High-precision Time Synchronization Research Articles

High-Precision Time Synchronization Based on Timestamp Mapping in Datacenter Networks

In datacenter networks, it is necessary to determine whether the path is congested according to the one-way delay of packets. The accurate measurement of one-way delay depends on the high-precision time synchronization of the source device and destination device. We have proposed a time synchronization method based on timestamp mapping, combined with in-band network telemetry technology to obtain the packet send timestamp and receive timestamp on devices. The results show that the maximum synchronization error is 19 ns, and the standard deviation is 7.8 ns with a 100 ms time synchronization period and offset adjustment strategy. The proposed time synchronization method achieves outstanding synchronization accuracy and stability.

Ultrahigh-Performance Radio Frequency System-on-Chip Implementation of a Kalman Filter-Based High-Precision Time and Frequency Synchronization for Networked Integrated Sensing and Communication Systems

Ultrahigh-Performance Radio Frequency System-on-Chip Implementation of a Kalman Filter-Based High-Precision Time and Frequency Synchronization for Networked Integrated Sensing and Communication Systems

Dual-comb-enhanced microwave clock synchronization over commercial fiber

The large-scale clock network is the key ingredient to obtain high precision in many scenarios, from fundamental research to cutting-edge applications. The advantage of the time synchronization among microwave clocks is their cost, size, and accessibility. Here, we demonstrate a femtosecond-level time synchronization of microwave clocks through a commercial link of 205.86 km via dual-comb-enhanced optical two-way time transfer, which achieves a 6.23-fs residual time deviation between synchronized timescales at 1 s and an instability below 6×10−18 at 10,000 s. Further, the high-precision time synchronization of microwave clocks significantly enhances the probe ability of subtle reciprocity changes of fiber to the sub-picosecond level. This work provides a path toward secure fiber time-frequency networks to support future microwave-clock-based precise timing and sensing systems.

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.

High-Precision Time Difference of Arrival Estimation Method Based on Phase Measurement

In unmanned aerial vehicle (UAV)-based time difference of arrival (TDOA) positioning technique, baselines are limited due to communication constraints. In this case, the accuracy is highly sensitive to the TDOA measurements’ error. This article primarily addresses the problem of short-baseline high-precision time synchronization and TDOA measurement. We conducted a detailed analysis of error models in TDOA systems, considering both the time and phase measurement. We utilize the frequency division wireless phase synchronization technique in TDOA systems. Building upon this synchronization scheme, we propose a novel time delay estimation method that relies on phase measurements based on the integer least squares method. The performance of this method is demonstrated through Monte Carlo simulations and outdoor experiments. The standard deviations of synchronization and TDOA measurements in experiments are 1.12 ps and 1.66 ps, respectively. Furthermore, the circular error probable (CEP) accuracy is improved from 0.33%R to 0.02%R, offering support for the practical application of distributed short-baseline high-precision passive location techniques.

Time transfer over 113 km free space laser communication channel.

The space time frequency transfer plays a crucial role in applications such as space optical clock networks, navigation, satellite ranging, and space quantum communication. Here, we propose a high-precision space time frequency transfer and time synchronization scheme based on a simple intensity modulation/direct detection (IM/DD) laser communication system, which occupies a communication bandwidth of approximately 0.2%. Furthermore, utilizing an optical-frequency comb time frequency transfer system as an out-of-loop reference, experimental verification was conducted on a 113 km horizontal atmospheric link, with a long-term stability approximately 8.3 × 10-16 over a duration of 7800 seconds. Over an 11-hour period, the peak-to-peak wander is approximately 100 ps. Our work establishes the foundation of the time frequency transfer, based on the space laser communication channel, for future ground-to-space and inter-satellite links.

