Search Results (77)

As foundational infrastructure for spatiotemporal information, the Global Navigation Satellite System (GNSS) delivers high-precision positioning, navigation, and timing (PNT) services worldwide. However, satellite atomic clock drift causes satellite clock bias, degrading PNT service quality. Compared to post-processed clock bias products and real-time estimation, satellite clock bias prediction offers a key advantage: it provides high-precision real-time clock bias even in scenarios with limited real-time data or poor communication. Through analysis and summarization of error sources in prediction models, this paper proposed generalized modeling frameworks for both classical and AI-based approaches. We reviewed current research on classical mathematical models—including polynomial, grey, Kalman filter, and time series models—and AI-based models such as machine learning (ML), multilayer perceptron (MLP), recurrent neural networks (RNN), and Transformer architectures. Technical characteristics, applicability, and limitations of each model were discussed. While AI-based models demonstrate superior flexibility and adaptability in complex scenarios compared to classical approaches, they require extensive datasets and computational resources. In conclusion, we summarized the advantages, disadvantages, and future research directions, offering insights for developing next-generation real-time high-precision GNSS PNT services. Full article

This work proposes a 5G-based synchronized measurement method for urban distribution networks. First, downlink frequency synchronization is achieved by cross-correlating the Primary and Secondary Synchronization Signals (PSSs/SSSs) within gNB-broadcast Synchronization Signal Blocks (SSBs), enabling accurate alignment with the 5G system clock. Then, uplink phase synchronization is refined using Timing Advance (TA) feedback to compensate for propagation delays. Based on the recovered 5G Pulse Per Second (PPS) signal, a dynamic compensation algorithm is applied to discipline the SAR ADC sampling process. This algorithm tracks crystal oscillator drift, accumulates sub-cycle deviations, and corrects integer timer counts only when the error exceeds $\pm 0.5$. Simulations under a 228 MHz oscillator and 1200 samples per cycle demonstrate that the accumulated phase error remains below 0.00008°, satisfying IEEE C37.118 precision requirements. Compared with traditional GPS-based synchronization methods, the proposed solution offers greater deployment flexibility and can operate reliably in GPS-denied environments such as indoors and urban canyons. Full article

The rapid expansion of deep-sea sensing networks underscores the critical need for accurate underwater positioning of observation base stations. However, achieving precise acoustic localization, particularly at depths exceeding 4 km, remains a significant challenge due to systematic ranging errors, clock drift, and inaccuracies in sound speed modeling. This study proposes and validates a three-tier calibration framework consisting of a Dynamic Single-Difference (DSD) solver, a geometrically optimized reference buoy selection algorithm, and a Virtual Beacon (VB) depth inversion method based on sound speed profiles. Through simulations under varying noise conditions, the DSD method effectively mitigates common ranging and clock errors. The geometric reference optimization algorithm enhances the selection of optimal buoy layouts and reference points. At a depth of 4 km, the VB method improves vertical positioning accuracy by 15% compared to the DSD method alone, and nearly doubles vertical accuracy compared to traditional non-differential approaches. This research demonstrates that deep-sea underwater target calibration can be achieved without high-precision time synchronization and in the presence of fixed ranging errors. The proposed framework has the potential to lower technological barriers for large-scale deep-sea network deployments and provides a robust foundation for autonomous deep-sea exploration. Full article

After the interruption of the timing service, the increase in clock offset is a critical issue for the global navigation satellite system (GNSS)-disciplined oven-controlled crystal oscillator (OCXO). Current timekeeping methods for GNSS-disciplined OCXO have some drawbacks, such as high computational complexity, inadequate consideration of temperature effects, and insufficient separation of the impacts of temperature and aging. To address this issue, this study proposes a timekeeping method using a dual iterative algorithm. First, the external iteration separates the clock offset caused by temperature and aging. Then, the internal Gauss–Seidel iterative algorithm estimates the temperature and aging coefficients. During the timing service interruption phase, the model estimates and compensates for the frequency offset in real time using the coefficients. The proposed method demonstrates improved performance compared with OCXO in the free state and compensated by a second-order polynomial model, with better accuracy, drift rate, and long-term stability. The time offset is better than 4 μs over 24 h, representing an improvement of over 95% compared with the OCXO in the free state. Full article

