Monostatic Synthetic Aperture Radar Research Articles

Using an Equivalence-Based Approach to Derive 2-D Spectrum of BiSAR Data and Implementation Into an RDA Processor

An equivalent range equation with a monostatic equivalent component and a bistatic synthetic aperture radar (BiSAR) compensation component was proposed. There were five monostatic equivalent parameters (MEPs) in the both components. With the MEPs, a BiSAR range equation can be expressed in a form similar to the monostatic SAR (MoSAR) expression. Then, by using the range equation and the principle of stationary phase (POSP), we analytically derive the 2-D spectrum of a point target, and this 2-D spectrum is implemented into the range Doppler algorithm (RDA) that processes the translational invariant (TI) BiSAR data. In our RDA, spatially variant range cell migration (RCM) correction and azimuth compression (AC), both of which are related to the spatially variant MEPs, are required to focus all targets in the whole scene of different range cells. Simulations under a wide range of imaging parameters are conducted to assess the equivalent range equation and the 2-D spectrums. Satisfactory results are obtained. In addition, traditional methods for motion compensation can be directly applied to our equivalent range equation. Finally, the RDA is evaluated through the analysis of acquired raw BiSAR data. Well-focused images are obtained. Therefore, the proposed equivalent range equation, derived 2-D spectrum, and RDA have been validated for forming imagery using raw BiSAR data.

Image Distortion Characterization due to Equivalent Monostatic Approximation in Near-Field Bistatic SAR Imaging

The $\omega $ – $k$ synthetic aperture radar (SAR) algorithm is a computationally efficient algorithm for near-field 3-D monostatic SAR imaging in nondestructive testing (NDT) applications. However, bistatic measurements are preferred in order to obtain high dynamic range, in particular when real-time imaging arrays are used. This article investigates the image distortion caused by using an equivalent monostatic imaging algorithm for bistatic measurements. Simulations and measurements at millimeter-wave frequencies in the Ka-band (26.5–40 GHz) are used to investigate the resultant image distortion. Furthermore, the image distortion is quantified through the root-mean-square (rms) error, which is calculated as a function of the bistatic transmitter–receiver separation distance, range, and noise power. Simulations and measurements are conducted for imaging using the raster scanning of a pair of antennas and for nonuniform imaging arrays. In addition, an approximate method for phase compensation is introduced to improve the image error from the monostatic approximation of bistatic measurement.

Design of an end-to-end demonstration mission of a Formation-Flying Synthetic Aperture Radar (FF-SAR) based on microsatellites

The paper focuses on space system design aspects related to an end-to-end demonstration mission, aiming at showing the feasibility of a Formation Flying Synthetic Aperture Radar (FF-SAR) with microsatellite class platforms (~100 kg). Trajectory design approaches that can fulfil payload requirements are addressed to enable selected FF-SAR applications. The exploitation of these applications relies on suitable combinations of FF-SAR techniques like Signal-to-Noise Ratio (SNR) enhancement, High-Resolution Wide Swath (HRWS) SAR imaging, and Coherence Resolution Enhancement (CRE). In this framework, a cluster of 3 micro-satellites, working in X-band, flying in a Low Earth Orbit (LEO) close-formation, has been designed as a candidate end-to-end system demonstration mission. One satellite embarks a Transmitting-Receiving (Tx/Rx) radar, i.e. it is a monostatic SAR. The other two satellites are Receiving-only platforms. Critical design aspects related to spacecraft subsystems and formation-flying analysis are addressed to confirm the technical feasibility of the spaceborne distributed system implementing the FF-SAR principle.

Above-Ground Biomass Retrieval over Tropical Forests: A Novel GNSS-R Approach with CyGNSS

An assessment of the National Aeronautics and Space Administration NASA’s Cyclone Global Navigation Satellite System (CyGNSS) mission for biomass studies is presented in this work on rain, coniferous, dry, and moist tropical forests. The main objective is to investigate the capability of Global Navigation Satellite Systems Reflectometry (GNSS-R) for biomass retrieval over dense forest canopies from a space-borne platform. The potential advantage of CyGNSS, as compared to monostatic Synthetic Aperture Radar (SAR) missions, relies on the increasing signal attenuation by the vegetation cover, which gradually reduces the coherent scattering component σ coh , 0 . This term can only be collected in a bistatic radar geometry. This point motivates the study of the relationship between several observables derived from Delay Doppler Maps (DDMs) with Above-Ground Biomass (AGB). This assessment is performed at different elevation angles θ e as a function of Canopy Height (CH). The selected biomass products are obtained from data collected by the Geoscience Laser Altimeter System (GLAS) instrument on-board the Ice, Cloud, and land Elevation Satellite (ICESat-1). An analysis based on the first derivative of the experimentally derived polynomial fitting functions shows that the sensitivity requirements of the Trailing Edge TE and the reflectivity Γ reduce with increasing biomass up to ~ 350 and ~ 250 ton/ha over the Congo and Amazon rainforests, respectively. The empirical relationship between TE and Γ with AGB is further evaluated at optimum angular ranges using Soil Moisture Active Passive (SMAP)-derived Vegetation Optical Depth ( VOD ), and the Polarization Index ( PI ). Additionally, the potential influence of Soil Moisture Content (SMC) is investigated over forests with low AGB.