Dynamic phasor measurement algorithm based on high-precision time synchronization

Ensuring the swift and precise tracking of power system signal parameters, especially the frequency, is imperative for the secure and stable operation of power grids. In instances of faults within the distribution network, abrupt changes in frequency may occur, presenting a challenge for existing algorithms that struggle to effectively track such signal variations. Addressing the need for enhanced performance in the face of frequency mutations, this paper introduces an innovative approach—the Covariance Reconstruction Extended Kalman Filter (CREKF) algorithm. Initially, the dynamic signal model of electric power is meticulously analyzed, establishing a dynamic signal relationship based on high-precision time source sampling tailored to the signal model’s characteristics. Subsequently, the filter gain, covariance matrix, and variance iteration equation are determined based on the signal relationship among three sampling points. In a final step, recognizing the impact of the covariance matrix on algorithmic tracking ability, the paper proposes a covariance matrix reset mechanism utilizing hysteresis induced by output errors. Through extensive verification with simulated signals, the results conclusively demonstrate that the CREKF algorithm exhibits superior measurement accuracy and accelerated tracking speed when confronted with mutating signals.

Estimating Bridge Natural Frequencies Based on Modal Analysis of Vehicle-Bridge Synchronized Vibration Data.

This paper presents a method for accurately estimating the natural frequencies of bridges by simultaneously measuring the acceleration vibration data of vehicles and bridges and applying modal analysis theory. Vibration sensors synchronized with GPS timing were installed on both vehicles and bridges, achieving stable and high-precision time synchronization. This enabled the computation of the bridge's Frequency Response Functions (FRFs) for each mode, leading to a refined estimation of natural frequencies. The validity of the theory was confirmed through numerical simulations and experimental tests. The simulations confirmed its effectiveness, and similar trends were observed in actual bridge measurements. Consequently, this method significantly enhances the feasibility of bridge health monitoring systems. The proposed method is suitable for road bridges with spans ranging from short- to medium-span length, where the vehicle is capable of exciting the bridge.

Research on high-precision time synchronization technology for sea mobile platforms

Research on high-precision time synchronization technology for sea mobile platforms

Novel national high-precision time synchronization network architecture and first field trial with 2150km span for 5G communication

Novel national high-precision time synchronization network architecture and first field trial with 2150km span for 5G communication

High-precision fiber-optic time transfer with an unlimited compensation range.

We present a fiber-optic time transfer system with high transfer stability and an unlimited compensation range of the delay variation. We first stably transmit a frequency signal from a voltage-controlled oscillator to the remote site. The time signal is then embedded in the frequency signal by simply selecting its one cycle per second with a tunable gate signal. Therefore, the proposed time transfer system inherits both the stability and the unlimited adjustment range of the frequency transfer yet with no need for demodulation. The time deviation of 1.93 ps is achieved at 1000-s averaging. This simple and demodulation-free time transfer system is applicable for scalable distributed applications that require high-precision time synchronization and wide-range delay compensation.

High-precision Time Synchronization Algorithm for Unmanned Aerial Vehicle Ad Hoc Networks based on Bidirectional Pseudo-range Measurements

The increasing demand for missions involving unmanned aerial vehicles (UAVs) raises the need for reliable and accurate time synchronization in ad hoc networks. Time synchronization plays a crucial role in coordinating the actions of multiple UAVs, particularly when it comes to positioning, coordination, and data fusion. However, achieving high-precision time synchronization is challenging due to the difficult-to-estimate communication transmission delay, measurement noise, relative motion, and clock source characteristics in wireless ad hoc networks formed by UAVs. To address this issue, this paper proposes a fully distributed high-precision network time synchronization algorithm that is suitable for multi-hop dynamic ad hoc networks. The proposed algorithm is based on bidirectional pseudo-range measurements, graph theory, and random matrix correlation theory. It is composed of relative clock skew estimation, logic clock skew compensation and logic clock offset compensation. The algorithm is rigorously derived, requiring no central node, and it is capable of obtaining relative clock skew even when nodes broadcast clock information asynchronously. In addition, the proposed algorithm enables real-time elimination of transmission delays and has noise-resistant capabilities. Simulation results validate the effectiveness of the algorithm, demonstrating that the time synchronization accuracy in UAV ad hoc networks can reach the sub-nanosecond level.