The nocturnal, flightless camel crickets (Rhaphidophoridae) have a global distribution and are believed to have originated prior to the breakup of Pangea. We investigated the phylogeny and the timing of the radiation of East Asian species with mitogenomic data. Initially we analyzed a large taxon dataset (n = 117) using available partial mitochondrial and nuclear DNA sequences to confirm the monophyly of subfamilies and current taxonomy. Our findings support the monophyly of each genus within the subfamily Aemodogryllinae, with a minor inconsistency between taxonomy and phylogeny resolved by resurrection of the genus Gymnaeta Adelung. Fossil-calibrated molecular clock analysis used 11,124 bp alignment of 13 complete mitochondrial protein-coding genes for 20 species of Rhaphidophoridae, with a focus on the neglected Rhaphidophorinae and Aemodogryllinae lineages. Divergence time estimates suggest that the most recent common ancestor of the family lived during the Early Jurassic (189 Mya ± 23 Mya) before Pangea broke into the supercontinents or possibly during the early stage of breakup when Gondwana and Laurasia were still connected by land. The two subfamilies, Rhaphidophorinae and Aemodogryllinae, that overlap in Asia are estimated to have diverged 138 Mya ± 17 Mya, well before the Late Cretaceous northern connection between America and Asia (the Bering Land Bridge). Thus, our extended sampling of species from East Asia and Oceania refutes the importance of continental drift in the evolution of this wingless orthopteran family. Full article

Aging and cancer, though distinct biological processes, share overlapping molecular pathways, particularly in epigenetic regulation. Among these, DNA methylation is central to mediating gene expression, maintaining cellular identity, and regulating genome stability. This review explores how age-associated changes in DNA methylation, characterized by both global hypomethylation and focal hypermethylation, contribute to the emergence of cancer. We discuss mechanisms of DNA methylation drift, the development of epigenetic clocks, and the role of entropy and epigenetic mosaicism, in aging and tumorigenesis. Emphasis is placed on how stochastic methylation errors accumulate in aging cells and lead to epiallelic shifts and gene silencing, predisposing tissues to malignant transformation, even despite recently increased cancer incidences at younger ages. We also highlight the translational potential of DNA methylation-based biomarkers, and therapeutic targets, in age-related diseases. By framing cancer as a disease of accelerated epigenetic aging, this review offers a unifying perspective and calls for age-aware approaches to both basic research and clinical oncology. Full article

Two-photon vapor cell-based optical clocks are strong candidates for next-generation portable atomic standards, offering simplicity, compactness, and high performance. Their narrow clock transitions with counter-propagating beams enable first-order Doppler-free operation. However, systematic perturbations such as the AC Stark shift, temperature-induced shift, and drifts resulting from the laser system pose challenges cause instabilities to medium- to long-term performance. This paper provides a comprehensive overview of Rb two-photon vapor cell optical standards, focusing on the long-term performance-limiting effects and potential mitigation strategies, aiming for clock stabilities better than 1 × 10⁻¹⁵ over the averaging time of a day and beyond. Full article

The ESA DIVERGENCE (Diversity Architecture for Robust GNSS Receivers in Launcher Applications) project is focused on the design of a GNSS/INS hybrid navigation system and an appropriate FDIR/FDE algorithm for GNC applications in launchers and re-entry vehicles. The main goal is to demonstrate architecture robustness with respect to possible threats and weaknesses introduced by GNSS and INS technology. A baseline navigation system architecture has been developed through a sensor fusion algorithm, which combines IMU, GNSS/DGNSS, a radar altimeter, and a star sensor to cover the accuracy requirements for all the flight phases. The navigation system has been designed to be easily adaptable to multiple applications, such as expendable launch vehicles, micro-launchers, reusable first stage boosters and unmanned re-entry vehicles. The most critical threats/failures were considered for the development of the FDIR/FDE algorithm, comprising GNSS signal outages, spoofing, satellite/receiver clock bias/drift discontinuities, IMU failures, saturation, vibration rectification, coning and sculling, and INS software numerical failures. A preliminary description of the implemented robust FDIR/FDE techniques is reported, and an analysis is conducted to compare the performance before and after FDIR/FDE algorithm implementation in a representative launcher scenario. Full article

