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This paper describes a software tool for performance analysis and parameter prediction for conventional and special SAR modes. Several aspects of design, performance analysis and parameter optimization for Spotlight and interferom...
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This paper describes a software tool for performance analysis and parameter prediction for conventional and special SAR modes. Several aspects of design, performance analysis and parameter optimization for Spotlight and interferometric SAR systems are examined. After the description of relevant principles the synthesis of antenna dimensions and efficient modeling of parabolic reflector antenna patterns are demonstrated. The influences of range migration and yaw steering on the PRF selection are shown along with a design example. The tool presented here was used on the Shuttle Radar Topography Mission (SRTM) flown on the Shuttler Endeavour for the X-SAR performance prediction in the preparation phase and was also used in near realtime during the mission.
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This letter addresses the efficient evaluation of Fourier-based kernels for synthetic aperture radar (SAR) image formation. The goal is to evaluate the quality of the focused impulse response function and the residual phase errors...
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This letter addresses the efficient evaluation of Fourier-based kernels for synthetic aperture radar (SAR) image formation. The goal is to evaluate the quality of the focused impulse response function and the residual phase errors of the kernel without having to implement the processor itself nor perform a costly point-target simulation followed by the processing. The proposed methodology is convenient for situations where the assumption of a hyperbolic range history does not hold anymore, and hence, a compact analytic expression of the point target spectrum is not available. Examples where the hyperbolic range history does not apply include very high resolution spaceborne SAR imaging or bistatic SAR imaging. The approach first numerically computes the 2-D spectrum of a point target and then uses the transfer function of the focusing kernel to match it. The spectral support is then computed to adapt the spectrum to the output imaging geometry, so that the impulse response function (IRF) is finally obtained. The proposed approach is valid under the assumption of a large time-bandwidth product, as is usually the case for current air- and spaceborne SAR sensors. The methodology is validated by comparing the matched IRFs with the ones obtained using point-target simulations.
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This paper presents the analysis and simulation results of a ground stationary passive synthetic aperture radar (SAR) receiver using a spaceborne SAR as an illuminator. In the paper, the bistatic geometry of passive SAR is present...
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This paper presents the analysis and simulation results of a ground stationary passive synthetic aperture radar (SAR) receiver using a spaceborne SAR as an illuminator. In the paper, the bistatic geometry of passive SAR is presented and the expected resolutions are discussed and compared to the monostatic case. Finally, the results of the signal processing for both active and passive SAR imaging are presented. The main goal of the analysis performed was to develop and test new passive SAR algorithms dedicated for the passive SAR receiver developed at the Warsaw University of Technology. The results obtained verify the feasibility of the proposed passive SAR imaging algorithm.
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The paper proposes a flexible and efficient wavenumber domain processing scheme suited for close formations of low earth orbiting (LEO) synthetic aperture radar (SAR) sensors hosted on micro-satellites or CubeSats. Such systems ai...
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The paper proposes a flexible and efficient wavenumber domain processing scheme suited for close formations of low earth orbiting (LEO) synthetic aperture radar (SAR) sensors hosted on micro-satellites or CubeSats. Such systems aim to generate a high-resolution image by combining data acquired by each sensor with a low pulse repetition frequency (PRF). This is usually performed by first merging the different channels in the wavenumber domain, followed by bulk focusing. In this paper, we reverse this paradigm by first upsampling and focusing each acquisition and then combining the focused images to form a high-resolution, unambiguous image. Such a procedure is suited to estimate and mitigate artifacts generated by incorrect positioning of the sensors. An efficient wave–number method is proposed to focus data by adequately coping with the orbit curvature. Two implementations are provided with different quality/efficiency. The image quality in phase preservation, resolution, sidelobes, and ambiguities suppression is evaluated by simulating both point and distributed scatterers. Finally, a demonstration of the capability to compensate for ambiguities due to a small across-track baseline between sensors is provided by simulating a realistic X-band multi-sensor acquisition starting from a stack of COSMO-SkyMed images.
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Transporter proteins are a vital interface between cells and their environment. In nutrient-limited environments, microbes with transporters that are effective at bringing substrates into their cells will gain a competitive advant...
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Transporter proteins are a vital interface between cells and their environment. In nutrient-limited environments, microbes with transporters that are effective at bringing substrates into their cells will gain a competitive advantage over variants with reduced transport function. Microbial ammonium transporters (Amt) bring ammonium into the cytoplasm from the surrounding periplasm space, but diagnosing Amt adaptations to low nutrient environments solely from sequence data has been elusive. Here, we report altered Amt sequence amino acid distribution from deep marine samples compared to variants sampled from shallow water in two important microbial lineages of the marine water column community—Marine Group I Archaea (Thermoproteota) and the uncultivated gammaproteobacterial lineage SAR86. This pattern indicates an evolutionary pressure towards an increasing dipole in Amt for these clades in deep ocean environments and is predicted to generate stronger electric fields facilitating ammonium acquisition. This pattern of increasing dipole charge with depth was not observed in lineages capable of accessing alternative nitrogen sources, including the abundant alphaproteobacterial clade SAR11. We speculate that competition for ammonium in the deep ocean drives transporter sequence evolution. The low concentration of ammonium in the deep ocean is therefore likely due to rapid uptake by Amts concurrent with decreasing nutrient flux.
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The Severe acute respiratory syndrome coronavirus (SARS-CoV) main protease (Mpro) cleaves two virion polyproteins (pp1a and pp1ab); this essential process represents an attractive target for the development of anti-SARS drugs. The...
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The Severe acute respiratory syndrome coronavirus (SARS-CoV) main protease (Mpro) cleaves two virion polyproteins (pp1a and pp1ab); this essential process represents an attractive target for the development of anti-SARS drugs. The functional unit of Mpro is a homodimer and each subunit contains a His41/Cys145 catalytic dyad. Large amounts of biochemical and structural information are available on Mpro; nevertheless, the mechanism by which monomeric Mpro is converted into a dimer during maturation still remains poorly understood. Previous studies have suggested that a C-terminal residue, Arg298, interacts with Ser123 of the other monomer in the dimer, and mutation of Arg298 results in a monomeric structure with a collapsed substrate-binding pocket. Interestingly, the R298A mutant of Mpro shows a reversible substrate-induced dimerization that is essential for catalysis. Here, the conformational change that occurs during substrate-induced dimerization is delineated by X-ray crystallography. A dimer with a mutual orientation of the monomers that differs from that of the wild-type protease is present in the asymmetric unit. The presence of a complete substrate-binding pocket and oxyanion hole in both protomers suggests that they are both catalytically active, while the two domain IIIs show minor reorganization. This structural information offers valuable insights into the molecular mechanism associated with substrate-induced dimerization and has important implications with respect to the maturation of the enzyme.
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