A Monostatic/Bistatic Ground-Based Synthetic Aperture Radar System for Target Imaging and 2-D Displacement Estimation

A monostatic/bistatic ground-based synthetic aperture radar (GB-SAR) system using an optical electric field sensor (OEFS) as the bistatic receiving unit has been designed for target imaging and 2-D displacement estimation purposes. The designed system has the capability of acquiring the monostatic and bistatic SAR images simultaneously from different looking angles, enabling the displacement vector estimation of the illuminated area. The spatial baseline between the monostatic and bistatic receivers is selected according to the estimation precision analysis. Experimental results are presented to evaluate the performance of the designed system and the proposed method. The displacement estimation accuracies in x- and y-directions can reach up to millimeter level.

Focusing Arc-Array Bistatic Synthetic Aperture Radar Data Based on Keystone Transform

Arc-array synthetic aperture radar (AA-SAR) has become a novel imaging scheme for full azimuth observation. However, the exiting arc-array monostatic SAR system is placed on a single platform and is easy to detect. Arc-array bistatic SAR (AA-BiSAR) with a stationary transmitter is proposed in this paper, which can obtain high data acquisition efficiency and reduce vulnerability of arc-array monostatic SAR. Furthermore, since the azimuth resolution with full azimuth observation is not related to the location of the stationary transmitter, the transmitter can be placed far away from the receiver. Compared with imaging algorithms for other modes, the key points of AA-BiSAR imaging algorithms are a square root in the bistatic slant range equation and an arc synthetic array in azimuth. According to the imaging geometry of AA-BiSAR, a novel imaging approach for AA-BiSAR based on keystone transform (KT) is proposed, and the KT implements range-cell migration correction (RCMC) in conditions of trigonometric function under square root in the range history and arc synthetic array in azimuth via reformatting the AA-BiSAR raw data. Besides presenting the proposed imaging approach, a complete resolution analysis of AA-BiSAR is given. Results of numerical simulation experiments on point targets validate the proposed imaging approach.

A Frequency-Domain Imaging Algorithm for Translational Variant Bistatic Forward-Looking SAR

Bistatic forward-looking synthetic aperture radar (BFSAR) breaks through the limitations of the conventional monostatic SAR on the forward-looking imaging. However, the problems of range cell migration (RCM) caused by the linear range walk and 2-D spatial variability of Doppler parameters become more serious and complicated in translational variant BFSAR. In this article, a keystone transform is introduced to correct the linear RCM. Based on the characteristics of a small aperture, the nonlinear chirp scaling (NCS) is discussed in the frequency domain to equalize the azimuth-range-dependent Doppler parameters. The improved NCS in our newly proposed BFSAR imaging algorithm, especially the re-definition of range direction and the model of spatial variant phase, differentiates this article from all the existing studies in the literature on BFSAR signal processing. Simulation results and real data processing further validate the effectiveness of the proposed algorithm.

Collaborative forward‐looking imaging and reconnaissance technology for manned/unmanned aerial vehicles

With the imaging advantage of bistatic forward-looking synthetic aperture radar, collaborative forward-looking imaging and reconnaissance technology for manned/unmanned aerial vehicles could be performed, in which unmanned aerial vehicle can realise two-dimensional (2-D) and high-resolution imaging and further attacking targets in its straight-ahead position. However, there exists more complicated space-variance property in this special configuration than traditional mono-static SAR. Such property will lead to performance deterioration of imaging if not corrected effectively. To address this problem, 2-D frequency spectrum with high precision is first obtained based on squint minimisation method here, and then a novel-phase space-variance correction method is developed through polynomial fitting. Imaging focus performance on targets could be improved significantly with authors’ method. Several simulations for scattering targets confirm its validity.

Spatial Baseline Optimization for Spaceborne Multistatic SAR Tomography Systems.

