Fast Backprojection Algorithms Research Articles

The Adaptive Streaming SAR Back-Projection Algorithm Based on Half-Precision in GPU

The back-projection (BP) algorithm is completely accurate in the imaging principle, but the computational complexity is extremely high. The single-precision arithmetic used in the traditional graphics processing unit (GPU) acceleration scheme has low throughput and its usage of the video memory is large. An adaptive asynchronous streaming scheme for the BP algorithm based on half-precision is proposed in this study, and then it is extended to the fast back-projection (FBP) algorithm. In this scheme, the adaptive loss factors selection strategy ensures the dynamic range of data, the asynchronous streaming structure ensures the efficiency of large scene imaging, and the mixed-precision data processing ensures the imaging quality. The schemes proposed in this paper are compared with BP, FBP, and fast factorized back-projection (FFBP) algorithms of single-precision in GPU. The experimental results show that the two half-precision acceleration schemes in this paper reduce the video memory usage to 74% and 59% of the single-precision schemes with guaranteed image quality. The efficiency improvements of the proposed schemes are almost one and 0.5 times greater than that of the corresponding single-precision scheme, and the advantage can be more obvious when dealing with large computations.

A Modified Cartesian Factorized Back-Projection Algorithm for Highly Squint Spotlight Synthetic Aperture Radar Imaging

Highly squint synthetic aperture radar (SAR) increases the flexibility and aspect-information of observation but poses several challenges to frequency-domain algorithms. The back-projection (BP) algorithm is recognized to produce the best results for highly squint mode but at high computational expense. Several fast BP algorithms have been developed to enhance the efficiency of the BP integral. The Cartesian factorized BP (CFBP) algorithm has been proposed to improve the performance. However, CFBP is ineffective in the highly squint case because its range model (RM) is imprecise. In this letter, we propose a modified CFBP (MCFBP) algorithm for highly squint spotlight SAR imaging. We employ a transformed Cartesian coordinate system to treat the transceiver mode as approximately side-looking mode. According to the transformed coordinate system, we propose a modified RM (MRM) to reduce the range error. Based on the MRM, we derive modified image spectrum compression steps and the Nyquist sampling rate of the subaperture image. MCFBP inherits CFBP's accuracy and efficiency advantages. Results of simulations and experiments performed by the X-band airborne SAR system with a maximum bandwidth of 1.2 GHz validate the improved performance of the proposed algorithm relative to BP and fast factorized BP.

An Autofocus Cartesian Factorized Backprojection Algorithm for Spotlight Synthetic Aperture Radar Imaging

A backprojection (BP) algorithm is recognized as an ideal method for high-resolution synthetic aperture radar (SAR) imaging. Several fast BP algorithms have been developed to enhance the efficiency of the BP integral. The Cartesian factorized BP (CFBP) algorithm is proposed recently to avoid massive interpolations and improve the performance. However, integrating autofocus techniques with the CFBP has not been discussed. In this letter, an autofocus CFBP algorithm is proposed to compatibly combine the autofocus processing within the CFBP. After modifying the spectrum compression step in the CFBP, the approximate Fourier transformation (FT) relationship between the modified compensated subaperture images and the corresponding range-compressed phase history data in the Cartesian coordinate is revealed. The phase error is obtained by the multiple aperture map drift method, and the singular value decomposition total least square method is combined to improve the estimate robustness. Employing the range blocking method, the range variance of the phase error is compensated. The proposed algorithm inherits the advantages of the CFBP. Experiments performed by the X-band airborne SAR system with a maximum bandwidth of 1.2 GHz validate the proposed approaches.

Fast Backprojection Algorithms Based on Subapertures and Local Polar Coordinates for General Bistatic Airborne SAR Systems

This paper introduces a bistatic fast backprojection synthetic aperture radar (SAR) algorithm that is available for different bistatic geometries. The proposed algorithm is tested with the bistatic CARABAS-like data, and the results indicate that the algorithm is a good candidate for bistatic SAR data processing. The image quality measurements are quite similar to the referenced values obtained with the bistatic global backprojection algorithm. That is, the peak sidelobe ratio (PSLR) is -13.7 dB in comparison to the referenced PSLR of -13.8 dB. The half-power beamwidths (HPBWs) measured on the cuts in x and y and the direction where the peak sidelobes locate are 2.8, 3.7, and 3.5 m, respectively, which are approximate to the referenced HPBWs. The small differences in the measured results mainly come from the interpolation step of the algorithm.

Synthetic Aperture Inversion for Statistically Nonstationary Target and Clutter Scenes

We introduce a class of nonstationary stochastic processes suitable for modeling nonstationary radar targets and clutter. This class of processes can be characterized by pseudodifferential operators driven by the Wiener process. We refer to these processes as pseudostationary processes and define a concept of space-varying spectral density function using the symbol of the underlying operators. We then derive an analytic filtered-backprojection- and backprojection-filtering-type formulas based on the minimum mean square error criterion for synthetic aperture image reconstruction. The inversion formulas depend on the space-varying spectral density function of target and clutter. We present an algorithm for the simultaneous estimation of target a priori information and reconstruction of SAR images. The algorithm is computationally efficient and can be implemented with the computational complexity of fast-backprojection algorithms. Extensive numerical simulations demonstrate the performance of the method.

A Fast BP Algorithm With Wavenumber Spectrum Fusion for High-Resolution Spotlight SAR Imaging

This letter presents the accelerated fast backprojection (AFBP) algorithm for high-resolution spotlight synthetic aperture radar (SAR) imaging. In conventional fast backprojection (FBP) algorithms, image-domain interpolation is employed in the subaperture (SA) fusion. However, in AFBP, by using a unified polar coordinate (UPC) system, the interpolation-based fusion is substituted by fusing the SA spectra in the wavenumber (WN) spectrum domain. The WN-domain SA fusion is efficiently implemented by fast Fourier transform and circular shifting. In this letter, an accurate impulse response function and the WN spectrum expression of the backprojected image in the UPC are explicitly derived, and furthermore, the implementations of AFBP are investigated in detail. Compared with conventional FBP algorithms, the AFBP can precisely focus on high-resolution SAR data with dramatically improved efficiency. Both simulation and real-measured data experiments validate the superiorities of AFBP by comparing it with the fast factorization backprojection (FFBP) algorithm.

Polar format algorithm for bistatic SAR

Matched filtering (MF) of phase history data is a mathematically ideal but computationally expensive approach to bistatic synthetic aperture radar (SAR) image formation. Fast backprojection algorithms (BPAs) for image formation have recently been shown to give improved O(N/sup 2/ log/sub 2/N) performance. An O(N/sup 2/ log/sub 2/N) bistatic polar format algorithm (PFA) based on a bistatic far-field assumption is derived. This algorithm is a generalization of the popular PFA for monostatic SAR image formation and is highly amenable to implementation with existing monostatic image formation processors. Limits on the size of an imaged scene, analogous to those in monostatic systems, are derived for the bistatic PFA.