Clock Difference Research Articles

The investigation of real-time LEO timing via GPS PPP with different RT products

Low Earth Orbit (LEO) satellites mostly utilize crystal oscillators instead of high-performance atomic clocks mounted on medium to high-orbit satellites. Therefore, keeping high-precision time synchronization between LEO and GNSS satellites is the premise of realizing the high-precision application of LEO satellites. In this contribution, GPS precise point positioning (PPP) technology was employed to release precise timing for LEO satellites with different Real-time (RT) products. GRACE-FO C and GRACE-FO D satellites and five precise RT products from different analysis centers, which are CNES, GFZ, GMV, WHU, and CAS were chosen to achieve precise timing. The results indicated that the overall trend of clock difference fluctuations presented with all real-time products is generally consistent, fluctuating within the range of ± 4 ns. Furthermore, real-time precise products from GMV and GFZ exhibit significant fluctuations during the daily initial time period. Additionally, real-time products from WHU outperform other products and demonstrate greater stability. In terms of frequency stability, the clock stability of various analysis centers is generally consistent, with short-term stability (1280 s) reaching approximately 10−13 and long-term stability (10240 s) reaching around 10−14. WHU's timing stability is notably superior to other real-time products. In the time synchronization experiment, real-time products fluctuate within the range of ± 2 ns, while IGS final products fluctuate within ± 1 ns. In terms of time synchronization stability comparison, short-term stability (1280 s) still fluctuates around 10−13, and long-term stability (10240 s) fluctuates around 10−14. WHU exhibits relatively good overall stability, while GFZ shows comparatively lower overall stability.

Biological Clock Genes are Crucial and Promising Biomarkers for the Therapeutic Targets and Prognostic Assessment in Gastric Cancer.

Gastric cancer is one of the major public health problems worldwide. Circadian rhythm disturbances driven by circadian clock genes play a role in the development of cancer. However, whether circadian clock genes can serve as potential therapeutic targets and prognostic biomarkers for gastric cancer remains elusive. In this study, we comprehensively analyzed the potential relationship between circadian clock genes and gastric cancer using online bioinformatics databases such as GEPIA, cBioPortal, STRING, GeneMANIA, Metascape, TIMER, TRRUST, and GEDS. Biological clock genes are expressed differently in human tumors. Compared with normal tissues, only PER1, CLOCK, and TIMELESS expression differences were statistically significant in gastric cancer (p < 0.05). PER1 (p = 0.0169) and CLOCK (p = 0.0414) were associated with gastric cancer pathological stage (p < 0.05). Gastric cancer patients with high expression of PER1 (p = 0.0028) and NR1D1 (p = 0.016) had longer overall survival, while those with high expression of PER1 (p = 0.042) and NR1D1 (p = 0.016) had longer disease-free survival. The main function of the biological clock gene is related to the circadian rhythms and melatonin metabolism and effects. CLOCK, NPAS2, and KAT2B were key transcription factors for circadian clock genes. In addition, we also found important correlations between circadian clock genes and various immune cells in the gastric cancer microenvironment. This study may establish a new gastric cancer prognostic indicator based on the biological clock gene and develop new drugs for the treatment of gastric cancer using biological clock gene targets.

Research on establishing a joint time-scale of pulsar time and atomic time based on a wavelet analysis method

ABSTRACT Pulsar time has different physical mechanisms from atomic time, and these two time-scales are complementary in terms of stability performance. The joint timekeeping of pulsar and atomic time is currently an important application direction. Our research utilizes a wavelet analysis method to perform multiscale analysis of time signals in both the time and frequency domains, and assigns corresponding weights to different frequency bands for synthesis, which solves the problem of balancing long- and short-term stability in unified weighted synthesis. In this paper, a joint time-scale APT (atomic pulsar time) is obtained by a fusion of pulsar ensemble time (EPT) and atomic time TA(NTSC) (AT) based on wavelet analysis. The results show that APT effectively integrates the stability advantages of both EPT and AT, and the short-term stability of APT at 0.4 yr is consistent with that of AT, reaching 5.6 × 10−15 as evaluated by σz variance. The stability of APT remains consistent with EPT after 3.2 yr, and the long-term stability at 13.1 yr is close to 1.3 × 10−16, perfectly retaining the excellent long-term stability characteristics of pulsar time. The root mean square of APT’s clock difference relative to TT(BIPM) is much lower than that of TA(NTSC). Moreover, we propose ways to improve the stability of APT through experimental comparisons. The results indicate that APT has the potential to serve as a reference for steering UTC(NTSC) in the future, improving the long-term timekeeping capability of local atomic time in China, thereby comprehensively improving the stability and reliability of China’s time reference.