Spaceborne multistatic synthetic aperture radar (SAR) tomography (SMS-TomoSAR) systems take full advantage of the flexible configuration of multistatic SAR in the space, time, phase, and frequency dimensions, and simultaneously achieve high-precision height resolution and low-deformation measurement of three-dimensional ground scenes. SMS-TomoSAR currently poses a series of key issues to solve, such as baseline optimization, spatial transmission error estimation and compensation, and the choice of imaging algorithm, which directly affects the performance of height-dimensional imaging and surface deformation measurement. This paper explores the impact of baseline distribution on height-dimensional imaging performance for the baseline optimization issue, and proposes a feasible baseline optimization method. Firstly, the multi-base multi-pass baselines of an SMS-TomoSAR system are considered equivalent to a group of multi-pass baselines from monostatic SAR. Secondly, we establish the equivalent baselines as a symmetric-geometric model to characterize the non-uniform characteristic of baseline distribution. Through experimental simulation and model analysis, an approximately uniform baseline distribution is shown to have better SMS-TomoSAR imaging performance in the height direction. Further, a baseline design method under uniform-perturbation sampling with Gaussian distribution error is proposed. Finally, the imaging performance of different levels of perturbation is compared, and the maximum baseline perturbation allowed by the system is given.

MIMO Ground Penetrating Radar Imaging Through Multilayered Subsurface Using Total Variation Minimization

In most of the existing ground penetrating radar (GPR) imaging algorithms, using either full or sparse data collection, the ground is modeled as a single half-space layer. In this paper, a generalized sparse imaging approach with total variation minimization (TVM) for multiple-input multiple-output (MIMO) GPR imaging through multilayered subsurface is proposed. The multilayered media Green’s function is incorporated in the imaging algorithm to take into account the complex wave propagation effects under multilayered subsurface. An analytical expression of the layered subsurface Green’s function is derived using the stationary-phase method, which significantly reduces the computation time and complexity. On the other hand, as TVM minimizes the gradient of the image, its incorporation in the imaging algorithm results in a reconstruction that preserves the geometry and edges of the targets better than the standard L1-minimization-based sparsity-driven imaging. The number of antenna elements and frequency measurements in MIMO GPR system can be significantly reduced using the proposed technique without degradation of the image quality. Although MIMO configuration is investigated in this paper, the presented approach can be simply applied to monostatic synthetic aperture radar.

Bistatic Forward-Looking SAR Focusing Using $\omega\hbox{--}k$ Based on Spectrum Modeling and Optimization

Bistatic synthetic aperture radar (SAR) is able to break through the limitation of monostatic SAR on forward-looking area imaging with appropriate geometry configurations. Thanks to such an ability, bistatic forward-looking SAR (BFSAR) has extensive potential practical applications. For the focusing problem of conventional side-looking SAR, $\omega\hbox{--}k$ algorithm is accepted as the ideal solution. In this paper, the $\omega\hbox{--}k$ algorithm will be discussed in BFSAR geometry. As for the bistatic configuration, spatial domain linearization procedure should be carried out to extract a range variable from the point target reference spectrum (PTRS) in the existing $\omega\hbox{--}k$ algorithms. With respect to the BFSAR geometry, nevertheless, the linearization procedure reduces the accuracy of PTRS seriously. To cope with such a problem, a novel $\omega\hbox{--}k$ algorithm for BFSAR is proposed. In the proposed method, the range variable is modeled as a parameterized polynomial, and the corresponding PTRS with respect to two-dimensional frequencies is established. Then, the parameters are estimated by differential evolution to minimize the PTRS errors for each range coordinate and frequency point. Based on the estimated PTRS, the BFSAR data can be focused well by the proposed $\omega\hbox{--}k$ algorithm. Simulation results verify the effectiveness of the proposed method.

Analysis and Simulation on Imaging Performance of Backward and Forward Bistatic Synthetic Aperture Radar

