National Time Service Center

The enhanced long-range navigation (eLoran) system serves as an important backup method for the global navigation satellite system (GNSS) system. In long-distance transmission scenarios, the signal propagation delay of the eLoran system is affected by fluctuations in meteorological factors along the path. Regarding these issues, such as the potential timing system errors caused by meteorological factors and the limitation on the accuracy of the timing system, in this paper, an innovative prediction model is proposed to predict the propagation delay data by fusing the propagation delay data of multiple differential reference stations on the path and the path-weighted meteorological data. By collecting and processing actual data, four types of prediction tasks were designed. Comparative analyses of the prediction performance of eight common models were conducted on a unified dataset. The results show that the Pucheng–Zhengzhou path-weighted ten-factor back-propagation neural network (PZWT-BPNN) model performs the best, achieving a balance between prediction accuracy and training efficiency. This model effectively suppresses the timing errors caused by meteorological fluctuations and improves the prediction accuracy of the propagation delay of the system, providing corresponding technical support for key fields such as low-altitude economy and transportation.

HighlightsWhat are the main findings?A Matched Correlation method integrating Notch Filtering (MC-NF) is proposed to effectively suppress random noise and in-band continuous wave interference.The Pattern Modulation combined version (PMC-NF) further improves demodulation performance by approximately 2.8 dB compared to MC-NF.What are the implications of the main findings?The proposed algorithms provide superior robustness and accuracy for eLoran systems operating in complex, high-interference environments.Demodulation is a key technology for the enhanced Loran (eLoran) system to achieve positioning and timing, and it affects the final performance of the system. Based on the traditional matched correlation algorithm, this paper proposes a new matched correlation demodulation method with notch processing. Furthermore, by combining it with the pattern modulation of the eLoran system, the matched correlation integrate notch demodulation method is further modified to improve demodulation performance. Firstly, the data link of the eLoran system is introduced in detail, including the encoding and modulation processes, the influencing factors of received signals, and the evaluation methods in the demodulation process. Secondly, on the basis of the principle of the matched correlation (MC) demodulation algorithm, a matched correlation demodulation algorithm integrating notch processing (MC-NF) and a demodulation correlation algorithm combined with modulation patterns (PMC-NF) are proposed. And, an analysis of the key factors affecting demodulation performance is given. Next, the demodulation performance of the mentioned algorithms under the conditions of random noise, skywave, and in-band continuous wave interference is calculated in detail. A large number of experimental results show that notch processing performs excellently in suppressing random noise and in-band continuous wave interference, and it can greatly improve the demodulation performance of the traditional matched correlation algorithm. Moreover, PMC-NF is superior to MC-NF; approximately 2.8 dB at decoding the critical point.

Magneto-electric (ME) antennas operating at very low frequency (VLF, 3–30[Formula: see text]kHz) are known as a new solution to communication applications. In actual tests, the clamping effect is a significant factor affecting the performance. In this study, an extended model is developed based on the modified Butterworth-Van Dyke (MBVD) model by incorporating multiple clamping methods into the ME antenna. The experimental results show that the proper clamping position and stress can significantly improve the antenna receiving and radiating performance, so that the peak magneto-electric coefficient (MEC) and the converse magneto-electric coefficient (CMEC) are increased by 5.223% and 33.91%, respectively. In addition, the resonant frequency of the ME antenna is modulated by the clamping effect. Under stress ranging from 0 to 25[Formula: see text]MPa, the operating frequencies for transmitting and receiving are tunable in the range of 20.31–21.79[Formula: see text]kHz and 19.22–20.66[Formula: see text]kHz. Compared to single-clamp studies, multi-clamp systems enable flexible adjustment of boundary conditions, which significantly broadens the tunable range of ME antenna bandwidth and systematically studies the dependence of ME antenna performance on clamping position and stress.

To enhance the denoising performance of satellite signals at a low signal-to-noise ratio (SNR), we propose an improved wavelet threshold denoising correlation (IWTDC) method that utilizes wavelet decomposition to analyze the received satellite signals. By dynamically selecting the threshold based on the transformation of the wavelet coefficients of each layer, we reconstruct the signal using processed coefficients for effective denoising. To validate the denoising effect of the proposed IWTDC method, we compare it with methods such as traditional cross-correlation, adaptive filtering correlation, and wavelet threshold correlation. Simulation results indicate that compared to other methods, SNR can be improved by up to 5 dB using the IWTDC method, leading to superior signal correlation and enhanced detection performance for the related peaks. Furthermore, experimental data collected during the study were consistent with the simulation results, confirming the performance of the IWTDC method. Thus, our proposed method can overcome the limitations of using traditional threshold functions through adaptive threshold selection, preserve signal details, and significantly improve the denoising performance.

