Synthetic Aperture Radar Image Formation Algorithm Research Articles

A Review of Synthetic-Aperture Radar Image Formation Algorithms and Implementations: A Computational Perspective

Designing synthetic-aperture radar image formation systems can be challenging due to the numerous options of algorithms and devices that can be used. There are many SAR image formation algorithms, such as backprojection, matched-filter, polar format, Range–Doppler and chirp scaling algorithms. Each algorithm presents its own advantages and disadvantages considering efficiency and image quality; thus, we aim to introduce some of the most common SAR image formation algorithms and compare them based on these two aspects. Depending on the requisites of each individual system and implementation, there are many device options to choose from, for instance, FPGAs, GPUs, CPUs, many-core CPUs, and microcontrollers. We present a review of the state of the art of SAR imaging systems implementations. We also compare such implementations in terms of power consumption, execution time, and image quality for the different algorithms used.

New SAR Algorithm for Sidelobe Reduction in Range direction

Sidelobes degrade radar performance by placing energy up and down range from its source. Range sidelobes associated with a large radar cross section (RCS) target may mask the presence of a smaller target located within a pulse width of the larger target; thus, “low” sidelobes are a desirable property. All the synthetic aperture radar (SAR) Sidelobe reduction algorithms which using matched filtering (windowing) like hamming window reduce the sidelobes on the expense of degrading the resolution. Our proposed algorithm using an optimum filter is reduced the sidelobes in range direction whilekeeping range resolution. In this paper, drive the formula for filter that reduce the sidelobe after linear frequency modulation matched filter then check the ambiguity function for proposed filter. Finally applied simulated and real raw data of SAR to proposal algorithm for SAR image formation and range Doppler algorithm and measure the quality of focused image.

Spectrum-Oriented FFBP Algorithm in Quasi-Polar Grid for SAR Imaging on Maneuvering Platform

In this letter, a new spectrum-oriented fast factorized backprojection (FFBP) algorithm is proposed for synthetic aperture radar (SAR) imaging on a maneuvering platform. Specifically, an analytical SAR image spectrum is derived in a novel quasi-polar coordinate system based on the FFBP, which makes it easy to incorporate with an autocalibration process for both systematic and nonsystematic errors. Different from the conventional FFBP algorithms developed in polar grid, the proposed algorithm devised in quasi-polar gird conducts the motion-induced phase error as a space-invariant component, which will definitely facilitate the phase autofocusing process during the FFBP recursions. Subsequently, a phase autofocusing process is incorporated in the resultant SAR image formation algorithm. Simulations and discussions are presented to show the focusing quality improvement made by the proposed algorithm.

A TOTAL QUALITY ASSESSMENT SOLUTION FOR SYNTHETIC APERTURE RADAR NLFM WAVEFORM GENERATION AND EVALUATION IN A COMPLEX RANDOM MEDIA

Abstract A Design, simulation and optimal selection of non-linear frequency modulation waveforms (NLFM) based on correlated ambiguity function (AF) quality analysis for the purpose of Synthetic Aperture Radar (SAR) is done in this article. The selected optimum CNLFM waveform in contribution with other waveforms are applied directly into a SAR image formation algorithm (IFA) and their quality metrics in comparison to other waveforms are derived and analyzed in a complex random media (CRM). The total quality performance analysis includes both the qualitative AF diagrams and the objective image quality metrics assessments. The simulation results not only verify the robustness of the proposed NLFM waveforms as a suitable alternative for LFM waveform but also introduce NLFM as a proper method of modulation for SAR in CRM.

Synthetic aperture radar imaging algorithm customized for programmable optronic processor in the application of full-scene synthetic aperture radar image formation

With the high programmability of a spatial light modulator (SLM), a newly developed synthetic aperture radar (SAR) optronic processor is capable of focusing SAR data with different parameters. The embedded SLM, encoding SAR data into light signal in the processor, has a limited loading resolution of 1920×1080. When the dimension of processed SAR data increases to tens of thousands in either range or azimuth direction, SAR data should be input and focused block by block. And then, part of the imaging results is mosaicked to offer a full-scene SAR image. In squint mode, however, Doppler centroid will shift signal spectrum in the azimuth direction and make phase filters, loaded by another SLM, unable to cover the entire signal spectrum. It brings about a poor imaging result. Meanwhile, the imaging result, shifted away from the center of light output, will cause difficulties in subsequent image mosaic. We present an SAR image formation algorithm designed to solve these problems when processing SAR data of a large volume in low-squint case. It could not only obtain high-quality imaging results, but also optimize the subsequent process of image mosaic with optimal system cost and efficiency. Experimental results validate the performance of this proposed algorithm in optical full-scene SAR imaging.