In recent years, bistatic synthetic aperture radar (SAR) technique has attracted considerable and increasing attention. Compared to monostatic SAR for which only the backscattering is measured, bistatic SAR expands the scattering measurements in aspects of angular region and polarization, and greatly enhances the capability of remote sensing over terrain and sea. It has been pointed out in recent theoretical researches that bistatic scattering measured in the forward region is preferable to that measured in the backward region in lines of surface parameters retrieval. In the forward region, both dynamic range and signal sensitivity increase to a great extent. For these reasons, bistatic SAR imaging is desirable. However, because of the separated positions of the transmitter and receiver, the degrees of freedom in the parameter space is increased and the forward bistatic imaging is more complicated than the backward bistatic SAR in the aspects of bistatic range history, Doppler parameter estimation and motion compensation, et, al. In this study, we analyze bistatic SAR in terms of ground range resolution, azimuth resolution, bistatic range history and signal to noise ratio (SNR) in different bistatic configurations. Effects of system motion parameters on bistatic SAR imaging are investigated through analytical modeling and numerical simulations. The results indicate that the range resolution is extremely degraded in some cases in forward bistatic SAR imaging. In addition, due to the different imaging projection rules between backward and forward bistatic SAR, the ghost point is produced in the forward imaging. To avoid the above problems, the forward bistatic imaging geometry must be carefully considered. For a given application requirement with the desired imaging performances, the design of the motion parameters can be considered as a question of solving the nonlinear equation system (NES). Then the improved chaos particle swarm optimization (CPSO) is introduced to solve the NES and obtain the optimal solutions. And the simulated imaging results are used to test and verify the effectiveness of CPSO. The results help to deepen understanding of the constraints and properties of bistatic SAR imaging and provide the reference to the optimal design of the motion parameters for a specific requirement, especially in forward bistatic configurations.

Feasibility, Design, and Deployment Requirements of TCR for Bistatic SAR Radiometric Calibration

The trihedral corner reflector (TCR) is widely used as the calibration device in monostatic synthetic aperture radar (SAR) calibration, and the performance of the TCR in radiometric calibration has been studied and verified in depth. As for the bistatic SAR system calibration problem, there have been few published studies. There is a lack of knowledge regarding the exact bistatic radar cross-section (RCS) pattern of TCR with different bistatic angles, and it is also not clear whether the TCR can be used as the calibration target in bistatic SAR. Moreover, the bistatic and monostatic radar cross-section (RCS) characteristics of the TCR are different, even if the bistatic angle is very small. Therefore, the feasibility, design, and deployment requirements of the TCR for bistatic SAR calibration should be carefully investigated. In this paper, we outline the theoretical and practical requirements that need to be satisfied when choosing appropriate calibration devices for bistatic radiometric calibration. Based on these requirements, we analyzed the bistatic RCS patterns using electromagnetic simulation, and concluded that the TCR is feasible for bistatic SAR calibration under relatively small bistatic angles (less than 6°). The change of TCR boresight with the bistatic angle is not considered generally. However, we found that the TCR boresight and peak RCS will change with the bistatic angle. We have also proposed that the bistatic angle can be extended to 20° by taking the change of the TCR boresight into account. In this condition, we should get the TCR boresight according to the bistatic angle and then align it during the deployment. Both of these two conditions have their own unique advantages. Different error sources of TCR RCS from manufacture, misalignment, and deformation were investigated quantitatively with simulations, which can provide a theoretical basis for how to design a suitable TCR and guarantee the calibration accuracy for bistatic calibration. In addition, simulation results are different from those of monostatic calibration. Through experiments, we have further verified the feasibility by comparing the quality of bistatic SAR images and point target energy with several typical bistatic angles as the TCR boresight is considered or not. If the bistatic angle is larger than 6°, taking the TCR optimum boresight into account can improve imaging quality and point target energy.

Airborne Bistatic Forward-Looking SAR Using the Polynomial NCS Algorithm

Airborne bistatic forward-looking synthetic aperture radar (ABFL-SAR) breaks through the limitations of the conventional monostatic SAR on the forward-looking imaging. However, an ABFL-SAR is faced with the severe 2-D coupling, owing to the mechanism of translational variability, which is especially obvious in the azimuth compression compared with the conventional monostatic SAR. In this article, we improve the nonlinear chirp scalling (NCS) algorithm on the basis of polynomial fitting for ABFL-SAR with the cross trajectory. Simulation results further validate the effectiveness of the proposed algorithm.

Terahertz Bistatic Synthetic Aperture Radar for 1-D Near-Field High-Resolution Imaging

Considering the difficult transceiver-isolation problem of the monostatic synthetic aperture radar (SAR) in the terahertz (THz) band, this paper proposes a compact THz bistatic SAR (BiSAR) geometry. The system allows the separately distributed transmitter and receivers. At the receiving end, there are a direct-wave receiver and an echo receiver, both operating at the heterodyne and in-phase mode. The echo receiver runs along a linear rail to fulfill the scene scanning, while the direct-wave one is fixed as a reference. Furthermore, assuming that the receivers are synchronized, both the problem of synchronization between the separated transmitter and receivers and the problem of timing at the signal acquisition would be solved by utilizing the high coherence between the echo and the direct wave. Based on such a system, the application of THz BiSAR for one-dimensional imaging is taken into consideration. Then, a high-resolution imaging algorithm is proposed benefitting from the total least squares estimating signal parameters via rotational invariance techniques (TLS-ESPRIT) and the spatial smoothing process (SSP). The imaging performance is then demonstrated by both simulations and the experiments in the 0.183 THz.

