An Efficient Polyphase Filter Based Resampling Method for Unifying the PRFs in SAR Data

Variable and higher pulse repetition frequencies (PRFs) are increasingly being used to meet the stricter requirements and complexities of current airborne and spaceborne synthetic aperture radar (SAR) systems associated with higher resolution and wider area products. POLYPHASE, the proposed resampling scheme, downsamples and unifies variable PRFs within a single look complex (SLC) SAR acquisition and across a repeat pass sequence of acquisitions down to an effective lower PRF. A sparsity condition of the received SAR data ensures that the uniformly resampled data approximates the spectral properties of a decimated densely sampled version of the received SAR data. While experiments conducted with both synthetically generated and real airborne SAR data show that POLYPHASE retains comparable performance to the state-of-the-art BLUI scheme in image quality, a polyphase filter-based implementation of POLYPHASE offers significant computational savings for arbitrary (not necessarily periodic) input PRF variations, thus allowing fully on-board, in-place, and real-time implementation

Optimized Nonlinear PRI Variation Strategy Using Knowledge-Guided Genetic Algorithm for Staggered SAR Imaging

Staggered synthetic aperture radar (SAR), which operates with variable pulse repetition interval (PRI), staggers blind areas to solve the blind range problem caused by constant PRI in conventional high-resolution wide-swath SAR imaging. The PRI variation strategy determines the blind area distribution, and thus has a significant influence on the imaging performance in staggered mode. Generally, the existing strategies based on linear PRI variation can control the blind areas in a straightforward way, which has achieved impressive results. However, the linearity of the PRI variation imposes regularity or even periodicity on the locations of the blind areas, which limits the distribution of the blind areas. The imaging performance has the potential to be further improved by introducing much more irregularity into the PRI sequences. To this end, this article proposes an optimized nonlinear PRI variation strategy for staggered SAR mode. First, a novel objective function is defined that quantitatively measures the uniformity of the blind area distribution along the slant range and the discontinuity of the blind area distribution along the azimuth. Subsequently, the optimum nonlinear PRI variation strategy is found using an optimization problem and the proposed objective function. A knowledge-guided genetic algorithm is proposed to solve the optimization problem. Comparisons with the existing linear variation strategies show that the proposed strategy can provide a superior imaging performance after reconstruction with a lower objective function value. Simulations and experiments on raw data generated in staggered SAR mode are performed to verify the effectiveness of the optimized nonlinear PRI variation strategy

Advanced Concepts for Ultra-Wide-Swath SAR Imaging

This paper reviews advanced multi-channel SAR system concepts for the imaging of ultra-wide swaths with high azimuth resolution. Novel system architectures and operational modes are introduced and compared to each other with regard to their performance

High-Temporal-Resolution High-Spatial-Resolution Spaceborne SAR Based on Continuously Varying PRF

Synthetic Aperture Radar (SAR) is a well-established and powerful imaging technique for acquiring high-spatial-resolution images of the Earth's surface. With the development of beam steering techniques, sliding spotlight and staring spotlight modes have been employed to support high-spatial-resolution applications. In addition to this strengthened high-spatial-resolution and wide-swath capability, high-temporal-resolution (short repeat-observation interval) represents a key capability for numerous applications. However, conventional SAR systems are limited in that the same patch can only be illuminated for several seconds within a single pass. This paper considers a novel high-squint-angle system intended to acquire high-spatial-resolution spaceborne SAR images with repeat-observation intervals varying from tens of seconds to several minutes within a single pass. However, an exponentially increased range cell migration would arise and lead to a conflict between the receive window and 'blind ranges'. An efficient data acquisition technique for high-temporal-resolution, high-spatial-resolution and high-squint-angle spaceborne SAR, in which the pulse repetition frequency (PRF) is continuously varied according to the changing slant range, is presented in this paper. This technique allows echo data to remain in the receive window instead of conflicting with the transmitted pulse or nadir echo. Considering the precision of hardware, a compromise and practical strategy is also proposed. Furthermore, a detailed performance analysis of range ambiguities is provided with respect to parameters of TerraSAR-X. For strong point-like targets, the range ambiguity of this technique would be better than that of uniform PRF technique. For this innovative technique, a resampling strategy and modified imaging algorithm have been developed to handle the non-uniformly sampled echo data. Simulations are performed to validate the efficiency of the proposed technique and the associated imaging algorithm

A novel generation of 3D SAR-based passive micromixer: efficient mixing and low pressure drop at low Reynolds number

Abstract This study introduces a novel generation of 3D splitting and recombination (SAR) passive micromixer with microstructures placed on the top and bottom floors of microchannels called a ‘chain mixer’. Both experimental verification and numerical analysis of the flow structure of this type of passive micromixer have been performed to evaluate the mixing performance and pressure drop of the microchannel, respectively. We propose here two types of chain mixer—chain 1 and chain 2—and compare their mixing performance and pressure drop with other micromixers, T-, O- and tear-drop micromixers. Experimental tests carried out in the laminar flow regime with a low Reynolds number range, 0.083 Re 4.166, and image-based techniques are used to evaluate the mixing efficiency. Also, the computational fluid dynamics code, ANSYS FLUENT-13.0 has been used to analyze the flow and pressure drop in the microchannel. Experimental results show that the chain and tear-drop mixer’s efficiency is very high because of the SAR process: specifically, an efficiency of up to 98% can be achieved at the tested Reynolds number. The results also show that chain mixers have a lower required pressure drop in comparison with a tear-drop micromixer

