The results of the parametric model suggest that a flat array is suitable for imaging AAA shapes with limited curvature, but that it is not suitable for imaging larger AAA shapes with more curvature. According to the model, tilted multi-aperture configurations combined with bistatic imaging can achieve a larger region with high contrast compared to non-tilted configurations. The findings of the model are in agreement with experimental findings. To conclude, this study demonstrates the vast improvements in field of view and AAA wall visibility that a large, sparsely populated 3D array can potentially achieve when imaging AAAs compared to single or dual aperture imaging. In the future, larger arrays, less thermal noise, more steering, and more channel-to-element connections combined with carefully chosen orientations of (sub-) apertures will likely advance 3D imaging of AAAs. [ABSTRACT FROM AUTHOR]

The quantum radar cross section (QRCS) describes how much return one gets when illuminating an object with a handful of photons. However, the previous studies mainly focused on the monostatic scattering of quantum radar. In this study, in response to the four key questions raised by ourselves in bistatic quantum radar cross section (BIQRCS), we calculate and analyze the BIQRCS for the typical tow-dimensional plate. First, as mentioned by Brandsema, the further derived analytical solution for the rectangular plate is obtained. In addition, the influence of incident frequency and the number of photons on the BIQRCS for the plate were obtained. Besides, we first reveal that the envelope curves of sidelobes in BIQRCS at the given incidence display the invariance in frequency. Finally, we show some comparison results among the BIQRCS, classical radar cross section (CRCS), and monostatic QRCS for the plate and find the advantage of sidelobe of the BIQRCS in given direction. We anticipate that these research results will find use in the detection and discrimination of stealthy platforms.

Ground-based synthetic aperture radar (GB-SAR) is a useful tool to simulate advanced SAR systems with its flexibility on RF system and SAR configuration. This paper reports an indoor experiment of bistatic/multistatic GB-SAR operated in Ku-band with two antennae: one antenna was stationary on the ground and the other was moving along a linear rail. Multiple bistatic GB-SAR images were taken with various stationary antenna positions, and then averaged to simulate a multistatic GB-SAR configuration composed of a moving Tx antenna along a rail and multiple stationary Rx antennae with various viewing angles. This configuration simulates the use of a spaceborne/airborne SAR system as a transmitting antenna and multiple ground-based stationary antennae as receiving antennae to obtain omni-directional scattering images. This SAR geometry with one-stationary and one-moving antennae configuration was analyzed and a time-domain SAR focusing algorithm was adjusted to this geometry. Being stationary for one antenna, the Doppler rate was analyzed to be half of the monostatic case, and the azimuth resolution was doubled. Image quality was enhanced by identifying and reducing azimuth ambiguity. By averaging multiple bistatic images from various stationary antenna positions, a multistatic GB-SAR image was achieved to have better image swath and reduced speckle noise.

The high-resolution bistatic lidar developed at the Physikalisch-Technische Bundesanstalt (PTB) aims to overcome the limitations of conventional monostatic lidar technology, which is widely used for wind velocity measurements in wind energy and meteorology applications. Due to the large measurement volume of a combined optical transmitter and receiver tilting in multiple directions, monostatic lidar generally has poor spatial and temporal resolution. It also exhibits large measurement uncertainty when operated in inhomogeneous flow; for instance, over complex terrain. In contrast, PTB’s bistatic lidar uses three dedicated receivers arranged around a central transmitter, resulting in an exceptionally small measurement volume. The coherent detection and modulation schemes used allow the detection of backscattered, Doppler shifted light down to the scale of single aerosols, realising the simultaneous measurement of all three wind velocity components. This paper outlines the design details and theory of operation of PTB’s bistatic lidar and provides an overview of selected comparative measurements. The results of these measurements show that the measurement uncertainty of PTB’s bistatic lidar is well within the measurement uncertainty of traditional cup anemometers while being fully independent of its site and traceable to the SI units. This allows its use as a transfer standard for the calibration of other remote sensing devices. Overall, PTB’s bistatic lidar shows great potential to improve the capability and accuracy of wind velocity measurements, such as for the investigation of highly dynamic flow processes upstream and in the wake of wind turbines.

Monitoring large built structures, landslides, volcanoes, or glaciers implies relative height measurements and line-of-sight (LOS) displacement measurements, which are typically performed with monostatic spaceborne synthetic aperture radar (SAR) sensors. In this case, for each satellite that illuminates a given location, we can usually exploit only the information available from one ascending/descending orbit, and if the satellite uses multiple subswaths, the information is obtained only from one subswath per orbit. A bistatic configuration with spaceborne transmitter and fixed ground-based (GB) receiver opens the possibility to exploit the information available from more than one orbit and one subswath per orbit. Additionally, such a configuration offers new perspectives for target tracking and characterization (e.g., multiple LOSs and bistatic scattering signatures). This article presents an opportunistic C-band bistatic SAR differential interferometry architecture that uses a multichannel GB stationary receiver and a separate transmitter, which can be either the Sentinel-1A/B satellite, used opportunistically, or a specially designed GB sliding transmitter. The combination of the operation modes based on spaceborne and GB transmitter allows monitoring a small critical area of interest with many acquisitions triggered by the GB transmitter and surveying of the whole surrounding area with a small number of acquisitions corresponding to the satellites passes. The hardware platforms are presented along with the bistatic interferometric processing flow, and the potential of the proposed architecture is assessed in the context of monitoring large built structures. [ABSTRACT FROM AUTHOR]