Spaceborne SIMO-SAR for three-dimensional ionospheric irregularity sounding

Ionospheric irregularity sounding is of significant importance in compensating for ionospheric scintillation effects on satellite communication, navigation, and remote sensing systems at lower frequencies, i.e., L band, P band, etc. A novel spaceborne nadir-looking single input and multiple outputs synthetic aperture radar (SAR) scheme is proposed for three-dimensional (3D) sounding of ionospheric irregularities. The 3D radar echo signal model is developed by examining the ionospheric effects on electromagnetic radar waveforms. These echoes can be used to produce 3D global maps of the distribution of ionospheric irregularities by means of a novel 3D SAR image formation algorithm. The effectiveness of the proposed SAR system was verified by means of computer simulations. The simulation results show that the proposed system has the capability to generate a 3D high-precision image of ionospheric irregularities with an altitude resolution as high as 0.9 km, an along-track resolution as high as 0.4 km, and an across-track angular resolution of 0.25.

COMPLETE COMPLEMENTARY SEQUENCE CODING WAVEFORM BASED AZIMUTH MULTI-CHANNEL SPACE-BORNE SAR WITH ULTRA-LOW RANGE SIDELOBE RATIO PERFORMANCE

Sidelobes of strong targets substantially impact image quality of synthetic aperture radar (SAR) using linear frequency modulation (LFM) waveform, especially in urban areas. A novel space-borne azimuth multi-channel SAR scheme with ultra-low range sidelobe-ratio (RSLR) performance was proposed, employing complete complementary sequence (CC-S) coding waveform. The CC-S waveform was utilized to acquire ultra-low RSLR performance in range direction. Azimuth multi-channel scheme was introduced, to compensate reduction of efiective PRF due to employing CC-S, and to mitigate azimuth resolution lost resulted from strong azimuth weighting, in order to implement low side-lobe performance in both range and azimuth direction. The method for pre-processing the CC- S based multi-channel SAR data was proposed, which would both compensate receiving time difierence of sub-sequences and reconstruct azimuth spectrum of multi-channel SAR data. Furthermore, the corresponding image formation algorithm for accurately focusing raw data of the SAR system was also proposed. Computer simulation results were presented, which demonstrated the validity of the proposed SAR scheme and image formation algorithm.

Efficient Time-Domain Image Formation with Precise Topography Accommodation for General Bistatic SAR Configurations

Due to the lack of an appropriate symmetry in the acquisition geometry, general bistatic synthetic aperture radar (SAR) cannot benefit from the two main properties of low-to-moderate resolution monostatic SAR: azimuth-invariance and topography-insensitivity. The precise accommodation of azimuth-variance and topography is a real challenge for efficent image formation algorithms working in the Fourier domain, but can be quite naturally handled by time-domain approaches. We present an efficient and practical implementation of a generalised bistatic SAR image formation algorithm with an accurate accommodation of these two effects. The algorithm has a common structure with the monostatic fast-factorised backprojection (FFBP), and is therefore based on subaperture processing. The images computed over the different subapertures are displayed in an advantageous elliptical coordinate system capable of incorporating the topographic information of the imaged scene in an analogous manner as topography-dependent monostatic SAR algorithms do. Analytical expressions for the Nyquist requirements using this coordinate system are derived. The overall discussion includes practical implementation hints and a realistic computational burden estimation. The algorithm is tested with both simulated and actual bistatic SAR data. The actual data correspond to the spaceborne-airborne experiment between TerraSAR-X and F-SAR performed in 2007 and to the DLR-ONERA airborne experiment carried out in 2003. The presented approach proves its suitability for the precise SAR focussing of the data acquired in general bistatic configurations.