A Precise and Stable Space-Based Time System for Navigation in Smart Cities.

The high-accuracy and high-stability space-based time system is necessary for satellite navigation systems to achieve high quality of service (QoS) on navigation and positioning in smart city applications. This paper proposes a precise and high-stability space-based time system established under the autonomous time scale of navigation satellites. The generation, maintenance, and transfer of high-precision space-based time references are researched. A centralized time comparison method based on the ALGOS algorithm conducts the two-way time comparison of the inter-satellite link. Specifically, using the relative clock difference observations of all links between satellites for a certain period of time, the clock difference, clock speed, and clock drift parameters of n-1 stars in a constellation of n stars relative to the same reference can be estimated simultaneously. Simulations are conducted on real collected data from the Beidou navigation systems when providing services to smart cities around the world. The simulation results show the high accuracy and stability of the proposed space-based time system under the autonomous time scale reference. Moreover, the clock offset monitoring arc coverage is much higher than the satellite clock offset obtained by the direct observation of the satellite and the anchor station. It proves the efficiency of the proposed space-based time system to be used for satellite clock offset modeling and prediction.

A NavCom Signal Authentication Scheme Based on Twice Two-Way Satellite Time Transfer

Low Earth Orbit (LEO) satellite communication systems typically achieve identity authentication through the encryption and decryption of two-way information, which requires complex key management systems. In contrast, the integration of navigation and communication (NavCom) signals provides novel opportunities for physical observation and authentication solutions due to its measurement functions. This paper introduces a novel signal authentication scheme based on twice two-way satellite time transfer (TWSTT) for LEO satellite systems. It leverages the non-mutated nature of the clock difference to ascertain the legitimacy of the signal by measuring the clock difference of signals at different instances. Unlike traditional authentication methods, this approach directly exploits the temporal and spatial characteristics of the signal, negating the necessity for intricate authorization key systems. Additionally, it adeptly tackles the challenges posed by spoofing interference. The performance analysis indicates that this scheme can achieve a high detection probability for the repeater spoofing signal in the low carrier-to-noise ratio conditions.

Demonstration of 75 km-fiber quantum clock synchronization in quantum entanglement distribution network

The quantum entanglement distribution network, serviced as the communication infrastructure which distributes quantum information among remote users, enables many applications beyond the reach of classical networks. Recently, the applications such as quantum key distribution and quantum secure direct communication, have been successfully demonstrated in the quantum entanglement distribution network. In this article, we propose a multi-user round-trip quantum clock synchronization (QCS) scheme in the quantum network, which can be implemented with one single entangled photon source located at the server. The server distributes the entangled photons to remote multiple users with the wavelength division multiplexing strategy, and each user feeds partial received photons back to the server. The clock difference between the server and each user is calculated from the one-way and round-trip propagation times, which are determined according to the time correlation of entangled photons. Afterwards, the demonstration has been conducted between the server and a user over a 75-km-long fiber link, where the measured clock difference uncertainty is 4.45 ps, and the time deviation is 426 fs with an average time of 4000 s. Furthermore, the proposed QCS scheme is linearly scalable to many users, with respect to user hardware and number of deployed fibers.

