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In this paper, feasibility study along with conceptual design of missile-borne synthetic aperture radar (SAR) has been accomplished, which can effectively improve the detection capability and the blow precision of a missile. First, SAR echo signal modeling, geometry configuration for missile-borne SAR scenario, and an appropriate image formation algorithm are presented. Then, a system block diagram along with complete discussions about different parts and system design considerations of the SAR seeker are explained. Finally, simulation of the whole system along with raw data generation considering the imaging geometry of the missile-borne SAR seeker has been performed. Experimental results show that the proposed system is capable of imaging and targeting in the complicated geometry of missile-borne SAR.

Clutter suppression for non-side-looking airborne radar operating at high pulse repetition frequency is an intractable problem because the clutter statistical properties vary with the range, especially in the near range. In addition, the near-range clutter is always coupled with the far-range clutter. Elevation prefiltering is recognised as an effective method to suppress the range-dependent near-range clutter, but the elevation training samples required must be independent and identically distributed (IID) and not contaminated by the far-range clutter. Here, an elevation-compensation-prefiltering space-time adaptive processing (STAP) algorithm is developed. In the proposed algorithm, the elevation training samples are selected from the elevation snapshots corresponding to the first pulse, which are not contaminated by the far-range clutter. An elevation compensation method is presented to alleviate the range dependence, and then there are adequate IID training samples to estimate the elevation covariance matrix. The performance of the near-range clutter suppression is improved. As for the residual range-independent far-range clutter, it can be suppressed by the cascaded azimuth-Doppler STAP. Numerical examples demonstrate the effectiveness of the proposed algorithm.

During the past few decades, there has been growing interest in the use of microwave and millimeter wave radiometers for remote sensing of the Earth. Due to the need of large antennas and scanning mechanism, the conventional real aperture radiometer becomes infeasible for high spatial resolution application. Interferometric aperture synthesis was suggested as an alternative to real aperture radiometry for earth observation [Ruf et al., 1988]. Aperture synthesis radiometers (ASR) can synthesize a large aperture by sparsely arranging a number of small aperture antennas to achieve high spatial resolution without requiring very large and massive mechanical scanning antenna. The fundamental theory behind aperture synthesis technique is the same as the one used for decades in radio astronomy [Thompson et al., 2001], in which the product of pairs of small antennas and signal processing is used in place of a single large aperture. In aperture synthesis, the coherent product (correlation) of the signal from pairs of antennas is measured at different antenna-pair spacings (also called baselines). The product at each baseline yields a sample point in the Fourier transform of the brightness temperature map of the scene, and the scene itself is reconstructed by inverting the sampled transform. This chapter addresses the subject of antenna array design in ASR, which plays an important role in radiometric imaging of ASR. The chapter is organized as follows. In section 2, the basic principle of synthetic aperture radiometers is briefly formulated. In section 3, the topology optimization of the antenna array is concerned, aiming at minimum redundancy arrays (MRAs) for high spatial resolution. For one-dimensional case, different optimization methods for finding out minimum redundancy linear arrays (MRLAs) such as numerical algorithms and combinatorial methods are summarized, including their advantages and disadvantages. We also propose an effective restricted search method by exploiting the general structure of MRLAs. For two-dimensional case, different antenna array configurations as well as their spatial sampling patterns are compared, including rectangular sampling arrays, hexagonal sampling arrays, and nonuniform sampling arrays. Some original work on the design of thinned circular arrays is also described. In section 4, a novel antenna array for our HUST-ASR prototype is presented, which is a sparse antenna array with an offset parabolic cylinder reflector at millimeter wave band. 9

A workload limit forms an essential part of most order release methods designed for high-variety make-to-order contexts, but this mechanism does not necessarily lead to the lowest possible direct load buffer. To counter this, the Superfluous Load Avoidance Release (SLAR) procedure was developed that avoided the use of a workload limit altogether. SLAR significantly improves performance compared to alternative release methods in the literature but has been criticized for being impractical as it can lead to uncontrolled loads at stations downstream in the routing of an order. This criticism can be overcome by introducing a workload limit. Using simulation, this study shows that introducing a limit to SLAR further reduces the superfluous direct load, specifically during high load periods. This not only controls the load at downstream stations but also yields a further reduction in the percentage of tardy jobs whilst maintaining SLAR's good mean tardiness and shop floor throughput time performance. Meanwhile, introducing an additional load-based trigger further improves mean tardiness performance. These results partly question one of SLAR's original design principles and extend the theory upon which SLAR is built, by linking each of SLAR's two release triggers to periods of low and of high load. Finally, by gaining control over the workload in periods of high load, the practical applicability of SLAR is substantially enhanced.