Study and Development of New Passive Micromixers Based on Split and Recombination Principle

Micromixers have been a major topic of research in the past decade and progress on recent development of micromixers has been reviewed by many researchers. Developing devices for microfluidic technology has been a major concern for industry and microfluidic devices offer many advantages over conventional techniques. Compared to conventional macroscopic methods, microfluidic devices have the advantages of reduced solvent, reagent and cell consumption, shorter reaction times, portability, low cost and low power consumption. Also, micromixers are key elements in microfluidic technology and have been addressed by a large body of research. Interestingly, the historical development of microfluidics and its preoccupation with micromixing are the main fields of microtechnology. Micromixers have a wide variety of potential applications in industry. In modern technology, micromixers are applied in microtechnologies such as biological systems, as microreactors for chemical reactions, and as MEMS and lab-on-a-chip devices. This means that the community of engineers and scientist now engaged in microfluidic devices and also mixing process in micro scales. Indeed, they have entered the field from a variety of different backgrounds and they would have been confronted by the problems of mixing processing and mass transport at the micro scales. According to the survey carried out in my research, the main driving forces for this investigation are applications in incompressible mixing processing at low Reynolds number range, 0.08<Re<4.16. As far as we know, the technology and science of microfluidics cover a wide spectrum ranging from fundamental studies to real applications in laboratories and industries. This research focuses on an important subject of microfluidics, namely mixing processing at the microscale. The science of such mixing has carried out on newly fabricated micro scale devices on an extensive collection of established knowledge. Due to its applied nature, my research discuss practical outcome in the design and characterization of micromixers. In this thesis, first and foremost, I describe the method that I've used for analyzing the experimental data. The laminar flow regime (0.08<Re<4.16) was considered during tests and image-based techniques are used to evaluate mixing efficiency. This study propose a novel generation of 3D splitting and recombination passive micromixers. Mixing characteristics of two species are elucidated via experimental and numerical studies associated with microchannels with various inlet flow rates (velocities) and results compared with the previous well-known micromixers. It was found that mixing performance is significantly affected by the split and recombination (SAR) flows and depends on Reynolds number (inlet velocities). As well as the efficiencies of my proposed mixer are almost quite the same with the well-known basic mixers at each desired region, the required pressure drop is approximately two times less than previous mixers. This is a good particular result that with higher efficiency the required pressure drop decreases. Hence, this new geometries satisfies both of targets in micromixer design which are higher mixing efficiency and lower pressure drop in comparison with previous well-known mixers. These results open the new operating windows for rapid mixing in the microchannel to overcome the fluid mixing which strongly limited to laminar regime with lower required pressure dro

Earth-Abundant Tin Sulfide-Based Photocathodes for Solar Hydrogen Production.

Tin-based chalcogenide semiconductors, though attractive materials for photovoltaics, have to date exhibited poor performance and stability for photoelectrochemical applications. Here, a novel strategy is reported to improve performance and stability of tin monosulfide (SnS) nanoplatelet thin films for H2 production in acidic media without any use of sacrificial reagent. P-type SnS nanoplatelet films are coated with the n-CdS buffer layer and the TiO2 passivation layer to form type II heterojunction photocathodes. These photocathodes with subsequent deposition of Pt nanoparticles generate a photovoltage of 300 mV and a photocurrent density of 2.4 mA cm-2 at 0 V versus reversible hydrogen electrode (RHE) for water splitting under simulated visible-light illumination (λ &gt; 500 nm, Pin = 80 mW cm-2). The incident photon-to-current efficiency at 0 V versus RHE for H2 production reach a maximum of 12.7% at 575 nm with internal quantum efficiency of 13.8%. The faradaic efficiency for hydrogen evolution remains close to unity after 6000 s of illumination, confirming the robustness of the heterojunction for solar H2 production

On a Dual-Sequence Stripmap Imaging Mode as Alternative for High-Resolution Wide-Swath SAR

Future spaceborne synthetic aperture radar (SAR) systems are subject to conflicting high-resolution and wideswath imaging requirements. As several studies show, this fundamental conflict can be resolved by advanced instrument modes employing multiple receive channels in elevation and/or azimuth (MAPS). Currently, SAR system concepts include MAPS-ScanSAR systems and Staggered-SAR systems with Scan-on-Receive (SCORE). These show disadvantages: either scalloping or increased sampling rate/on-board complexity need to be tolerated. This paper discusses a method to achieve high-resolution imaging using complementary coverage of a wide swath with two (interleaved) constant-PRI sequences, motivated by the goal of avoiding Doppler spectral gaps but also limiting system-complexity