ABSTRACT The BeiDou global satellite navigation system (BDS) is an important tool for precise remote time and frequency transfer. The current day-boundary discontinuity in the data process of the BDS-3 carrier phase (CP) time transfer limits the exploitation of the full potential of the BeiDou system, particularly over extended periods (e.g. >1 day). This study focused on the daily discontinuities in BDS-3 ground time and frequency transfer induced by the current satellite orbit and clock products from the individual Multi-GNSS experiment (MGEX) analysis center. An interpolation approach that bridges the daily jump of satellite products is proposed to improve the continuity of BDS-3 time and frequency transfer when using the CP technique. Only third-order polynomial is required to smooth and bridge the impact of the daily jump for medium earth orbit (MEO) and inclined geosynchronous satellite orbit (IGSO) satellite products. Eight MGEX network stations equipped with various types of receivers and antennas with dual-frequency BDS-3 signals were used to establish four time transfer links (i.e. BRUX – PTBB, AMC4–HOB2, LCK4–USUD, and ONS1–LCK3) and evaluate their effectiveness. These results reveal that the current BDS-3 satellite product discontinuity from individual analysis center easily caused the drift of phase residuals near the day-boundary epoch, with mean amplitudes of 0.081 m and 0.434 m for MEO and IGSO satellite products, respectively. The proposed approach can contribute to effectively eliminating this drift in time and frequency transfer. Furthermore, it achieved a noticeable reduction in the percentage of negative day-boundary discontinuities in BDS-3 ground time and frequency transfer results. The average values of the jump were 0.143 ns, 0.367 ns, 0.138 ns, and 0.131 ns for the BRUX – PTBB, AMC4–HOB2, LCK4–USUD, and ONS1–LCK3 time links, respectively, representing improvements of 11.4%, 40.5%, 4.5%, and 12.0%, respectively, compared with the traditional approach.

: Time Difference of Arrival (TDOA) estimation is a pivotal technique with extensive applications across various domains, including passive detection and indoor positioning. For signals characterized by unknown modulation types and originating from non-cooperative sources, achieving high-precision TDOA estimation traditionally necessitates a substantial volume of sampling data. Conventional approaches, such as cross-correlation, require high sampling rates and extended durations, posing significant challenges in terms of data acquisition, transmission, and storage. To surmount these obstacles, this paper delves into an enhanced inexact reconstruction-based compressed sensing method for TDOA estimation (EIRCS), presents an optimized algorithmic procedure that further reduces the computational complexity of the EIRCS method. Experimental results substantiate that the EIRCS method is an unbiased estimation technique. It accomplishes high-precision TDOA estimation concurrent with efficient data compression. These insights suggest that the EIRCS method can yield dependable TDOA estimates with minimal error, even at elevated compression ratios. Its capacity to sustain high accuracy across a range of compression ratios renders it particularly apt for applications demanding efficient data processing and reliable TDOA estimation.

Rubidium atomic fountain clocks represent the next generation atomic clocks for timekeeping. They offer a short-term frequency stability that is an order of magnitude higher than that of commercial cesium beam atomic clocks. In terms of frequency drift, they outperform hydrogen masers by an order of magnitude. What's more, rubidium atomic fountain clocks have achieved the highest weighting in the International Atomic Time. The five main types of noise that affect the short-term stability of rubidium atomic fountain clocks are quantum projection noise, photon shot noise, atomic number fluctuation noise, detection laser noise, and local oscillator noise. In this paper, the contributions of quantum projection noise, atomic number fluctuation noise, and local oscillator noise to the short-term frequency stability of the clock are measured experimentally.

To address the challenges of user site aggregation and noise accumulation interference in multi-user time synchronization, this study proposes an optical fiber time transfer system based on a star topology. The system distributes time and frequency signals to multiple slave users through a master terminal device combined with Time Division Multiple Access (TDMA) technology and optical splitters. Each slave site implements compensation within its time transfer equipment to synchronize its local time signal with the master reference signal. Through TDMA-based two-way time comparison, the system ensures the mutual independence of slave devices while eliminating multi-user signal noise accumulation. Experimental results show that all slave devices achieve pslevel stability and a time difference of less than 100 ps with no cross-interference. The root mean square (RMS) and standard deviation (STD) are less than 10 ps. This scheme provides a scalable solution and feasibility for distributed user nodes requiring high-precision time synchronization.