Shallow Clock Tree Pre-Estimation for Designing Clock Tree Synthesizable Verilog RTLs

Clock tree synthesis (CTS) is an important process in determining overall chip timing and power consumption. The CTS is also a time-consuming process for checking the clock tree. If the chip design and specification do not match, the CTS result will be wrong. Many users use licensed electronic design automation (EDA) tools like Synopsys, and Cadence to carry out accurate chip verification. However, when using a licensed EDA tool, it is difficult to change the function and confirm the overall process in detail. If the design is wrong, the expected cost is doubled, as it will be necessary to modify the design and check all processes for verification. Currently, it cannot check the synthesizability of the clock tree on the placement and route process using only RTL. The main purpose of this study is to predict the CTS result of pre-estimation roughly using an RTL source placing temporary logics using random buffer insertion before the route process: then the incorrectly designed part can be freely modified because the CTS result can be known in advance. Experimental results showed that this research achieves an increase in inserted buffer area by about 10%, the standard deviation of clock skew achieves zero clock skew after shallow CTS, and clock frequency increases by about 10%. This paper contributes to optimizing clock tree implementation by conducting the pre-route process before using the CTS tool. Also, our approach not only minimizes resource usage but also optimizes CTS for the RTL structure. It holds considerable value in enhancing the efficiency and performance of integrated circuits.

Evaluation of receiver calibration based on clock-steering for time–frequency transfer

Calibrating the global navigation satellite system (GNSS) time–frequency transfer system is crucial for accurate GNSS time–frequency transfer. The most commonly used relative calibration method relies on the common-clock to eliminate clock differences. However, it cannot eliminate the changes in clock signal reference point delay (REFDLY). The changes in REFDLY during calibration could not be ignored, especially for ordinary users in an unstable environment. To address this issue, we use the GNSS real-time single-difference time synchronization calibration based on clock-steering, which can detect the change in REFDLY. According to this method, the clock differences and REFDLY are eliminated simultaneously. We installed two GNSS antennas on the roof of the Xinghu Building of Wuhan University and designed a clock synchronization terminal to evaluate the performance of this method. The data were collected on Day of Year 152–156, 167–171, 222–226, 231–235, 244, 258, 260, and 262 in 2022. In accordance with the calibration results, the standard deviation better than 100 ps, and the stability reaches 10−15@10000s . Also, the time variance better than 60 ps, and the uncertainty uCAL better than 0.5 ns. Compared to the common-clock calibration, one of the most significant benefits is the elimination of REFDLY, which leads to decreased uncertainty and better stability over both the short-term and long-term.

Undifferenced precise time transfer with information‐constraints

AbstractSatellite navigation systems enable long‐distance and high‐precision time transfers. The authors proposed an undifferenced precise time transfer method with information constraints in this study. A more accurate receiver clock offset can be obtained by constraining the station coordinate parameters and receiver clock offset parameters to achieve a precise time transfer. In this example, the research object is the time transfer link established by the Global Positioning System observation station using a caesium clock, rubidium clock, and hydrogen clock. The clock difference sequence of the time‐transfer link was calculated by combining the observation data and precise products. Following analysis of the examples, the following conclusions were reached. The undifferenced precise time transfer method based on information constraints can effectively be used to smooth the clock difference sequence of the receiver and the clock difference sequence of the time transfer link. To some extent, this method can also be used to reflect the performance of atomic clocks at Global Positioning System stations and is better suited for the precise time transfer of the equal‐clock long‐baseline time transfer link. Simultaneously, the frequency stability of this method is improved significantly in the short term compared to the traditional scheme without any constraints. The improvement of frequency stability is helpful to improve the accuracy of time transfer.

Exploring Perinatal Biopsychosocial Factors and Epigenetic Age in 1-Year-Old Offspring

Background: Little is known about the determinants of epigenetic aging in pediatric populations. Methods: Epigenetic age was estimated from 258 1-year-olds, using pediatric buccal epigenetic and Horvath clocks. We explored associations between epigenetic age and maternal indicators of mental and relational health, substance use and general physical health assessed during trimester three. Results: Higher anxiety and stress, BMI and higher parent–parent relationship quality were associated with pediatric buccal epigenetic clock differences. High blood pressure during pregnancy was associated with Horvath age acceleration. Third-trimester smoking and pre-pregnancy weight were associated with acceleration and deceleration respectively, and concordant across clocks. Conclusion: A broad range of maternal factors may shape epigenetic age in infancy; further research is needed to explore the possible effects on health and development.

