To address the multiple requirements of low cost, low power consumption, and high accuracy (sub-nanosecond level)for time-frequency systems in large-scale low Earth orbit (LEO) navigation constellations, a precision frequency control method based on micro-step adjustment is proposed for oven-controlled crystal oscillators (OCXOs), targeting the mitigation of long-term frequency drift. A quantization noise model was established, and a 48-bit direct digital synthesizer (DDS) was employed to achieve high-resolution real-time frequency correction at 5×10^-13 resolution. A constraint framework and frequency adjustment procedure compatible with rapid precise position services were constructed. Experimental results demonstrate that during 24-hour continuous operation, a clock bias accuracy (RMS) of 0.24 ns was achieved. Compared to free-running state, a long-term frequency stability was significantly improved by three orders of magnitude, with a slight degradation of 25% in short-term stability (1 s-20 s). This approach provides a cost-effective, high-precision, and stable time-frequency reference solution for LEO constellation construction.

With the development of electronic technology and motor PWM control technology, the electric servo system has gradually changed from analog system to digital system. Closed-loop control cycle is a key parameter in digital servo system, which is restricted by system maneuverability, hardware resources, power supply capacity, function performance and so on. At present,there are few literatures on the closed-loop cycle of servo system at home and abroad. This paper takes servo system of electric actuator as the research object. Firstly, the composition, working principle and signal transmission link of electric actuator servo system are analyzed in detail. Then the mathematical modeling of the servo system of the electric actuator is carried out, and the P-D control law is designed. Then the servo system simulation model is established based on Simulink to analyze the influence of different closed-loop control cycles on the servo system performance. Finally, an experimental platform is built to verify the effects of different closed-loop cycles on servo system performance. The simulation results show that the step response speed decreases with the increase of the closed-loop period. Appropriately increasing the closed-loop period can reduce the transient current to a certain extent,and avoid the risk of computer reset caused by large transient current pulling down the system voltage. The research in this paper has certain guiding significance for servo system and the platform system interacting with them.

This paper conducts an engineering feasibility study on the dielectric absorption coefficient test requirements stipulated by relevant fixed capacitor standards. The study includes equivalent circuit analysis and physical verification, achieving satisfactory results: confirming the reasonable feasibility of the single-branch model, which effectively simplifies circuit analysis. The residual voltage and current measured after capacitor charge/discharge cycles exhibit distinct peak characteristics, clearly reflecting the presence and variation pattern of the absorption effect. Analysis of the peak conditions readily reveals the necessity for test capacitors to have identical capacitance values, facilitating comparative grading. Expressions for the dielectric absorption coefficient include voltage-based and current-based types. High-input-impedance test instruments must be matched to the source impedance.

With the swift advancement of radar jamming techniques, the variety of active jamming types and the diversity of jamming strategies have surged, urging for accurate identification of jamming types. Conventional active jamming identification methods lack efficiency and universality. Meanwhile, current deep learning-based approaches are encumbered by large-scale parameters and the need for extensive data, which significantly limit their practical applications. To enhance recognition capabilities under conditions with limited parameters and data, a lightweight few-shot radar active jamming identification method based on multi-modality fusion is proposed. Lightweight fusion is achieved by leveraging the temporal locality of time-frequency features and the high-resolution range profile features. Additionally, few-shot classification performance is improved through exploiting metric learning and feature retrieval techniques. Experiments conducted on both simulated and measured datasets demonstrate the superior performance of the proposed method under a variety of conditions.

Due to the impact of global climate change, extreme weather events have become more frequent in recent years, with a corresponding increase in both the frequency and intensity of typhoons. As typhoons have extremely strong destructive power, they can have a significant impact on economic and social development, human life and property, as well as maritime activities. Therefore, the real-time tracking and positioning of typhoons is critical for mitigating their adverse impacts. Based on the infrared cloud image data from the multi-channel scanning imaging radiometer aboard the Fengyun-4 geostationary satellite, combined with the tropical cyclone best track dataset, this study uses the YOLOv8 object detection algorithm to achieve automatic identification and rapid positioning of typhoons. The verification results show that the recognition accuracy for typhoons with strong tropical storm intensity and above exceeds 83%, with a precision rate of over 88%. This achievement provides robust data support for maritime activities, maritime transportation, and oceanographic research, effectively improving the accuracy and timeliness of typhoon monitoring and enhancing safety in related fields.