Omega-K Algorithm for Bistatic SAR With Arbitrary Geometry Configuration

Compared to the common monostatic synthetic aperture radar (SAR), the bistatic SAR data are more challenging to process because of their complicated flight geometry. Although several bistatic SAR image formation algorithms have been presented, most of them are only able to handle azimuth-invariant bistatic configuration where the transmitter and receiver platforms are moving in parallel tracks with identical velocities. This paper proposes an Omega-K algorithm to handle a general case of bistatic SAR data. The two-dimensional frequency spectrum expression derived by the method of series reversion is rewritten to a new form with explicit geometric significance. Based on this new frequency spectrum expression, the frequency mapping function can be derived by making use of the multivariable Taylor's theorem. After data mapping operation, the coupling between the range and azimuth can be eliminated, and the final focused image can be obtained using FFT technology. Numerical simulations are performed to validate the proposed algorithm.

COMPARATIVE STUDY OF RMA AND PFA ON THEIR RESPONSES TO MOVING TARGET

The synthetic aperture radar (SAR) signatures of moving target are the basis of ground moving target indication and imaging (GMTI&Im) in the framework of SAR systems. However, previous studies are mainly based on the 2-D separable SAR processing, and little work has been done to investigate the signatures of moving target after the application of a particular flne resolution SAR image formation algorithm. In this paper, we derive the image spectrum of moving target after two representative flne resolution SAR image formation algorithms, i.e., the range migration algorithm (RMA) and polar format algorithm (PFA), respectively. Based on the spectrum derived, detailed analysis on the SAR signatures of moving target, including the geometric displacement, residual range migration, and defocusing efiect in both the range and azimuth dimensions are performed. The presented work might be helpful when considering a SAR system with the capability of ground moving target indication and imaging (GMTI&Im).

Multistatic Synthetic Aperture Radar Image Formation

In this paper, we consider a multistatic synthetic aperture radar (SAR) imaging scenario where a swarm of airborne antennas, some of which are transmitting, receiving or both, are traversing arbitrary flight trajectories and transmitting arbitrary waveforms without any form of multiplexing. The received signal at each receiving antenna may be interfered by the scattered signal due to multiple transmitters and additive thermal noise at the receiver. In this scenario, standard bistatic SAR image reconstruction algorithms result in artifacts in reconstructed images due to these interferences. In this paper, we use microlocal analysis in a statistical setting to develop a filtered-backprojection (FBP) type analytic image formation method that suppresses artifacts due to interference while preserving the location and orientation of edges of the scene in the reconstructed image. Our FBP-type algorithm exploits the second-order statistics of the target and noise to suppress the artifacts due to interference in a mean-square sense. We present numerical simulations to demonstrate the performance of our multistatic SAR image formation algorithm with the FBP-type bistatic SAR image reconstruction algorithm. While we mainly focus on radar applications, our image formation method is also applicable to other problems arising in fields such as acoustic, geophysical and medical imaging.

SIGNATURES OF MOVING TARGET IN POLAR FORMAT SPOTLIGHT SAR IMAGE

The synthetic aperture radar (SAR) signatures of moving target are the basis of ground moving target detection and imaging (GMTI&Im). However, previous studies are mainly based on the 2-D separable SAR processing, and little work has been done to investigate the signatures of moving target after the application of a particular fine resolution SAR image formation algorithm. In this paper, the spectrum of moving target after polar format algorithm (PFA) processing is derived. Based on this spectrum, detailed analysis on the SAR signatures of moving target, including the geometric displacement, residual range migration, and the defocusing effect in both range and azimuth dimensions are performed. Simulation results validate the theoretical analysis.

Born inversion of surface-scattered SAR (synthetic aperture radar) wave field

A new approach to synthetic aperture radar (SAR) digital image formation, based on inverse scattering theory, is derived as an alternative to the conventional method of imaging surface-scattered wave fields. The conventional image formation technique for high-resolution SAR data utilized azimuth compression using correlation in the range-Doppler domain. More recent approaches in SAR image formation algorithms exploit downward extrapolation of the wave field in the frequency–wavenumber (f–k) domain to perform not only the azimuth compression but also the range curvature correction at the same time, with improved quality of the final image. In this paper, imaging of the SAR wave field is formulated with the Born inversion approach, which includes a range-curvature-correction term that is valid at all ranges of image formation. This new inversion formula is established to exploit f–k domain computation, from which the complex backscattering coefficient, defined by the ratio of the backscattered wave field to the incident wave field, can be accurately estimated from the observed back-scattered wave field.