Inconsistent pseudorange biases in time links and their effect on BDS and GPS PPP frequency transfer

Precise point positioning (PPP) using the Global Navigation Satellite System (GNSS) phase and code measurements has become the primary technique for time and frequency comparisons. Several scholars have studied multi-GNSS PPP clock comparisons; however, the inconsistent pseudorange bias from receivers with different correlator spaces and front-end designs in pseudorange observations has not been considered. In this study, we analyze the characteristics of the inconsistent biases of the Global Positioning System (GPS) and BeiDou Navigation Satellite System (BDS) and their effects on PPP frequency transfer. The biases are confirmed to exist in receivers from Septentrio, Trimble, Leica and differ by manufacturer. To explicitly investigate the pseudorange bias effects of GPS and BDS on the PPP time and frequency comparisons, nine stations with receivers from three different manufacturers that could track the BDS-2 and BDS-3 signals were selected. Regarding PPP frequency transfer with inhomogeneous receivers, the Modified Allan Deviations (MDEVs) of the GPS and BDS PPP frequency stabilities were significantly optimized after using bias corrections. According to the receiver type classification strategy, the overall improvements in frequency transfer with Trimble-Septentrio and Trimble-Leica can be reached up to 31%, 29%, and 39%, and 38%, 29%, and 31% for GPS, BDS-2, and BDS-3, respectively. Moreover, the convergence time of the clock comparisons was shortened after bias corrections were applied. For GOPE-BRUX and GOPE-MATE links, the corrected cases yielded average clock difference stability improvements of 38%, 35%, and 53% and 35%, 35%, and 23% for GPS, BDS-2, and BDS-3, respectively. The results show that bias corrections are vital and allow for more stable time links for PPP frequency transfer.

Chronic HDM exposure shows time-of-day and sex-based differences in inflammatory response associated with lung circadian clock disruption

Circadian rhythms and sex differences are involved in the pathophysiology of asthma. Yet, there are no reports that simultaneously address the role of the circadian clock and sex-based differences in chronic house dust mite (HDM)-induced asthma. Here, we sought to determine if chronic HDM exposure during the resting phase (zeitgeber time: ZT0/6:00 a.m.) versus the active phase (ZT12/6:00 p.m.) differentially affects the circadian clock and alters asthma pathobiology in female and male mice. HDM exposure at ZT12 exaggerated infiltration of eosinophil subtypes and associated chemokines in females compared to males. Furthermore, HDM exposure augmented eosinophil chemokines, Th2 gene expression and cytokine release, and humoral immune response in females compared to males at ZT12. Concurrently, histopathological evaluation confirmed increased airway inflammation at ZT12 in both females and males. Overall, we showed a time-of-day response and sex-based differences in HDM-induced exaggerated asthmatic phenotypes (inflammation/remodeling) and circadian clock disruption in females compared to males.

IARMTS: Design of an Interference-Aware Routing Model with Time Synchronization Capabilities for Dense Wireless Sensor Network Deployments

Performance of dense wireless sensor networks is often degraded due to communication interference and time synchronization issues. Existing machine learning &amp; deep learning models that propose bioinspired &amp; pre-emptive packet-analysis solutions for these tasks either have high complexity, or high deployment costs. Moreover, these models cannot be scaled for heterogeneous node &amp; traffic types, which limits their applicability when applied to real-time scenarios. To overcome these issues, this text proposes design of an interference-aware routing model with time synchronization capabilities for dense wireless sensor network deployments. The network initially collects temporal clock states &amp; packet delivery performance of different nodes on heterogeneous traffic scenarios. These traffic patterns are converted into frequency, entropy, Gabor, and Wavelet components. The converted components are used to train an ensemble set of Naïve Bayes (NB), k Nearest Neighbour (kNN), Multilayer Perceptron (MLP), and Support Vector Machine (SVM) classifiers. These classifiers assist in identification of optimal clock deviations and set of routing paths. These routing paths are further fine-tuned via use of a Bacterial Foraging Optimization (BFO) Model, which assists in identification of interference-aware paths. The BFO Model uses a temporal fitness function that fuses throughput, communication delay, energy levels, and packet delivery performance for different set of contextual communications. Due to which, the model is able to showcase lower end-to-end delay, higher throughput, lower energy consumption, and higher packet delivery performance when compared with existing routing methods under high density nodes &amp; heterogeneous network scenarios. The model showcases 99% PDR, 18.3% lower delay, 19.5% higher energy efficiency and 10.4% lower delay levels when compared with existing methods.