In this paper, a coherent multistatic radar network with a novel system architecture is presented, which circumvents the general problems of clock distribution and phase noise-related signal-to-noise ratio (SNR) issues. The proposed network consists of a variable number of multiple-input multiple-output (MIMO) radar sensors and a variable number of repeater tags, all of which operate incoherently on the hardware level. In a minimum configuration, the network only consists of one MIMO radar sensor and a repeater tag. The theory behind such a multistatic network is mathematically derived, and simulations are presented to show key aspects of the network, i.e., multistatic range and Doppler measurements, as well as high-resolution angle estimation, exploiting a very large virtual aperture spanning the whole network. Measurements with one sensor and one repeater tag at 77 GHz are carried out to verify the simulations. The measurements show that the bistatic path between the sensor and the repeater tag retains coherency. [ABSTRACT FROM AUTHOR]

SCoBi-Veg stands for Signals of opportunity Coherent Bistatic scattering model for Vegetated terrains. It simulates polarimetric reflectometry of vegetation canopy over a flat ground using a Monte Carlo scheme. The model is aimed at assessing the value of navigation and communication satellite Signals of Opportunity in a range of frequencies from P- to S-bands for remote sensing of a number of geophysical land parameters such as soil moisture and biomass. A fully polarimetric expression for bistatic scattering from a vegetation canopy is first formulated for a general case and is then specialized to the practical case of ground-based/low-altitude platforms with passive receivers overlooking vegetation using the signals transmitted from large distances. Using analytical wave theory in conjunction with distorted Born approximation, the transmit and receive antenna effects (i.e., polarization crosstalk/mismatch, orientation, and altitude) are explicitly accounted for. The forward model developed here enables the understanding of the effect of different geophysical parameters and system configurations on the coherent and incoherent components of the reflected signatures. It can thus help developing robust inverse algorithm for extraction of soil moisture and biomass. The model is applied to P-band signals of geostationary communication satellites to describe polarimetric reflections from tree canopies as observed from down-looking platforms at various altitudes. The relative contributions of diffuse and specular scattering on total reflected power and reflectivity are quantified for various observing scenarios. [ABSTRACT FROM AUTHOR]

This work offers both analog and digital tag modulation schemes and respective receiver designs, for ultra-low power, high performance, ambient backscatter communications. All proposed techniques are based on simple, but careful, switching frequency control at the tag, allowing for the easy frequency-domain multiple access. First, a digital modulation scheme is offered, namely pseudo-frequency shift keying, assuming illumination from constant envelope-modulated signals and a fully coherent detector is derived along with closed-form probability of error. A second digital modulation scheme is also offered, based on a frequency-shifted form of binary phase shift keying (S-BPSK), relaxing the constant-envelope requirement for the illuminator and an illumination-agnostic detector is derived. Based on S-BPSK, short packet error correction coding is utilized for ambient backscatter communication, for the first time in the literature. It is shown that the proposed coded scheme under modulated ambient signal illumination & wireless channel variation, offers tremendous performance gains, i.e., modulation of the ambient signal is helpful. Finally, a third, purely analog, modulation scheme is analyzed, based on FM remodulation principles. A low-cost tag is implemented, demonstrating tag-to-receiver ranges up to 26 meters outdoors, power consumption of ${24} ~\mu $ Watts in continuous operation, able to be interrogated by any conventional frequency modulation (FM) receiver. The proposed techniques cover a large variety of omnipresent wireless industry systems, enabling universal ambient backscatter and relevant wireless information and power transfer (WIPT) applications. [ABSTRACT FROM AUTHOR]

Recent advancement of satellite synthetic aperture radar (SAR) techniques require more sophisticated system configurations such as the use of bistatic antennas or multi-frequencies. A ground-based experiment is a cost-effective and efficient way to evaluate those new configurations especially in the early stage of the system development. In this paper, a ground-based synthetic aperture radar (GB-SAR) system was constructed and operated in a bistatic mode at Ku-band where a receiving antenna (Rx) follows a transmitting antenna (Tx) separated by a baseline B. A new bistatic GB-SAR focusing algorithm was developed by modifying a conventional range-Doppler algorithm (RDA), and its performance has been evaluated by comparing the results with those from a back-projection algorithm (BPA). The results showed good performance of RDA at far range approaching nominal resolutions of 9.4 cm in range and 4.5 cm in azimuth, but limited quality at near range due to the approximation used in RDA. Signals from three trihedral corner reflectors (CR) reduced with increasing B, showing a typical bidirectional scattering behavior of CR. This GB-SAR system will be a testbed for new SAR imaging configurations with variations in antenna positions and target properties.

