This paper introduces a new concept of slow-time spatial frequency space, also known as "slow-time <inline-formula><tex-math notation="LaTeX">k</tex-math></inline-formula>-space," and redefines the ...associated cross-range bandwidth, which arises naturally from Doppler radar tomography. The slow-time <inline-formula><tex-math notation="LaTeX">k</tex-math></inline-formula>-space is the most suitable for narrow-band radar imaging, and can be augmented for higher resolution imaging by appropriate signal processing. The respective relationships with the more traditional concepts of fast-time <inline-formula><tex-math notation="LaTeX">k</tex-math></inline-formula>-space and signal (or range) bandwidth are explained. We also discuss image resolution limits and demonstrate the concepts with simulated data examples.
The problem of obtaining high range resolution (HRR) profiles for non-cooperative target recognition by coherently combining data from narrowband radars was investigated using sparse reconstruction ...techniques. If the radars concerned operate within different frequency bands, then this process increases the overall effective bandwidth and consequently enhances resolution. The case of unknown range offsets occurring between the radars' range profiles due to incorrect temporal and spatial synchronisation between the radars was considered, and the use of both pruned orthogonal matching pursuit and refined l 1 -norm regularisation solvers was explored to estimate the offsets between the radars' channels so as to attain the necessary coherence for combining their data. The proposed techniques were demonstrated and compared using simulated radar data.
Sparse signal representation is used to coherently separate the main rotor blade components of a helicopter from the composite data of complex time series radar returns. Received signals of this type ...typically consist of returns from the rotating main and tail rotor blades, the helicopter body, and other residual components which may overlap in time and frequency, thus making it difficult for conventional time-frequency separation techniques to be applied for the signal separation. In the proposed algorithm, a sparse signal representation is applied with the use of tunable Q wavelet transform to construct the dictionary, and basis pursuit denoising is used for the signal reconstruction of the component of interest. The algorithm shows very effective separation of the main rotor blade component from the composite radar returns, which is demonstrated using both simulated and real radar data at X-band.
This article demonstrates the novel approach of multi-bistatic Doppler radar tomography for high-resolution imaging of a generally non-cooperative target using narrowband signals. An imaging system ...consisting of a single narrowband transmitter and multiple bistatic receivers is considered. The signal processing is performed centrally. This technique requires the bistatic angles to be estimated with sufficient accuracy before the final image inversion step. Results from an experimental dataset are used to demonstrate the imaging algorithm, which reaffirm the widely held expectation that the spatial diversity of a multi-bistatic narrowband radar system can provide a system resource equivalent to a wide signal bandwidth.
This paper presents a new parameter-refined orthogonal matching pursuit method to tackle the dictionary mismatch problem arising in compressive radar imaging of a rotating target with off-grid ...scatterers. The main idea is to replace the linear least-squares projection step of the conventional orthogonal matching pursuit (OMP) method with a nonlinear least-squares procedure which jointly estimates the off-grid positions and reflectivities of true scatterers. Simulations are presented to corroborate the performance advantages of the proposed method.
Water purification by solar distillation has been deemed to be a green and promising approach for the production of potable water from undrinkable water resources. Carbonaceous materials have been ...well-known for the application of the sorption layer in the solar-to-steam system. Herein, both carbon spheres (CSs) and carbon dots (CDs) were synthesized by a one-pot hydrothermal method at 180 °C for 12h and they were used for constructing a closed water treatment system, in which CSs served as solar-thermal membranes in harvesting water and purifying Fe3+ ions in condensed water, while CDs played a role in the determination of Fe3+ ions in water. The evaporation rate of the CS membrane reaches 1.046 kg m−2 h−1 under 1 sun illumination and the efficiency treatment of Fe3+ is about 80%. The limit of detection obtained in the evaluation of Fe3+ ions in water by CDs was 3.61 μM. This work represents a significant step forward, as it opens the concept of constructing a closed water treatment system.
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•Carbon materials for constructing a closed water treatment system was provided.•Carbon spheres are used as solar absorber materials in solar to steam system.•Carbon dots are a good ability for selectively detecting Fe3+ in condensed water.•The finding of this work has a potential application in many fields.
This paper presents a new image focusing algorithm for sparsity-driven radar imaging of rotating targets. In the general formulation of off-grid scatterers, the sparse reconstruction algorithms may ...result in blurred and low-contrast images due to dictionary mismatch. Motivated by the natural clustering of atoms in the sparsity-based reconstructed images, the proposed algorithm first partitions the atoms into separate clusters, and then the true off-grid scatterers associated with each cluster are estimated. Being a post-processing technique, the proposed algorithm is computationally simple, while at the same time being capable of producing a sharp and correct-contrast image, and attaining a scatterer parameter estimation performance close to the Cramér⁻Rao lower bound. Numerical simulations are presented to corroborate the effectiveness of the proposed algorithm.
Doppler Radar Tomography (DRT) relies on spatial diversity from rotational motion of a target rather than spectral diversity from wide bandwidth signals. The slow-time
-space is a novel form of the ...spatial frequency space generated by the relative rotational motion of a target at a single radar frequency, which can be exploited for high-resolution target imaging by a narrowband radar with Doppler tomographic signal processing. This paper builds on a previously published work and demonstrates, with real experimental data, a unique and interesting characteristic of the slow-time
-space: it can be augmented and significantly enhance imaging resolution by signal processing. High resolution can reveal finer details in the image, providing more information to identify unknown targets detected by the radar.
This paper introduces a new approach to bistatic radar tomographic imaging based on the concept of compressive sensing and sparse reconstruction. The field of compressive sensing has established a ...mathematical framework which guarantees sparse solutions for under-determined linear inverse problems. In this paper, we present a new formulation for the bistatic radar tomography problem based on sparse inversion, moving away from the conventional
-space tomography approach. The proposed sparse inversion approach allows high-quality images of the target to be obtained from limited narrowband radar data. In particular, we exploit the use of the parameter-refined orthogonal matching pursuit (PROMP) algorithm to obtain a sparse solution for the sparse-based tomography formulation. A key important feature of the PROMP algorithm is that it is capable of tackling the dictionary mismatch problem arising from off-grid scatterers by perturbing the dictionary atoms and allowing them to go off the grid. Performance evaluation studies involving both simulated and real data are presented to demonstrate the performance advantage of the proposed sparsity-based tomography method over the conventional
-space tomography method.
•Carbon dots-wrapped NiCo flower-like nanostructures were synthesized.•The developed materials provided excellent stability during long-term performance.•DSC with Cdot/NiCo provided 8.27 % ...efficiency.
Replacing traditional counter electrodes (CEs) with Pt-free electrodes is one of the challenges for developing dye-sensitized solar cells (DSCs). Herein, we successfully synthesized carbon dots-wrapped NiCo flower-like nanostructure (NiCo/Cdots) and evaluated its application as CEs for DSCs. To characterize as-prepared materials, we conducted FESEM and TEM-EDS. Cyclic Voltammetry (CV), electrochemical Impedance Spectroscopy (EIS), Bode, and Tafel plots were then used to investigate the electrocatalytic properties of NiCo/Cdots CE. As a result, the as-prepared sample exhibited high electrocatalytic activity, low charger-transfer resistance, and large exchange current density. DSC based on NiCo/Cdots CE was assembled and showed a slightly lower PCE than Pt CE-based DSCs (8.27 and 8.52 %, respectively). However, it provided excellent stability during a long-term performance. These results indicated that CoNi/Cdots is a low-cost, easy-to-fabricate material with high potential as a CE of DSCs.
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