Bi-directional PRN laser ranging and clock synchronization for TianQin mission

Laser frequency coupling noise due to unbalanced arm lengths is the dominant noise source for space-based gravitational wave detection. Time-delay interferometry is a proposed technique to eliminate the coupling noise by constructing a virtual equal-arm-length interferometer, where the real-time information of both absolute inter-satellite distance and clock synchronization is required. Inter-satellite laser ranging based on pseudo-random noise (PRN) sequence modulation has become the main technical means for extraction of the inter-satellite distance and clock difference information. This paper shows a bi-directional laser ranging scheme based on PRN code modulation and experimentally demonstrates its function by using an optical fiber of 10 km long. The measured results of the transmission time duration and the clock difference are compared with independent methods of pulse laser ranging and 1PPS signal measurement, respectively, both of which coincide well. It is also evaluated that the laser ranging accuracy is 1.2 m, and the clock synchronization accuracy reaches 6.7 ns, applicable to TianQin with the preliminarily determined requirement of 3 m and 10 ns. This laser ranging scheme can be applied to satellite navigation and space-based gravitational wave detection.

Phase Characterization and Correction in a Hardware Implementation of an OFDM-Based System for VLC Applications

Orthogonal frequency-division multiplexing (OFDM) systems have been widely implemented in guided and non-guided communications. Even though they are becoming a standard for popular technologies such as WiFi, their implementation in newly emerging ones is still in evolution. As the number of connected devices increases, the radio frequency (RF) channel gets crowded. Here is where visible light communication (VLC) links gain interest. VLC offers a wireless alternative to RF with a trade-off between the necessity of line of sight with the offer of a free, highly secure, and immune to electromagnetic interference communication channel. This paper proposes a programmable software implementation of an OFDM system using a novel phase correction technique for VLC channels. The new equation considers two types of phase shifts, one that will be non-deterministic (has a random variation) and another that will be deterministic and, therefore, can be estimated and corrected. This second can be split into two: one that comes from the clock differences and has more impact on higher subcarriers, and the second that appears from the electronics of the link and will directly impact the symbols' phase. The main difference with current phase correction techniques is the identification and estimation of the deterministic noise specific to VLC systems. The deterministic noise impact all subcarriers equally and must be subtracted before the measure of the shift induced by the differences between the transmitter and receiver clocks. Another difference is that the analysis and tests have been done with data collected in an actual hardware implementation.

Frequency steering of spaceborne clocks based on XPNAV-1 observations

Due to high stable rotations, timing of pulsars provides a natural tool to correct the frequency deviation of spaceborne atomic clocks. Based on processing the observational data about a year of Crab pulsar given by XPNAV-1 satellite, we study the possibility of correcting the frequency deviation of spaceborne atomic clocks using pulsar timing. According to the observational data in X-ray band and the timing model parameters from radio observations, the pre-fit timing residuals with a level of 67.66 µs are obtained. By fitting the slope of the timing residuals affected by the faked frequency-biased reference clock, we estimated successfully the relative frequency deviation of the reference clock. For a satellite clock with frequency deviation of the order about 10−12, a calibration accuracy with relative error of about 2% can be obtained from the Crab pulsar’s data for one year. The stability of the time scale based on Crab pulsar is about 10−12 for an interval of one year.