A multilayer module is incorporated into the Signals of Opportunity (SoOp) Coherent Bistatic Scattering model (SCoBi) for determining the reflections and propagation of electric fields within a series of multilayer dielectric slabs. This module can be used in conjunction with other SCoBi components to simulate complex, bistatic simulation schemes that include features such as surface roughness, vegetation, antenna effects, and multilayer soil moisture interactions on reflected signals. This paper introduces the physics underlying the multilayer module and utilizes it to perform a simulation study of the response of SoOp-R measurements with respect to subsurface soil moisture parameters. For a frequency range of 100–2400 MHz, it is seen that the SoOp-R response to a single dielectric slab is mostly frequency insensitive; however, the SoOp-R response to multilayer dielectric slabs will vary between frequencies. The relationship between SoOp-R reflectivity and the contributing depth is visualized, and the results show that SoOp-R measurements can display sensitivity to soil moisture below the penetration depth. By simulation of simple soil moisture profiles with different wetting and drying gradients, the dielectric contrast between layers is shown to be the greatest contributing factor to subsurface soil moisture sensitivity. Overall, it is observed that different frequencies can sense different areas of a soil moisture profile, and this behavior can enable subsurface soil moisture data products from SoOp-R observations.

In an increasing number of applications of focused ultrasound (FUS) therapy, such as opening of the blood–brain barrier or collapsing microbubbles in a tumor, elevation of tissue temperature is not involved. In these cases, real-time visualization of the field distribution of the FUS source would allow localization of the FUS beam within the targeted tissue and allow repositioning of the FUS beam during tissue motion. In this paper, in order to visualize the FUS beam in situ, a 6-MHz single-element transducer ($f$ /2) was used as the FUS source and aligned perpendicular to a linear array which passively received scattered ultrasound from the sample. An image of the reconstructed intensity field pattern of the FUS source using bistatic beamforming was then superimposed on a registered B-mode image of the sample acquired using the same linear array. The superimposed image is used to provide anatomical context of the FUS beam in the sample being treated. The intensity field pattern reconstructed from a homogeneous scattering phantom was compared with the field characteristics of the FUS source characterized by the wire technique. The beamwidth estimates at the FUS focus using the in situ reconstruction technique and the wire technique were 1.5 and 1.2 mm, respectively. The depth-of-field estimates for the in situ reconstruction technique and the wire technique were 11.8 and 16.8 mm, respectively. The FUS beams were also visualized in a two-layer phantom and a chicken breast. The novel reconstruction technique was able to accurately visualize the field of an FUS source in the context of the interrogated medium. [ABSTRACT FROM AUTHOR]

A modified omega-K algorithm for planar multiple-input-multiple-output (MIMO) array-based 3-D image reconstruction is proposed. By applying several proper approximations in wavenumber domain, the complex and time-consuming bistatic Stolt transformation within the traditional MIMO- $\omega \text{K}$ is changed into a relatively simple monostatic one, and therefore the amount of the interpolation given for the data is greatly reduced. Combining with multidimensional fast Fourier transformation, extremely high computation efficiency can be achieved. Implementation details are well described. The imaging accuracy and the high efficiency of the algorithm are demonstrated by a near-field imaging experiment with a flat mannequin target based on a planar MIMO array radar. [ABSTRACT FROM AUTHOR]

We extend the concept of monostatic aspect diversity for improved automatic target recognition (ATR) to a bistatic synthetic aperture radar (SAR) platform. We derive the probability of error with respect to the number of aspects used for a simple two-target template-matching classification system. The validity of the error prediction is confirmed using simulated bistatic SAR images. Our results demonstrate the ATR benefits of supplementing monostatic SAR images with one or more bistatic SAR images. [ABSTRACT FROM AUTHOR]

This letter introduces ground-range and cross-range resolution equations for the side-looking bistatic synthetic aperture radar (SAR). The derivation is based on the backprojection integral and the method of stationary phase. The ground-range and cross-range resolution equations are provided in closed form, making them easy for calculation. They are, therefore, helpful for bistatic SAR system development. The derived ground-range and cross-range resolution equations are validated with the bistatic data simulated mainly using the parameters of the LORA system. [ABSTRACT FROM PUBLISHER]

High Frequency (HF) passive radar is a typical bistatic radar, and the modeling and simulation of sea clutter in bistatic radar are worth researching for HF passive radar. The paper first establishes a time-domain signal model of the first-order sea clutter in bistatic radar with a detailed derivation process. Then, based on the signal model, a new simulation method for the first-order sea clutter using band-pass filter and Hilbert transform is proposed. Simulation result verifies the validity of the proposed signal model and simulation method.