The article discusses a method for getting high-resolution radar images (RI) based on a side looking airborn radar (SLAR), which consists of a set of single independent single-beam small-sized radar stations (SRS) and a synthetic-aperture radar (SAR) used for transmission and reception in any given polarization on board of the aircraft (AP). Probing signals are binary PM signals of one cluster L formed on binary sequences of the maximum linear recurrent period (MLRP) equal to L = 2m − 1 (m ≥ 2). The emission of probing PM signals and the coordinated reception of the reflected PM signals can be carried out simultaneously by all SRS. Each SRS has its own PM signal from the L cluster. Based on the representation of binary MLRP by a complex Markov circuit with two states and taking into account the same type of SRS, it is possible to synthesize a receiving device with an L-channel recurrence matched filter (RMF) common to all SRSwith the radar included in the MRLP for simultaneous reception of several or the entire cluster L reflected PM signals of the same azimuth in all beams of SAR by range.A characteristic feature of the RMF is the absence of side lobes at the RMF outputs of the autocorrelation functions of the received reflected PM signals of each beam. It allows to get simultaneous joint nonlinear filtering of the discrete parameter (phase) and continuous parameters (amplitude and delay) of the reflected PM signals that provide to obtain a higher evaluation of a discrete parameter and, accordingly, high-resolution radar images. The result of the reflected PM signals of each beam is recorded in the memory unit, where SLAR is formed. To assess the effectiveness of the proposed method, the authors carried out computer modeling of the formation of the radar image of an artificial pattern with various objects under the influence of Gaussian fluctuations in the amplitude and delay of the reflected PM signals relative to the time of their radiation. The simulation results demonstrate getting high-resolution radar images in resolution range and azimuth in real time.

This paper focuses on the problem of the optimization of nodes positions for sparse large aperture radar (SLAR) system. To overcome the deficiency of low target search efficiency in traditional SLAR system, a search scheme based on grating lobe controlling, in which the SLAR system searches multiple directions simultaneously by utilizing a transmit beampattern with multiple grating lobes, is provided. More specifically, the relationship between the nodes positions and grating lobes is mathematically discussed. On this basis, under the practical node position constraint, a design method of nodes positions for synthesizing a beampattern with desired grating lobes and low sidelobes is presented. Some representative simulations are provided to validate the effectiveness of the proposed scheme.

Helicopter-borne rotating synthetic aperture radar (ROSAR) enjoys fast imaging property as the synthetic aperture obtained by rotor rotation. However, ROSAR data processing is usually a challenging task due to severe range cell migration (RCM) and motion error. To solve this problem, an enhanced phase gradient autofocus (PGA) algorithm based on helicopter-borne ROSAR wavenumber-domain imaging approach is proposed in this study, which alleviates the imaging performance degradation due to the RCM and motion error. With even small motion error induced in the wavenumber domain, the influence of motion error will become evident after performing Stolt interpolation, which induces serious image defocusing and degradation. Hence, the authors further propose the PGA-based ROSAR motion compensation scheme, which combines quadratic term correction with phase gradient estimation, and well-focused ROSAR images can be obtained via iterative processing. Several results of simulated experiments are presented to validate the proposed method for helicopter-borne ROSAR imaging.

Sidescan imaging sonar performance is often described using metrics such as resolution, maximum range, and area coverage rate. These attributes focus on describing how finely and over what area a sensor can make imagery of the seafloor and targets. While these metrics are important, they are inadequate for fully characterizing the quality of sonar imagery. Shadow regions often carry as much, and sometimes even more, information than the direct return from a target. The shadow contrast within an image is therefore a critical property, and it is the result of a complex interaction between the sonar hardware, the environment, and the signal processing. This paper builds on key results from the synthetic aperture radar and sonar literature to develop a comprehensive, quantitative model for predicting the shadow contrast ratio. A model for the contrast ratio is constructed using an approach that is similar to the development of the sonar equation. This ratio describes the average relative level between the seafloor and a shadow. The model includes the effects of the transducer and array design, ambient noise, quantization noise, unwanted volume and surface reverberation, and the signal processing used to construct the synthetic aperture sonar (SAS) imagery. The shadow contrast predicted by the model is compared to the contrast measured from SAS imagery, where close agreement is observed. Examples are presented where the model is applied to a hypothetical high-frequency imaging SAS sensor in a typical operating environment. The shadow contrast ratio is estimated as a function of the sensor range and key parameters of interest, such as receiver channel spacing, sediment backscatter coefficient, and the time-bandwidth product of the transmitted signal. At long ranges, the contrast ratio is limited by additive noise interference, while at near ranges the effect of the receiver channel spacing dominates the contrast ratio through along-track ambiguities. The use of this tool for analysis and prediction of image quality has significant implications for system design, mission planning, and data processing.

The tunable band of ladar which is limited by the velocity of the laser’s tunable component contradict with the pulse repetition frequency (PRF), in SAL. Meanwhile, the swath of SAL is small for the micro-meter wavelength. This paper gives a novel imaging mode of SAL, which combines multichannel technique and Scan mode SAL. This system makes full use of information of the space domain with multichannel, and removes the ambiguity of the frequency domain in the azimuth direction. And then, controls the beam to scan in different swathes for a better width of the imaging scene, which could lead to a reasonable resolution loss. In view of the ambiguity in Scan mode, SPECAN analysis is utilized to resolve the ambiguity. Finally, the simulation proves the validity of the given method.