Decreased Probability of Error in Template-Matching Classification Using Aspect-Diverse Bistatic SAR

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.

Target Motion Estimation Ambiguities for Monostatic Synthetic Aperture Radar

A recent analysis demonstrates that ambiguities exist in attempting to estimate target motion for general bistatic synthetic aperture radar collections. However, monostatic geometries appear to be problematic, since two coordinate angles become indeterminate. The current investigation resolves these issues and provides new insights into the nature of these ambiguities.

Bistatic Synthetic Aperture Radar With Undersampling for Terahertz 2-D Near-Field Imaging

To avoid the difficult transceiver-isolation problem of a monostatic synthetic aperture radar (SAR) for terahertz (THz) near-field azimuth-range imaging, a compact bistatic SAR geometry is demonstrated in this paper. The system consists of one fixed transmitter, one moving echo receiver and one fixed direct-wave receiver. The transmitter emits wideband signals for range resolution, and the echo receiver moves along a designed trajectory for the cross-range resolution, and the direct-wave receiver is taken as a reference for synchronization. Since the wavelength of THz is in the order of millimeter or submillimeter, the requirement on subwavelength interval of spatial sampling by the Nyquist theory aggravates the measurement difficulty. To break this limitation, a compressed sensing-based imaging algorithm is presented. Allowing the undersampling both in spatial domain and frequency domain, the proposed algorithm can provide high-resolution performance with few measurements. Thus, the balance between the resolution and the amount of measurements can be made. The system geometry and imaging performance are then demonstrated by both the simulations and the experiments in the 0.178–0.188 THz band.

An Accelerated Backprojection Algorithm for Monostatic and Bistatic SAR Processing

The backprojection (BP) algorithm has been applied to every SAR mode due to its great focusing quality and adaptability. However, the BP algorithm suffers from immense computational complexity. To improve the efficiency of the conventional BP algorithm, several fast BP (FBP) algorithms, such as the fast factorization BP (FFBP) and Block_FFBP, have been developed in recent studies. In the derivation of Block_FFBP, range data are divided into blocks, and the upsampling process is performed using an interpolation kernel instead of a fast Fourier transform (FFT), which reduces the processing efficiency. To circumvent these limitations, an accelerated BP algorithm based on Block_FFBP is proposed. In this algorithm, a fixed number of pivots rather than the beam centers is applied to construct the relationship of the propagation time delay between the “new” and “old” subapertures. Partition in the range dimension is avoided, and the range data are processed as a bulk. This accelerated BP algorithm benefits from the integrated range processing scheme and is extended to bistatic SAR processing. In this sense, the proposed algorithm can be referred to simply as MoBulk_FFBP for the monostatic SAR case and BiBulk_FFBP for the bistatic SAR case. Furthermore, for monostatic and azimuth-invariant bistatic SAR cases where the platform runs along a straight trajectory, the slant range mapping can be expressed in a continuous and analytical form. Real data from the spaceborne/stationary bistatic SAR experiment with TerraSAR-X operating in the staring spotlight mode and from the airborne spotlight SAR experiment acquired in 2016 are used to validate the performances of BiBulk_FFBP and MoBulk_FFBP, respectively.

Monostatic and Bistatic Low Frequency Ultrawideband SAR Imaging Experiment

Low frequency ultrawideband (LF UWB) synthetic aperture radar (SAR) has lately become of a particular interest to SAR community. Monostatic and bistatic LF UWB SAR system has the well foliage penetrating capability, high-resolution imaging and providing the increased information. In 2015, a monostatic and bistatic LF UWB SAR imaging experiment was conducted. In this experiment, the monostatic and bistatic data were collected simultaneously by operating a moving vehicle-based radar in the SAR mode, in conjunction with a stationary ground-based receiver. The aim was to investigate the imaging property of the bistatic LF UWB SAR system. The one pulse per second (1 PPS) signal in combination with the global position system (GPS) disciplined 100 MHz oscillator from the GPS receivers was used to implement the time and frequency synchronization in this SAR system. The bistatic SAR image was obtained by the subaperture spectrum-equilibrium method integrated with the fast factorized back projection (FFBP) algorithm. Bistatic experiment results are show to prove the validity of the bistatic LF UWB SAR imaging experiment.