White Rabbit (WR) time synchronization has an accuracy up to a sub-nanosecond level. However, the current application scenario of WR is limited to wired transmission links. In this paper, we have proposed a time synchronization technique over a free space link using WR. In the WR-based free space synchronization scheme, we replace the original WDM (Wavelength Division Multiplexing) with single-wavelength transmission to reduce the asymmetry of the path and design a high-power optical transceiver module to improve the transmission power. With the scheme, a free space synchronization experiment with a transmission distance of 50 m is conducted. The experimental results show that the RMS (root mean square) time drift of this free space synchronization system is 20.5 ps over a 24 h period, and the TDEV (Time Deviation) of the time synchronization is 14.3 ps at 1 s and 3.9 ps at 20,000 s. The experiment proves that it will be convenient to complete the free space time synchronization network between clock sites with the proposed technique in the future application of complex environments. Full article

To address the limitations of Global Navigation Satellite Systems (GNSSs), such as vulnerability to electromagnetic interference and weak ground signal power, signal of opportunity (SOP) provided by low Earth orbit (LEO) satellites can serve as a backup positioning method. By simulating a LEO constellation, the impact of satellite visibility, Doppler geometric dilution of precision (DGDOP), and positioning accuracy was explored. Considering positioning errors such as satellite clock drift rate, ionospheric delay rate, tropospheric delay rate, and Earth rotation effects, the instantaneous positioning performance with satellite orbital errors and satellite velocity errors of different magnitudes was simulated. The results show that satellite visibility and DGDOP are negatively correlated. In a typical atmospheric environment with orbital errors of 10 m and satellite velocity errors of 0.1 m/s, positioning accuracy within 30 m can be achieved. This confirms that Doppler-based positioning with LEO satellites can be used as a backup method for GNSSs. Full article

This work presents a potentiometric readout circuit for a pH-sensing system in an oral healthcare device. For in vivo applications, noise, area, and power consumption of the readout electronics play critical roles. While CMOS amplifiers are commonly used in readout circuits for these applications, their applicability is limited due to non-deterministic noises such as flicker and thermal noise. To address these challenges, the Correlated Double Sampler (CDS) topology is widely employed as a sampled-data circuit for potentiometric readout, effectively eliminating DC offset and drift, thereby reducing overall noise. Therefore, this work introduces a novel potentiometric readout circuit realized with CDS and a switched-capacitor-based low-pass filter (SC-LPF) to enhance the noise characteristic of overall circuit. The proposed readout circuit is implemented in an integrated circuit using 0.18 µm CMOS process, which occupies an area of 990 µm × 216 µm. To validate the circuit performances, simulations were conducted with a 5 pF load and a 1 MHz input clock. The readout circuit operates with a supply voltage range ±1.65 V and linearly reproduces the pH sensor output of ±1.5 V. Noise measured with a 1 MHz sampling clock shows 0.683 µ$V_{rms}$, with a power consumption of 124.1 µW. Full article