Conventional space-time adaptive processing (STAP) technique can achieve perfect performance when applied to side-looking airborne radar (SLAR), where the clutter is relatively stationary, whereas it suffers significant degradation for non-SLAR due to severe range dependence, especially when range ambiguity is present. In theory, the range-dependent clutter is mainly located at the near range and can be eliminated via elevation non-adaptive or adaptive beamforming. However, the pure near-range clutter portion utilized for calculating elevation adaptive weights cannot be obtained due to range ambiguity in practice. In this paper, a novel method to adaptive extract the near-range clutter via sparsity-based technique is presented and the non-stationary clutter potion can be effectively eliminated by elevation adaptive beamforming. With this technique, the residual clutter becomes stationary, and thus, the corresponding STAP performance will be significantly improved.

This paper considers the problem of reconstructing true targets in a single-channel synthetic aperture radar (SAR) imaging system, which has been disturbed by deceptive jammings. Since the deceptive jammings are usually confined to the main lobe of an SAR antenna, their time–frequency distributions are different from those of the true echoes. This enables us to utilize a dynamic synthetic aperture (DSA) scheme to extract the characteristics of the true and false targets. Dictionaries about the true and false targets are constructed by taking interactions between scatterers into account. Then a sparsity-driven optimization problem is solved to reconstruct the true and false targets separately with super-resolution. Moreover, the deceptively jammed SAR data are divided into different areas to handle various scenarios efficiently, and strategies for DSA selection are addressed as well. Simulations are provided to verify the effectiveness of the proposed algorithm.

[EN] Multiple-input multiple-output (MIMO) radars have been shown to improve target detection for surveillance applications thanks to their proven high-performance properties. In this paper, the design, implementation, and results of a complete 3-D imaging frequency-modulated continuous-wave MIMO radar demonstrator are presented. The radar sensor working frequency range spans between 16 and 17 GHz, and the proposed solution is based on a 24-transmitter and 24-receiver MIMO radar architecture, implemented by timedivision multiplexing of the transmit signals. A modular approach based on conventional low-cost printed circuit boards is used for the transmit and receive systems. Using digital beamforming algorithms and radar processing techniques on the received signals, a high-resolution 3-D sensing of the range, azimuth, and elevation can be calculated. With the current antenna configuration, an angular resolution of 2.9° can be reached. Furthermore, by taking advantage of the 1-GHz bandwidth of the system, a range resolution of 0.5 m is achieved. The radio-frequency front-end, digital system and radar signal processing units are here presented. The medium-range surveillance potential and the high-resolution capabilities of the MIMO radar are proved with results in the form of radar images captured from the field measurements.

In this paper, the physical optics method is employed to study the problem of characteristics analysis and image processing for full-polarization synthetic aperture radar (SAR), where electromagnetic scattering from a flat horizontal perfect electric conducting (PEC) reflector is involved. The model of the full-polarization SAR echo from a small region of flat horizontal PEC reflector is deduced based on the dyadic Green’s function theory. With the available echo model, image processing and results are presented for the full-polarization SAR. For the horizontal co-polarization channel, the amplitude of imaging result descends with the squint angle, where the well-focused imaging result can always be obtained. On the contrary, the amplitude of cross-polarization channel imaging results is increased with the squint angle. When the radar works in the side-looking mode or low-squint mode, the cross-polarization channel echo cannot be well focused. For the high-squint mode, it can be well focused. For the horizontal co-polarization channel, the amplitude of well-focused result can be approximately regarded as a constant for all squint angles. The effectiveness of the characteristics analysis results is demonstrated via simulated and real measured Ka-band airborne full-polarization SAR data.

For circular trace scanning synthetic aperture radar (CTSSAR) with a circular track, the conventional hyperbolic equation becomes inadequate to express the range history of a point target accurately, and when it comes to the wide swath observation and imaging, the range variance makes it even harder to focus the target on the edge of the scene. Thus, an expression with high-order terms is needed to approximate the range history and the range variance should also be considered in the imaging algorithm. In this paper, based on the method of series reversion, a fourth-order approximated range model is established for the CTSSAR processing and the 2-D spectrum is derived for the echo signal in CTSSAR with circular trajectory. At the same time, in order to deal with the range-variant range cell migration problem in large-area CTSSAR imaging, a modified chirp scaling algorithm is proposed to realize precise wide swath CTSSAR focusing. Experiments and analyses are performed to validate the effectiveness of the proposed algorithm.

