Ship detection is important for a wide range of applications. In this article, an airborne passive interferometric microwave sensor (PIMS) is proposed as a powerful complementary tool for ship detection. The fundamental of ship detection by the airborne PIMS is introduced. A specification called ``the detection factor'' is addressed to quantitatively assess the performance of the airborne PIMS for ship detection. The detection probability is discussed and addressed based on the constant false-alarm rate (CFAR). Numerical simulations are performed to demonstrate the feasibility of ship detection by the airborne PIMS. The impacts of the atmosphere and the sea wind on the performance of the airborne PIMS for ship detection are also addressed in terms of theoretical analyses and numerical simulations. The airborne experiments are also carried out by an X-band PIMS to verify the practicability of ship detection by the airborne PIMS in a sunny weather and a cloudy weather. The data processing procedure and a ship detection algorithm in the experiments are introduced. The airborne experimental results reveal that the ships can be effectively detected and tracked by the airborne X-band PIMS in a sunny weather and a cloudy weather. It demonstrates the practicability of ship detection by the airborne PIMS. The ship detection probability is also analyzed, which agrees well with the theoretical detection probability.

In this work, we propose and experimentally demon-strate a real-time and cost-efficient underwater wireless optical communication (UWOC) system based on field programmable gate arrays (FPGAs) for practical application. A three-stage cascaded T-bridge equalizer (TCBE) is designed to enlarge 3 dB bandwidth of the light-emitting diode (LED). Also, a digital waveform shaping filter (DWSF) is proposed to assist the TCBE circuit by shortening the rise and fall time of the received waveform and adjusting equalization parameters to flexibly satisfy diverse communication scenarios. The 3 dB bandwidth of UWOC system is extended from 2.5 MHz to 44 MHz via TCBE. With the proposed scheme, a data rate of at least 50 Mbps with a bit error rate (BER) of $\mathbf{8} \times \mathbf{1}\mathbf{0}^{{\bf - 5}}$ without forward error correction (FEC) code can be guaranteed over an underwater channel of 1 m. On the basis of TCBE, the DWSF is capable of further reducing the BER by 15%, 22%, 26%, 28%, 95%, and 98% for the data rates of 10 Mbps, 20 Mbps, 30 Mbps, 40 Mbps, 45 Mbps, and 50 Mbps, respectively. The results show that the method can remarkably improve the performance of UWOC system, especially when data rates are greater than 40 Mbps.

Gravitational wave (GW) detection is an important topic in fundamental scientific research. Studying high-precision capacitive displacement sensor (CDS) is crucial for capturing weak gravitational wave signals. However, existing research primarily focuses on noise analysis and optimization of the front-end and back-end circuits of CDS, with limited articles specifically studying the sensitive probe (SP). Therefore, this paper focuses on the modeling, analysis, and optimization of the SP in CDS. We establish a general model of the SP in COMSOL and extract the capacitance matrix to derive the displacement-capacitance curve between the displacement of each degree of freedom and the differential capacitance. By analyzing the capacitance matrix, we propose an equivalent circuit for the SP and verify its accuracy. Additionally, we optimize the structure of the SP and innovatively explore the impact of different electrode sizes on injection efficiency and induction efficiency. Overall, the works in this paper provide new perspectives for the design of high-precision CDS, and serve as a theoretical basis and reference for practical engineering design in GW detection.

Airborne passive interferometric microwave sensors (PIMSs) were proposed as a powerful complementary tool for ship detection, especially in low visibility and high sea clutter environments. At present, there is only one algorithm for ship detection by airborne PIMSs, and this algorithm is only based on the “cold” brightness temperature (TB) characteristic of metal ships with wakes in passive interferometric microwave images (PIMIs). Moreover, it has been found that ship wakes always exhibit a “hot” TB characteristic in PIMIs. In this article, an improved algorithm is innovatively proposed for ship detection by airborne PIMSs to improve detection performance, and it is simultaneously based on the “cold” and “hot” TB characteristics of metal ships with wakes. The microwave TB model of metal ships with wakes is established to analyze its TB characteristics. Given the TB model and its characteristics, an improved algorithm is introduced, which comprises five steps: searching targets, improving TB maps, clustering targets, estimating ship headings, and estimating ship trajectories. The improved algorithm can detect ships with a low false-alarm probability (FAR) and accurately estimate ship headings using only a single snapshot and the ship trajectory. The practicability and detection performance of the improved algorithm is verified by airborne experiments using an X-band PIMS. The experimental results demonstrate that the improved algorithm can accurately detect ships, estimate ship headings using only a single snapshot, and estimate ship trajectories. The impact of the algorithm parameters on the detection performance is analyzed and discussed. Moreover, the detection performance of the improved algorithm is analyzed and compared with the performance of the only existing algorithm, and the experimental results indicate that the improved algorithm has a better detection performance with a lower FAR value than the only existing algorithm, as expected.

Mirrored interferometric aperture synthesis (MIAS) is a novel interferometry that can improve spatial resolution compared with that of conventional IAS. In one-dimensional (1-D) MIAS, antenna array with low redundancy has the potential to achieve a high spatial resolution. This Letter presents a technique for the direct construction of low-redundancy linear arrays (LRLAs) in MIAS and derives two regular analytical patterns that can yield various LRLAs in short computation time. Moreover, for a better estimation of the observed scene, a bi-measurement method is proposed to handle the rank defect associated with the transmatrix of those LRLAs. The results of imaging simulation demonstrate the effectiveness of the proposed method.

In this paper, we demonstrate a real-time, multi-user downlink underwater wireless optical communication (UWOC) system for practical applications via field programmable gate arrays (FPGAs). The performance of the established UWOC system is experimentally investigated under diverse channel conditions. The established UWOC system utilizes arrayed light emitting diodes (LEDs) as the transmitter and employs optical superimposition-based non-orthogonal multiple access (NOMA). The feasibility of employing the arrayed LEDs as the transmitter under different channel conditions is validated by simulating the optical intensity distributions of LED groups. Both the simulation and experimental results reveal that the bit error rate (BER) of user 1 (higher power user) decreases with increasing power allocation ratio (PAR), while the BER of user 2 (lower power user) increases with the increase in PAR. Uniform post-equalization is employed to extend the bandwidth of each LED group. Besides, a metal-oxide-semiconductor field-effect transistor (MOSFET)-based driver circuit and a remaining carrier sweep-out circuit unit (CSCU) are proposed to facilitate the independent control over different LED groups and enhance the response speed of LEDs, which outperform bias-tee and the single MOSFET-based driver circuit. Experimental results also indicate that NOMA exhibits superior spectral efficiency compared to the conventional time division multiple access (TDMA). A PAR of approximately 2:1 is appropriate to ensure both users operate within the forward error correction (FEC) threshold. Based on the proposed schemes, the real-time UWOC system achieves a data rate of 40 Mbps for both users with BER below FEC limit under different channel conditions. The results highlight the significant potential of the designed UWOC system to effectively meet diverse real-time, multi-user UWOC application requirements.