Accurate time synchronization of power reference station based on BD3 system

A Beidou 3 (BD3) system-based power reference station can provide high-precision time synchronization for power distribution systems by sending synchronization data packets to devices in a multi-hop routing fashion. However, optimizing route selection to reduce both time synchronization error and delay is a challenging problem. In this paper, we establish a software-defined network-enabled power reference station time synchronization framework based on BD3. Then, we formulate the joint problem to minimize cumulative synchronization error and delay through multi-hop route selection optimization. A back propagation (BP) neural network-improved intelligent time synchronization route selection algorithm named BP-RS is proposed to learn the optimal route selection, which uses a BP neural network to dynamically adjust the exploration factor to achieve rapid convergence. Simulation results show the superior performance of BP-RS in synchronization delay, synchronization error, and adaptability with changing routing topologies.

Application-Layer Time Synchronization and Data Alignment Method for Multichannel Biosignal Sensors Using BLE Protocol.

Wearable wireless biomedical sensors have emerged as a rapidly growing research field. For many biomedical signals, multiple sensors distributed about the body without local wired connections are required. However, designing multisite systems at low cost with low latency and high precision time synchronization of acquired data is an unsolved problem. Current solutions use custom wireless protocols or extra hardware for synchronization, forming custom systems with high power consumption that prohibit migration between commercial microcontrollers. We aimed to develop a better solution. We successfully developed a low-latency, Bluetooth low energy (BLE)-based data alignment method, implemented in the BLE application layer, making it transferable between manufacturer devices. The time synchronization method was tested on two commercial BLE platforms by inputting common sinusoidal input signals (over a range of frequencies) to evaluate time alignment performance between two independent peripheral nodes. Our best time synchronization and data alignment method achieved absolute time differences of 69 ± 71 μs for a Texas Instruments (TI) platform and 477 ± 490 μs for a Nordic platform. Their 95th percentile absolute errors were more comparable-under 1.8 ms for each. Our method is transferable between commercial microcontrollers and is sufficient for many biomedical applications.

Wireless Picosecond Time Synchronization for Distributed Antenna Arrays

Distributed antenna arrays have been proposed for many applications ranging from space-based observatories to automated vehicles. Achieving good performance in distributed antenna systems requires stringent synchronization at the wavelength and information levels to ensure that the transmitted signals arrive coherently at the target, or scattered and received signals can be appropriately processed via distributed algorithms. In this article, we address the challenge of high-precision time synchronization to align the operations of elements in a distributed antenna array and overcome time-varying bias between platforms primarily due to oscillator drift. We use a spectrally sparse two-tone waveform, which obtains approximately optimal time estimation accuracy, in a two-way time transfer process. We also describe a technique for determining the true time delay using the ambiguous two-tone matched filter output, and we compare the time synchronization precision of the two-tone waveform with the more common linear frequency modulation (LFM) waveform. We experimentally demonstrate wireless time synchronization using a single-pulse 40-MHz two-tone waveform over a 90-cm 5.8-GHz wireless link in a laboratory setting, obtaining a timing precision of 2.26 ps.

GNSS Spoofer Localization for Vehicles Based on Doppler and Clock Drift Double Difference