Two novel aspects of polarimetric calibration for fully polarimetric imaging radar systems are addressed. First, the radar system model is formulated in the context of two generic transmitter designs, either a single amplifier followed by a high-power switch or a low-power switch followed by two amplifiers. In the latter case, it is shown that a particular factorization of the polarimetric distortion matrix leads to a significant simplification of the cross-talk representation, from the standard four parameters to two reciprocal parameters, one for each of the antennas. Various system models from the literature are thus placed in a unified framework. Second, calibration techniques for circularly polarized antennas are derived, using either corner reflectors or clutter. However, where standard linear-basis algorithms estimate the cross-talk by its first-order distortion of reflection-symmetric clutter, no equivalent algorithm has been found for the circular basis; indeed, it is shown that the distortion caused, to first-order, by circular-basis cross-talk does not permit the individual cross-talk parameters to be identified. The calibration techniques are applied to fully polarimetric data acquired by the Ingara L-band radar using left- and right-polarized helical antennas.

A novel array antenna with a flat-shoulder shaped radiation pattern is proposed as the transmitting antenna for 77 GHz automotive radar application. When it is used in an automotive radar comprising one transmitter and multiple identical receivers, it can meet the demands of both long-range and medium-range detections without switching the operation mode back and forth between the long-range radar scenario and the medium-range radar scenario. The proposed antenna concept is fully introduced to explain its mechanism. An unconventional array was synthesized to achieve the desired flat-shoulder shaped radiation pattern. Prototype of the proposed array antenna was also designed according to the array synthesis result, which consists of several linear series-fed patch arrays and a substrate-integrated waveguide power and phase distributing network. Fabrication and measurement of the prototype array antenna were completed, showing a good agreement between the measured data and the synthesized as well as the simulation results, thereby validating the design.

The major impediment to accurate airborne repeat-pass differential synthetic aperture radar (SAR) interferometry (DInSAR) is compensating for aircraft motion caused by air turbulence. Various motion compensation (MoCo) procedures have been used in the airborne DInSAR processing to acquire reliable deformation mapping. In this paper, we present the use of time-domain backprojection (BP) algorithm for SAR focusing in an airborne DInSAR survey: No MoCo procedure is needed because the BP algorithm is inherently able to compensate for platform motion. In this study, we present the results of a pilot study aimed at demonstrating the feasibility of deformation mapping with an airborne SAR system based on the monitoring of the Slumgullion landslide in Colorado, USA between July 3 and 10 of 2015. The employed airborne SAR system is an Artemis SlimSAR that is a compact, modular, and multi-frequency radar system. Airborne light detection and ranging and global navigation satellite system (GNSS) observations, as well as spaceborne DInSAR results using COSMO-SkyMed (CSK) images, were used to verify the performance of the airborne SAR system. The surface velocities of the landslide derived from the airborne DInSAR observations showed good agreement with the GNSS and spaceborne DInSAR estimates. A three-dimensional deformation map of the Slumgullion landslide was also generated, which displayed distinct correlation between the landslide motion and topographic variation. This study shows that an inexpensive airborne L -band DInSAR system has the potential to measure centimeter level deformation with flexible temporal and spatial baselines.

The reflection symmetry metric (RSM) and product of multilook amplitudes (PMA) detectors, which were proposed recently, have been demonstrated to be promising methods for processing dual-polarimetric synthetic aperture radar (SAR) data for ship detection. The improvements in ship detection performance by using the RSM, compared to that using the PMA, are investigated in this paper. As the ship-sea contrast (or the signal-clutter-ratio, SCR) is a central index to assess the performance of a detection method, the SCRs in the RSM and PMA are first defined and compared. Next, a theoretical explanation for why the RSM outperforms the PMA in detection performance is provided. The detection performance is then characterized by calculating the receiver operating characteristic (ROC) curves. The preliminary experimental results performed on measured RADARSAT-2, ALOS-PALSAR, and NASA/JPL AIRSAR images verify the accuracy of the theoretical analysis.

Recent spotlight synthetic aperture radar analyses predict the two-dimensional range migration signature smears induced by targets with arbitrary motion in the ground plane. These investigations were limited to a constant-velocity radar motion with level flight path. The current correspondence removes this constraint by including the radar trajectory ascent angle.

The DIscrete stepped strip (DI2S) technique (actually patent pending) introduces an innovative method to use a synthetic aperture radar in time-sharing allowing the acquisition of different images either to increase azimuth resolution (DI2S-improved resolution) or to have a multi-image system improving the system capability and flexibility (DI2S multiswath). In this letter, the approach used by the DI2S multiswath technique will be described highlighting the main advantages in terms of performance and application.