Background/Objectives: HBV infections can lead to serious liver complications that can have fatal consequences. In 2022, around 1.1 million individuals died from HBV-related cirrhosis and hepatocellular carcinoma. Vaccines allow us to save more than 2.5 million lives each year; however, up to 10% of vaccinated individuals may not develop sufficient protective antibody levels. The aim of this study was to investigate the epigenetic drift in the response to HBV vaccine in isolated B cells. Methods: Epigenetic drift was measured by counting rare DNA methylation variants. These epivariants were detected in epigenome-wide data collected from isolated B cell samples from 41 responders and 30 non-responders (age range 22–62 years) to vaccination against HBV. Results: We found an accumulation of epivariants in the NR group, with a significant increase in hyper-methylated aberrations. We identified the chromosomes (1, 3, 11, 12, and 14) and genes (e.g., RUSC1_AS1 or TROVE2) particularly enriched in epivariants in NRs. The literature search and pathway analysis indicate that such genes are involved in the correct functioning of the immune system. Moreover, we observed a correlation between epigenetic drift and DNA methylation entropy in the male population of the cohort. Finally, we confirmed the correlation between epivariant loads and age-related epigenetic clocks. Conclusions: Our findings support the idea that an age-related derangement of the epigenetic architecture is involved in unresponsiveness to the HBV vaccine. Furthermore, the overall results highlight the interconnection between various epigenetic dynamics (such as drift, clocks, and entropy), although these interconnections seem not to be involved in the altered immunological activity. Full article

The transition from text to images as the primary form of information transmission has recently increased the need for secure and effective encryption techniques due to the expanding information dimensions. The color picture encryption algorithm utilizing chaotic mapping is limited by a small chaotic range, unstable chaotic state, and lengthy encryption duration. This study integrates the Ackley function and the Styblinski–Tang function into a novel two-dimensional hyperchaotic map for optimization testing. A randomness test is run on the chaotic sequence created by the system to check that the new chaotic system can better sustain the chaotic state. This study introduces two techniques, genetic recombination and clock diffusion, to simultaneously disperse and mix images at the bit level. This study utilizes chaotic sequences in genetic recombination and clock drift to propose an image encryption technique. The data indicates that the method demonstrates high encryption efficiency. At the same time, the key also successfully passed the NIST randomness test, verifying its sensitivity and randomness. The algorithm’s dependability has been demonstrated and can be utilized for color image encryption. Full article

This paper presents an advanced Controller Area Network (CAN) bus transceiver designed to enhance security using frame-level authentication with the concept of a nonphysical virtual auxiliary data channel. We describe the newly conceived transceiver security features and provide results concerning the design, implementation, fabrication and test of the transceiver to validate its functionality and robust operation in the presence of systemic error sources including Process, Voltage, and Temperature (PVT) variations. The virtual auxiliary channel integrates CAN frame authentication signatures into the primary data payload via phase modulation while also providing compatibility with existing CAN protocols, interoperability with non-enhanced systems and requiring no network or software modifications. Enhanced rail converters are designed to facilitate single-rail to dual-rail data conversion and vice versa, preserving phase information and minimizing phase errors across various nonideal effects such as frequency drift, Process, Voltage, and Temperature (PVT) variations, and cable phase mismatch. This ensures reliable data transmission and robust authentication in the presence of adversarial cyberattacks such as packet injection. The receiver recovers both the CAN frame data and the security signature, comparing the latter with an authorized signature to provide a real-time “GO/NO_GO” signal for verifying packet authenticity and without exceeding the CAN clock jitter specifications. Full article

Proper timing synchronization is important when data from sensors are acquired by different devices. This paper proposes a simple but effective solution for System on Chip (SoC) architectures that integrates a general-purpose Field Programmable Gate Array (FPGA) with a CPU. The proposed approach relies on a network synchronization protocol implemented in software, such as Network Time Protocol (NTP) or Precision Time Protocol (PTP), and uses the FPGA to generate a clock reference that is maintained in step with the synchronized system clock. The clock generated by the FPGA is obtained from the FPGA oscillator via appropriate fractional clock division. Clock drift is avoided via a software program that periodically compares the FPGA and the system counters, respectively, and adjusts the fractional clock divider in order to slightly adjust the FPGA clock frequency using a Proportional Integral controller. A specific implementation is presented on the RedPitaya platform, generating a 1 MHz clock in step with the NTP synchronized system clock. The presented system has been used in a distributed data acquisition system for fast transient recording in the neutral beam test facility for the ITER nuclear fusion experiment. Full article