Global navigation satellite systems (GNSS) have been widely used in our daily lives. Meanwhile, GNSS are facing a severe threat of spoofing attacks, which can mislead users by false position, velocity and time (PVT) solutions, resulting in serious consequences. Especially, with the development of vehicle navigation, autonomous driving and unmanned aerial vehicles (UAV), increasing spoofing attacks have emerged towards vehicles, where anti-spoofing techniques are desperately needed. Among these techniques, spoofer localization is a necessary step to fundamentally address the spoofing attacks. Most of the conventional spoofer localization methods are based on time of arrival (TOA) or time difference of arrival (TDOA). These methods require measurements observed by multiple receivers at the same time, where time synchronization is indispensable. However, it is hard to apply these methods to the existing vehicles equipped with independent commercial receivers. To cope with this problem, in this paper we develop a flexible method for vehicles to locate a stationary spoofer based on Doppler and clock drift double difference (DCDD). DCDD is calculated using Doppler measurements and clock drift solutions of one vehicular receiver and another reference receiver. Generally, Doppler frequency of GNSS signals changes slowly with time, which releases the restriction for high-precision time synchronization. DCDD represents the Doppler frequency caused by the vehicle motion, from which the spoofer position can be estimated. In addition, with the elimination of irrelevant components in Doppler measurements, DCDD minimizes the user complexity required for localization, where at least two receivers and one vehicle is needed. Utilizing DCDD, an iterative weighted least squares (WLS) algorithm is used to solve the spoofer position, and a closed-form initial value is given to improve the convergence rate. We evaluate the localization performance by the derived Cramér-Rao lower bound (CRLB) in the presence of vehicle position and velocity errors. Simulations and field vehicle experiments are conducted to verify the feasibility and effective of the proposed method. The experimental results show that with an average vehicle velocity of 8 m/s, the horizontal localization error can be less than 40 m and 20 m, using DCDD observations of 4 epochs and 8 epochs respectively.

A mobile recording system featuring high-precision time synchronization

A mobile recording system featuring high-precision time synchronization

Passive UAV Tracking in Wireless Networks

Techniques for high accuracy tracking of the 3-D velocity, position, and cellular carrier frequency bias of unmanned aerial vehicles (UAVs) that communicate using a cellular network are presented. The tracking algorithm does <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">only</i> rely on asynchronous measurements of Doppler shift in a minimum of four base station sites. This number can be reduced to three, by combination with a round-trip time measurement from one site. The tracking is therefore independent of any capability of the UAV transceiver, like the global positioning system that may be disabled in the UAV by the pilot. Another significant advantage is that high precision time synchronization between base stations is not needed. The algorithm exploits interacting multiple model filtering, with the nonlinear measurement equations handled by extended Kalman filters. The low computational complexity, low sampling rate, and the uplink Doppler shift measurement resources enable joint tracking of several UAVs per cell. This makes the tracking technique relevant for supervision of UAV traffic using cellular connectivity, for example for border surveillance and surveillance of sensitive areas like airports where illegal flying may be extremely dangerous and costly. Theoretically, an observability analysis treats measurement configurations based on different combinations of Doppler shift and round-trip time. Simulations are used to illustrate the performance and to assess the initialization properties.

A high-precision detection method for laser time transfer based on a single-photon avalanche detector array.

Space-ground laser time transfer is of significant importance to effectively function a high-precision time synchronization system. Instead of the single-pixel single-photon avalanche detector (SPAD) commonly used in the laser time transfer system, the SPAD array device is capable of supporting the greater performance of the signal photon capture. In this paper, it is experimentally demonstrated that the detection precision and time deviation (TDEV) of the SPAD array can be extremely improved by an order of magnitude compared with those of a single-pixel SPAD. As a result, 2.4ps root mean square precision and 0.25ps over an averaging time of 1000s TDEV are achieved for the SPAD array, providing a new detection method for high-precision laser time transfer applications, such as space-ground optical clocks.

Research on dual-terminal synchronization and ranging for laser communication link

With the development of technologies such as spatial information networks, precision time synchronization and distance measurement have played a paramount role in space laser links. Time synchronization is the basis of bidirectional laser ranging, and its accuracy significantly affects the quality of laser communication and ranging. However, the only way for the current integrated communication and ranging system to improve the system’s time synchronization accuracy is to increase the communication rate. It is impossible to achieve higher-precision time synchronization. In this study, we propose a coarse and fine composite time synchronization algorithm for laser communication link. Based on a high-precision time synchronization protocol, a small amount of ranging timestamps are added to the communication data frame for dual terminals time exchange as a coarse measurement ruler. And use the precise mixing phase comparison of the communication data recovery clock as a fine measuring ruler. The exact phase difference between the dual-terminal of the laser link is obtained, and the experimental work of the synchronization algorithm is performed in an equivalent space link. The experiment shows that the double-ended time synchronization accuracy is 90ps(3σ), which can be well applied to laser communication and ranging links.