Highly squinted synthetic aperture radar (SAR) data focusing is a challenging problem with difficulty to correct the severe range-azimuth coupling and motion errors. Squint minimization processing with the range-walk correction is widely adapted to simplify the decoupling processing, while it destructs the azimuth-shift invariance of conventional SAR transfer function. In this paper, a two-stage focusing algorithm (TSFA) is proposed to generate a focused imagery for the highly squinted airborne SAR. In the proposed algorithm, conventional range cell migration correction and azimuth matched filtering are performed and a fine focusing stage is established to correct the azimuth variance. In the fine focusing procedure, the coarse-focused image is divided into azimuth blocks to accommodate the correction of azimuth-variant residual range migration and phase terms. Moreover, precise motion compensation is embedded into the TSFA procedure to form an accurate airborne SAR imagery, which may be called the extended TSFA. In order to balance the processing precision and computational load, optimal selection of block size is investigated in detail. Both simulated and real measured airborne SAR data sets are used to validate the proposed approaches.

The polar format algorithm, while being more efficient than backprojection for synthetic aperture radar image formation, produces images with distortion and defocus. Since these errors are caused by uncompensated phase, a correction may be applied in postprocessing. Recent work utilizes a slow-time Taylor expansion of the differential range to derive the residual quadratic phase error for broadside linear flight paths. In this letter, we extend this approach to squinted linear flight paths and present new distortion and defocus corrections.

We propose a formulation for predicting the cross-range characteristics of the synthetic aperture radar (SAR) point-spread function (PSF) for extremely high squints based on the concept of phased array factors. We demonstrate the validity of this approach by comparing theoretical predictions to PSF dimensions extracted from SAR images. The results show that the proposed approach is more robust than the conventional theory for extreme high-squint scenarios.

One of the drawbacks of airborne interferometric synthetic aperture radar is a relatively narrow swath compared to analogous space based systems. Increasing the swath with side view of the interferometer can be possible by increasing the flight altitude and angle of sight. At the same time the height measurement accuracy decreases due to slant range distance increase. Another possible way of swath increasing is using sector scan. The efficiency of sector scan using in interferometric synthetic aperture radar is analyzed in this paper. The mathematical model and geometry of height measurement at a sector scan have been discussed. There was made an analysis of the effect of terrain height and observation angle on received signal phase changing. Observation angle changing is shown to contribute to the phase changing. Potential height accuracy measurement was calculated. The calculation results show that increasing the observation angle reduces height accuracy measurement. The maximum accuracy decrease is obtained at the observation angle of 90°. Despite height accuracy measurement decrease applying the sector scan allow to expand the swath. The accuracy decrease can be limited by selecting optimal parameters of scanning.

So far multi-antenna techniques have been used in Synthetic Aperture Radar (SAR) to track moving targets. These techniques carry out the tracking of moving targets in an imaging area, using a combination of the data received by two or several antennas. The aim of this paper is single-target tracking in SAR Spotlight imaging mode based on the promoted PHD filter. In most applications, target tracking in densely cluttered environment using radar system demands robust filtering so as to increase the tracking efficiency. Therefore, tracking of moving targets in the presence of high density clutters in environment, as the particular capability of the PHD filter, has turned it into a robust approach in SAR to track moving targets. Also as the simulation results show, using Range Cell Migration Compensation (RCMC) on SAR raw data before tracking, makes it possible to track a moving target with high quality.

We proposed a new architecture of antenna, transmitter and receiver feeding configuration for small synthetic aperture radar (SAR) that is compatible with 100kg class satellite. Promising applications are constellations of earth observations together with optical sensors, and responsive, disaster monitoring missions. The SAR antenna is a deployable, passive, honeycomb panel antenna with slot array that can be stowed compactly. RF (radio frequency) instruments are in a satellite body and RF signal is fed to a deployable antenna through non-contacting choke flanges at deployable hinges. This paper describes its development strategy and the present development status of the small spaceborne SAR based on this architecture.

In this paper we present the CLUtter SIMulator (CLUSIM), a special program simulating radar side echoes from rough planetary surfaces using realistic topography data sets. A numerical model of realistic topography of the Martian surface, based on Mars Orbiter Laser Altimeter data, is developed. A specially developed computer routine for evaluation of wide band radar echoes reflected from rough surfaces, capable of aperture synthesis simulation, is described. A synthetic radargram for a portion of Mars Express (MEX) orbit 9466 is computed and validated against experimental data obtained by the MARSIS radar instrument. Finally, a previously developed ionospheric phase correction procedure is numerically tested with new simulated echo signals. Impact of the surface clutter on the ionospheric correction procedure is investigated with a direct numerical comparison to a known benchmark result, which shows robustness of the correction algorithm with respect to the surface clutter.

For highly squinted synthetic aperture radar (SAR) imaging, the Omega-k algorithm is generally accepted as an effective focusing processor. However, the performance of the Omega-k algorithm may be degraded by the trajectory deviations due to the precision limitation of the navigation system in airborne SAR. In this paper, we focus on the autofocusing approach, which is compatible with the Omega-k imaging for highly squinted SAR. This new autofocusing approach can be viewed as an extension version of the conventional map-drift (MD) algorithm, which has been widely used in the broadside mode. Two-step process is performed in the proposed autofocusing method: 1) the range-independent phase-error is first retrieved by a squinted MD (SMD) estimator; and 2) a squinted range-dependent MD (SRDMD) estimator is proposed to derive the residual range-variant phase-error. By correcting the phase-errors obtained from SMD and SRDMD, the highly squinted SAR data can be finally focused. The effectiveness of the proposed algorithm has been demonstrated by both simulated and real-measured SAR data in highly squinted mode.