The radial acceleration between the target and the observation platform causes Doppler frequency changes. High-precision estimation of Doppler change rate has broad application in the field of signal processing. To improve the processing gain,firstly, frequency domain accumulation is used within the pulse, the data for long time is diluted into limited spectral line data, then,the estimation of doppler rate of change is transformed into a least squares estimation through spectral correlation processing of interpulse. It is possible for inter pulse accumulation. The processing gain can be greatly improved, and the high estimation accuracy can be easily achieved by intra-pulse accumulation and multi-pulse least squares estimation. The difficulty of accurately estimating the frequency, time of arrival and initial phase is avoided by cleverly utilizing spectral correlation processing. This algorithm is insensitive to intra-pulse modulation and pulse repetition interval, and has strong universality. Finally, the validity, universality and high-precision estimation of the algorithm are verified through simulation in various scenarios.

The adaptive monopulse technology is a common method for phased array radar to achieve target detection and angle estimation. In order to solve the problem that the performance of the traditional adaptive beamforming algorithm is reduced and the monopulse angle estimation results are greatly deviated in the complex environment where the mainlobe and sidelobe interference coexists, a two-stage mainlobe and sidelobe interference suppression method of adaptive monopulse is designed. The first stage of adaptive processing uses the improved GSC structure to complete the suppression of sidelobe interference, and at the same time, the blocking matrix is constructed to protect the target signal and mainlobe interference from being cancelled in the suppression process of sidelobe interference, so as to improve the mainlobe maintenance effect remarkably. The second stage of adaptive processing uses the sum-difference four-channel mainlobe interference suppression to complete the directional suppression of mainlobe interference,that is, while the mainlobe interference in the elevation or azimuth direction is adaptively suppressed, the non-adaptive array pattern of the azimuth or elevation direction is maintained, and then the monopulse ratio maintains undistorted relative to the quiescent monopulse ratio. The simulation results demonstrate that the proposed method can effectively suppress the mainlobe and sidelobe interferences without distortion of monopulse ratio at the same time.

With the development of marine resources and the increasing demand for safety monitoring, the traditional underwater acoustic detection equipment has problems such as large size and difficult deployment. In order to meet the need of high precision underwater acoustic detection, a design method of seismic wave sensor based on micro-electro-mechanical system (MEMS)technology is proposed in this paper. By constructing the second-order dynamic model of the sensitive structure, the fully integrated closed-loop system control is realized by combining the ΣΔ modulator, and the behavior level modeling and non-ideal characteristic simulation optimization are completed by Simulink platform, and the chip integration is realized by 0.5 μm CMOS process. A fully differential precharge amplifier and a correlation double sampling circuit are designed. The test results show that the sensor sensitivity is up to 1.5V/g, the operating bandwidth is 300 Hz, the dynamic range is about 136 dB at 1 Hz bandwidth, the equivalent input noise is as low as 154 ng/√Hz, the nonlinearity is 0.065%, and the bias stability is about 28.2 μg, The FOM value is 204.5 μW·μg/Hz. The high-precision application potential of MEMS technology in the field of underwater acoustic detection is verified, and a miniaturized solution for seismic monitoring and marine environment perception is provided.

Considering the wide application and critical importance of satellite navigation systems, aiming at the requirements for precision and reliability of the next-generation BeiDou satellite navigation system, this study investigates the current development status of integrity technology from four dimensions: basic integrity of satellite navigation systems, integrity of satellite-based augmentation systems (SBAS), advanced receiver autonomous integrity monitoring (ARAIM), and precise point positioning (PPP)integrity, and compares the technical difficulties of each. Finally, it explores the development trends of future integrity technologies. This study is significant for the design and construction of the integrity system of the next-generation BeiDou satellite navigation system.

In order to improve the orbit determination accuracy of BDS satellites and solve the problem of limited construction of BDS ground stations, this paper proposes a solution using a LEO constellation as a space-based monitoring station. Two indicators were proposed to evaluate the monitoring performance of LEO constellations: Monitoring coverage factor and Satellite Position Dilution of Precision(SPDOP), and different configuration parameters of LEO constellations were optimized, including orbital altitude, number of orbital planes, orbital inclination angle, and number of satellites. An optimal LEO constellation was designed using the BDS-3 satellite as the monitoring object, and the monitoring capability of the LEO constellation was verified based on 30 tracking stations of the iGMAS system. The results show that the monitoring performance of the designed LEO constellation is superior to that of the iGMAS system with 30 tracking stations. The LEO constellation can achieve at least 6-fold of coverage over the entire arc of the BDS satellite with the minimum number of LEO satellites, with a good and stable geometric layout, which helps to improve the orbit determination accuracy of the BDS system and provide high-quality observation data for monitoring and evaluating the operating status and service performance of the BDS system.