Anti Interference Clock Error Correction Technology Based on BD3 System

Beidou-3 (BD3) uses the pseudo range transmission method to calculate the clock difference through the pseudo range. However, due to the influence of electromagnetic interference, the clock difference measurement error is too large, which affects the timing accuracy of the power system. This paper first introduces the principle of Beidou timing service; Secondly, it introduces the clock error calculation method; Finally, combined with the clock difference calculation method, this paper presents an anti-interference clock difference correction method based on Least Mean Square (LMS) filter. According to the principle of LMS filter, the influence of electromagnetic interference on pseudo distance can be eliminated, and the clock difference can be calculated. Then the clock difference can be corrected. The simulation results show that the method is feasible and effective.

GNSS PPP with added QZSS: Positioning and time-frequency transfer

Regional navigation system Quasi-Zenith Satellite System (QZSS) play an important role in positioning, time and frequency transfer for the Asia-Pacific region under high cut-off angle. This paper combines QZSS and the major navigation systems for Precise Point Positioning (PPP) positioning and time–frequency transfer. The time–frequency transfer experimental results show that with the addition of QZSS, the clock difference accuracy and the frequency stability of the corresponding time link are significantly improved for the case of cut-off above 15°. The corresponding range is between 5% and 30% of accuracy, and between 1% and 90% of frequency stability.

A combination of multi-GNSS time transfer based on the fault-tolerant federated Kalman filter

In order to make full use of multi-GNSS constellations for time transfer, we propose a multi-GNSS combination method based on the fault-tolerant federated Kalman filter. First, the single-constellation all-in-view (AV) or common view (CV) solutions were evaluated with their current constellation on long and short baselines. We show that Galileo basically provides time transfer performance that is better than other constellations. Then, taking the TAIPPP link as a reference, the bias determined as the average difference between each single-constellation AV/CV solution and TAIPPP was obtained. After all single-constellation solutions had been aligned to the TAIPPP link, the fault-tolerant federated Kalman filter was used to combine the constellations into one global solution, which was compared with the standard-deviation-weighted solutions, TAIPPP, and Galileo-only solutions. The results show that fault-tolerant federated Kalman combined solutions agree with the results of other time comparison methods, and its frequency stability is the highest for averaging times less than 1×104 s. On the long baseline of NTSC-PTB and NTSC-SP, the standard deviation gain factors for the double clock differences (DCD) of fault-tolerant federated Kalman combined solutions versus Galileo-only solutions against the TAIPPP link are 2.96 and 1.82, respectively. On the short baseline of SP-PTB and TP-PTB, the gain factors are 3.00 and 1.18, respectively. Finally, the robustness of the fault-tolerant federated Kalman filter was analyzed. And the results indicate that when the subsystem fails, the fault-tolerant federated Kalman filter can quickly detect and isolate the fault, which ensures the accuracy of the global combined solutions and improves the robustness of the system.

Research on variation test of atomic time based on pulsar observation

Pulsar time (PT) has high long-term stability, and its establishment process is completely different from that of atomic time (AT). Therefore, pulsar-based time scale can be used as an independent test for the fluctuation of atomic time scale. In this paper, the test results of the fluctuation of atomic time using pulsar time are presented in combination with the real clock difference data. In order to test the fluctuation of atomic time, the timing model parameters of four pulsars are used to simulate the pulse times of arrival (TOAs) data with TT(BIPM19) as the reference, and then the reference time is changed from TT(BIPM19) to TT(TAI). Based on this, the classical weighted average algorithm and the Wiener filtering algorithm are used to extract the variations of the atomic time relative to the pulsar time. The test results obtained by the two methods are compared, and it shows that the Wiener filtering algorithm is better than the weighted average algorithm for the extraction of the fluctuation of the atomic time. The wavelet threshold denoising method is added to the clock difference signal extracted by the Wiener filtering algorithm to deduct the high-frequency noise. After denoising, PT is closer to TT(BIPM19), which further improves the ability of PT to detect the fluctuation of AT. For the TOA measurement accuracy of 100 ns, the difference between PT and TT(BIPM19) can be kept roughly within 40 ns. This paper studies the method of further improving the accuracy of PT under the premise of given TOA measurement accuracy, which is of great significance for the next step to use PT for more effective application of time keeping.