This paper deals with the issue of ground moving target imaging and motion parameter estimation with an airborne dual-channel circular stripmap synthetic aperture radar (CSSAR) system. Although several methods of ground moving target motion parameter estimation have been proposed for the conventional airborne linear stripmap SAR, they cannot be applied to airborne CSSAR because the range history of a ground moving target for airborne CSSAR is different than that for airborne linear stripmap SAR. In this paper, the moving target’s range history for airborne dual-channel CSSAR and the target signal model after the displaced phase center antenna processing are derived, and a new ground moving target imaging and motion parameter estimation algorithm is developed. In this algorithm, the estimation of baseband Doppler centroid and its compensation are first performed. Then focusing is implemented in the 2-D frequency domain via phase multiplication, and the target is focused in the SAR image without azimuth displacement due to the compensation of the Doppler shift caused by its motion. Finally, the target’s motion parameters are estimated with its Doppler parameters and its position in the SAR image. Numerical simulations are conducted to validate the derived range history and the performance of the proposed algorithm.

This paper investigates a new and special imaging mode, bistatic forward-looking high-speed maneuvering platform synthetic aperture radar, in which both platforms travel curvilinear trajectories with high velocities and accelerations. A thorough understanding of its new properties of motion, 2-D resolution, and echo signal coupling is given. Applicability of gradient method neglecting time dependence of gradient to this special configuration is addressed. Moreover, 2-D coupling characteristic of echo signal at different imaging distances is investigated.

Since the trajectory deviations of a radar platform cause serious phase errors that degrade the focusing quality of synthetic aperture radar (SAR) imagery, an autofocus method is very important for high-resolution airborne SAR imaging. In this letter, an extended autofocus backprojection (EABP) algorithm is developed to accommodate the phase errors. Under the criterion of maximum image sharpness, the traditional ABP algorithm supports a broader class of collection and imaging geometries. However, it neglects the influence of SAR image energy distribution on the estimation of phase errors that make it inapplicable for SAR imaging, which has high dynamic range, such as low-frequency SAR imaging. By choosing regions and balancing the energy distribution of the data, the EABP algorithm is more efficient and useful to avoid the estimation error caused by the unbalanced energy distribution. Its performance has been demonstrated by using the experimental data that are acquired by a P-band airborne SAR system with a low-accuracy global positioning system.

A stepped-frequency radar that allows for adaptability in the radiated spectrum while maintaining high-resolution radar imagery has been developed. The spectrally agile frequency-incrementing reconfigurable (SAFIRE) radar system is a vehicle-mounted, ground-penetrating radar that is capable of producing high-resolution radar imagery for the detection of obscured targets (either buried or concealed surface targets). It can be easily transitioned between forward- and side-looking orientations. The SAFIRE system is capable of precisely excising subbands within its operating bandwidth, thus making the system “spectrally agile.” It is also highly reconfigurable thereby allowing for on-the-fly adjustment of many of the system parameters. The spectrally agile and reconfigurable aspects of the SAFIRE radar together with its enhanced IF processing scheme represent a novel contribution to the state of the art. This paper discusses the system design, implementation, and performance characteristics, and also presents preliminary high-resolution imagery.

Multichannel synthetic aperture radar (SAR) signal reconstruction methods can effectively suppress azimuth ambiguities and achieve high-resolution wide-swath imaging. However, due to the characteristics of the practical antenna patterns, there exist non-bandlimited Doppler spectra, which will result in residual azimuth ambiguities, especially for strong targets. This letter presents a novel method for the suppression of the azimuth ambiguities of the strong point-like targets. First, we find out the positions of the strong point-like targets from the multichannel reconstructed SAR image. Then, we locate the ambiguous range history of each strong point-like target. Finally, the ambiguous components in the range history are filtered out by an orthogonal projection method. Therefore, the spectra of the strong point-like targets will be converted into the bandlimited spectra, and then, the azimuth ambiguities can be effectively suppressed by the conventional multichannel SAR signal reconstruction methods. Theoretical analysis and experiments demonstrate the feasibility of the proposed methods.

To maintain sufficient signal-to-noise ratio (SNR) for image reconstruction and image interpretation, conventional synthetic aperture radar (SAR) systems must trade off resolution and scene size. This paper proposes a new SAR mode of operation, which improves resolution while maintaining good SNR and a large scene size. It leverages the unique properties of dynamic metasurface antennas (MSAs) to subsample a large virtual beamwidth utilizing multiple small distinct antenna beams. Due to this parallelization in scene sampling, the constraints on the azimuth sampling rate can be relaxed while maintaining an aliasing-free cross range. Due to the versatile properties of MSAs and their cost effective manufacturing process, this paper proposes SAR systems, which can obtain high resolution images over a wide scene size with lower cost and complexity than competing approaches. Point-spread functions and proof-of-concept SAR simulations are shown to verify this approach. In addition, laboratory experiments using a commercial prototype MSA are presented, which show an improvement of 62% in cross-range resolution of the proposed approach, compared with the cross-range resolution of stripmap mode SAR with the same aperture.

The rising population of space debris poses an enormous threat to all space vehicles, including space shuttles and other spacecraft with operators aboard; and their detection, tracking, and identification are of great importance. Using the spinning motion and the translational motion component that is parallel to its major axis, we have established a 3-D spiral synthetic aperture radar (SAR) imaging geometry and its corresponding signal model for space debris in this letter. Spatial offsets and coupling in the along-track and altitude directions, caused by the spiral motion of the radar, are compensated in the frequency domain. With these compensation procedures, the spiral SAR is simplified as a cylindrical scan mode. Then, a 3-D wavenumber domain algorithm is proposed to realize coherent imaging. The real data processing results are presented to demonstrate the effectiveness of the proposed algorithm for imaging space debris.

In a synthetic aperture radar (SAR) system, the radar platform may move with a curved trajectory due to the existence of vertical velocity and acceleration, which may result in the failure of the conventional imaging methods. In order to deal with this problem, this paper proposes an improved wavenumber-domain imaging algorithm for high-resolution-high-squint SAR with a curved trajectory. It mainly includes three aspects. First, a modified equivalent range model for a curved trajectory is derived. Second, an improved wavenumber domain imaging algorithm based on the proposed range model is analyzed in detail. Finally, the imaging distortion caused by vertical velocity and acceleration is corrected via geometry and inverse projection. Simulated results and Ku-band real SAR data processing are used to validate the proposed model and imaging algorithm.

Automatic target recognition (ATR) is the process where computer algorithms are used to detect and classify objects or regions of interest in sensor data [1]. ATR algorithms have been developed for a broad range of sensors, including electro-optic (EO), infrared (IR), and microwave sensors (radar). A key advantage of microwave radar is its utility for all weather and time of day compared with other types of sensors. One limitation of many radar systems' ATR performance is the poor spatial resolution in the range and cross-range dimensions. While the range resolution of the microwave radar system can be straightforwardly improved by increasing the system's radio frequency bandwidth, superior cross-range (or azimuth) resolution requires an increase in antenna size. In order to achieve resolution similar to EO or IR systems, an enormous antenna must be used for transmission and reception, hindering microwave radar system's general applicability to ATR problems. However, by operating a synthetic aperture radar (SAR), the large antenna requirement can be overcome, and very fine cross-range resolution can be obtained with a modest "real" aperture antenna. Indeed, SAR has been used to generate high-resolution 2-D or 3-D object images at a variety of different microwave frequencies. In particular, wideangle SAR images can contain important features of an object from a diverse set of observation angles collected by a radar system orbiting around a target or scene. These feature-rich wide-angle SAR images are useful for ATR due to their high resolution and comprehensive coverage of the target. Interference, jamming, or data dropouts, however, are commonplace in practical SAR environments, and result in an incomplete data set. These missing and corrupted data cause substantial degradation in the generated SAR imagery, hampering their utility for subsequent ATR processing especially when data-independent image formation algorithms are used. A prime example of these difficulties is operating a SAR system in the very high frequency (VHF) or ultrahigh frequency (UHF) bands, where the spectrum is frequently crowded [2].

An antenna system for high resolution airborne synthetic aperture radar (SAR) is developed at X-band. Basic specifications of the antenna are obtained from the swath and azimuth footprint requirements of the SAR system. Series-fed printed antenna is developed to achieve these specifications. To meet stringent requirement of isolation between the transmitting and receiving antenna, an optimized ground plane with horn flares is designed. Complete antenna system is simulated and measured for return loss, mutual coupling and radiation patterns. Measured results are in agreement with the simulated results as well as the requirements of the system. Achieved isolation between the transmitting and receiving antennas is better than 64 dB at the operating frequency.

In shipborne inverse synthetic aperture radar (ISAR), not only the target, but also the radar ship will have nonstationary 3-dimensional rotations induced by the irregular sea wave. Thus, how to compensate the radar ship’s rotations is the key problem in reconstructing the shipborne ISAR image of ship target successfully. In this letter, a novel received signal model of shipborne ISAR is proposed. On the basis of the proposed model, the Doppler frequency induced by the rotations of the target and the radar ship can be separated from the received signal, individually. An analysis of the effects of the radar ship’s rotations on the obtained image is given. Error analysis and imaging simulation are done and the results verified the correctness of the proposed model and the effects of the radar ship’s rotations.

In very high resolution spaceborne-squinted sliding spotlight synthetic aperture radar, the traditional imaging algorithms based on the equivalent squint range model (ESRM) cannot be applied, because the ESRM model is inaccurate in this case. For this problem, this paper proposes a squint equivalent acceleration range model to precisely take into account the spaceborne-squinted curved orbit. Then a full-aperture squint-imaging algorithm is proposed based on this new range model, which can handle the azimuth variation of the equivalent velocity and the range variation of the 2-D frequency spectrum. The results of the simulation validate the effectiveness of new range model and imaging algorithm.

The ionospheric scintillation induced by local ionospheric plasma anomalies could lead to significant degradation for geosynchronous earth orbit synthetic aperture radar (SAR) imaging. As radar signals pass through the ionosphere with locally variational plasma density, the signal amplitude and phase fluctuations are induced, which principally affect the azimuthal pulse response function. In this paper, the compensation of signal amplitude and phase fluctuations is studied. First, space-variance problem of scintillation is addressed by image segmentation. Then, SPECAN imaging algorithm is adopted for each image segment, because it is computationally efficient for small imaging scene. Furthermore, an iterative algorithm based on entropy minimum is derived to jointly compensate the signal amplitude and phase fluctuations. Finally, a real SAR scene simulation is used to validate our proposed method, where both the simulated scintillation using phase screen technique and the real GPS-derived scintillation data are adopted to degrade the imaging quality.

Currently, images obtained with microwave synthetic aperture radars (SARs) are widely used for measuring directionality of ocean waves on a global and continuous scale. However, with all the advantages of the microwave SAR systems, the effectiveness of wave spectrum retrieval from SAR images is still debated. In this paper, we demonstrate how the directional spectrum of relatively long sea waves can be measured using aperture synthesis method in conjunction with relatively low radio frequencies (HF and VHF bands). This approach has advantages over HF ground-based radars used for ocean wave studies in coastal zones. In this paper, we theoretically evaluate this technique for the system mounted on aircraft. A favorable combination of the parameters for both ocean surface waves and HF electromagnetic waves allows an accurate analytical description of scattering from the sea surface based on the first approximation of the small perturbation method. In this case, scattered electromagnetic field becomes a linear functional of sea surface wave elevations. An analysis of the signal processing pertaining to this situation is presented, and a practical example is discussed. The proposed approach can be used even in the case when the radar platform is moving nonsteadily and/or along the curvilinear trajectory.

For a target illuminated by a high-resolution and wide-swath synthetic aperture radar system, the velocity is estimated based on sparse direction-of-arrival estimation. Then, the space-time adaptive processing algorithm is utilized to suppress the clutter and reconstruct the spectra of the target, followed by the traditional target imaging. The performance of the proposed algorithm is analyzed in detail. The proposed algorithm can retain the power of the moving target, and is free of searching.

Conventional synthetic aperture radar (SAR) ground moving target detection is only effective for targets with high radar cross section (RCS), and its performance is degraded by focusing distortion caused by the motion of the target. This letter proposes a new low-RCS moving target detection method which exploits the target’s shadow using high-resolution sequential images captured by a Ka-band video SAR system. First, the characteristics of target shadow are investigated, which are mainly influenced by the size of the target, the incidence of the radar beam, and the target velocity. Then the presented method based on video SAR system is described, and its moving target detection performance is also analyzed. As a preliminary study, a simulated experiment is conducted in which the signal of a low-RCS moving target is simulated and inserted into real airborne Ka-band video SAR images. The results demonstrate the feasibility of the proposed method for low-RCS moving target detection.

Due to the accuracy limitation of the navigation system, deviations between the real trajectory and the measured one appear inevitably in airborne synthetic aperture radar (SAR), which degrades the image quality dramatically. To improve the focusing performance, these trajectory deviations should be well estimated and compensated. In this paper, a data-based autofocus approach is proposed to correct the residual 3-D trajectory deviations. This new approach mainly contains two processing stages. The first stage is the local phase error estimation procedure involving small images autofocusing. A gradient function considering smoothness regularization is developed to efficiently achieve the sharpness-maximizing local phase error functions. In the second stage, the local phase error functions are combined to retrieve the residual 3-D trajectory deviations by a proposed weighted total least square method. This approach has been applied on highly squinted and large-swath airborne SAR raw data, respectively. Both real data experiments generate well-focused SAR images by the estimated trajectory parameters, and thus, validate the effectiveness of the proposed autofocus approach.

Inverse synthetic aperture radar (ISAR) imaging of a precessing target, which is a kind of fast spinning target, is faced with migration through range cell when using traditional imaging algorithms. Theory of compressed sensing (CS) suggests that exact recovery of an unknown sparse signal with an overwhelming probability can be achieved from very limited number of samples. A cycle shift smoothed L0 algorithm based on CS is proposed in this paper for high-resolution ISAR imaging of precessing targets by exploiting sparse apertures. A precessing cone-shaped target model is built and a 3-D reconstruction method based on multistatic ISAR is proposed. Simulations and electromagnetic computation verify the validity of the proposed method.