Precise Time Synchronization Research Articles

Relativistic Effects on Satellite–Ground Two–Way Precise Time Synchronization

An ultrahigh precise clock (space optical clock) will be installed onboard a low-orbit spacecraft (a usual expression for a low-orbit satellite operating on an orbit at an altitude of less than 1000 km) in the future, which will be expected to obtain better time-frequency performance in a microgravity environment, and provide the possible realization of ultrahigh precise long-range time synchronization. The advancement of the microwave two-way time synchronization method can offer an effective solution for developing time-frequency transfer technology. In this study, we focus on a method of precise satellite-ground two-way time synchronization and present their key aspects. For reducing the relativistic effects on two-way precise time synchronization, we propose a high-precision correction method. We show the results of tests using simulated data with fully realistic effects such as atmospheric delays, orbit errors, and earth gravity, and demonstrate the satisfactory performance of the methods. The accuracy of the relativistic error correction method is investigated in terms of the spacecraft attitude error, phase center calibration error (the residual error after calibrating phase center offset), and precise orbit determination (POD) error. The results show that the phase center calibration error and POD error contribute greatly to the residual of relativistic correction, at approximately 0.1~0.3 ps, and time synchronization accuracy better than 0.6 ps can be achieved with our proposed methods. In conclusion, the relativistic error correction method is effective, and the satellite-ground two-way precise time synchronization method yields more accurate results. The results of Beidou two-way time synchronization system can only achieve sub-ns accuracy, while the final accuracy obtained by the methods in this paper can improved to ps-level.

Precise Time Synchronization and Clock Comparison Through a White Rabbit Network‐Based Optical Fiber Link

AbstractPrecise transfer of time and frequency signals over long distances as well as clock synchronization to an ultra‐stable reference are very crucial for many of the technological applications as well as for advanced scientific research. These reinforces a wide range of applications such as navigation, power grid management, mobile communication, and so on. In order to compare the performances of two highly stable and accurate atomic clocks, it is desirable that the link between the two clocks, that is, the transmission link has higher level of stability than those clocks. This article describes establishment of an ultra‐stable optical fiber link employing White Rabbit network for transfer of time and frequency signals and also for comparing performance of atomic clocks. Utilizing this link, time signals have been transferred within an uncertainty of ±130 ps at ambient temperature (30°C–40°C) and the instability of the link in terms of modified Allan‐deviation reaches to ∼10−16 within one day of integration time.

The space\u2013time references of BeiDou Navigation Satellite System

The BeiDou Navigation Satellite System (BDS) is essentially a precise time measurement and time synchronization system for a large-scale space near the Earth. General relativity is the basic theoretical framework for the information processing in the master control station of BDS. Having introduced the basic conceptions of relativistic space–time reference systems, the space–time references of BDS are analyzed and the function and acquisition method of the Earth Orientation Parameters (EOP) are briefly discussed. The basic space reference of BDS is BeiDou Coordinate System (BDCS), and the time standard is the BDS Time (BDT). BDCS and BDT are the realizations of the Geocentric Terrestrial Reference System (GTRS) and the Terrestrial Time (TT) for BDS, respectively. The station coordinates in the BDCS are consistent with those in International Terrestrial Reference Frame (ITRF)2014 at the cm–level and the difference in scale is about 1.1 times 10^{ - 8}. The time deviation of BDT relative to International Atomic Time (TAI) is less than 50 ns and the frequency deviation is less than 2 times 10^{ - 14}. The Geocentric Celestial Reference System (GCRS) and the solar Barycentric Celestial Reference System (BCRS) are also involved in the operation of BDS. The observation models for time synchronization and precise orbit determination are established within the GCRS framework. The coordinate transformation between BDCS and GCRS is consistent with the International Earth Rotation and Reference Systems Service (IERS). In the autonomous operation mode without the support of the ground master control station, Earth Orientation Parameters (EOP) is obtained by means of long-term prediction and on-board observation. The observation models for the on-board astrometry should be established within the BCRS framework.

Discovery of Random Delay Distribution Based on Complex WSNs Clock Synchronization

With the rapid development, Wireless Sensor Networks (WSNs) technology has been widely applied in many fields, such as military intelligence detection, medical condition detection, urban facility construction and smart home, etc. Clock synchronization is one of the key technologies in WSNs, precise node time synchronization is critical for many networked control applications in WSNs. Once the clock of sensor nodes loses synchronization, the information collected by nodes will lose timeliness and value for users. Therefore, the key technology of clock synchronization plays an important role in WSNs. The main purpose of clock synchronization is to control the interval time that between sending and receiving timestamps of wireless sensor nodes within acceptable clock synchronization accuracy. However, because there are many kinds of link delays in the process of information transmission, the transmission time is prolonged and the time precision is reduced. Therefore, it is necessary to study the exact causes and effects of these errors. In ZigBee wireless sensor network, through theoretical model and data experimental analysis, we got the result that the random delay distributions of the uplink and downlink obey normal distribution when the sensor nodes exchange some information.

A Clock Synchronization Algorithm for the Satellite System Based on MF-TDMA

In order to ensure microsecond slot scheduling, all terminals must maintain high precision of time synchronization in MF-TDMA(Multi-Frequency Time Division Multiple Access) satellite communication system. Due to the different frequency of crystal oscillator at the terminal, the absolute value of the clock deviation is constantly increasing, so a clock synchronization algorithm is required to keep the time synchronization of the satellite system. The traditional satellite clock synchronization algorithm minimizes the clock error through periodic synchronization, but it only considers the phase offset, not the crystal frequency offset, the synchronization accuracy is low and waste of bandwidth resources. Based on the traditional satellite clock synchronization, this paper proposes a gradient descent algorithm for the satellite system based on MF-TDMA(GDAS), adding clock frequency offset prediction and compensation mechanism to improve clock synchronization accuracy and bandwidth resource utilization rate. The simulation results of the proposed algorithm has optimized the precision performance of clock synchronization.

Automated two-dimensional localization of underwater acoustic transient impulses using vector sensor image processing (vector sensor localization).

Detecting acoustic transients by signal-to-noise ratio (SNR) becomes problematic in nonstationary ambient noise environments characteristic of coral reefs. An alternate approach presented here uses signal directionality to automatically detect and localize transient impulsive sounds collected on underwater vector sensors spaced tens of meters apart. The procedure, which does not require precise time synchronization, first constructs time-frequency representations of both the squared acoustic pressure (spectrogram) and dominant directionality of the active intensity (azigram) on each sensor. Within each azigram, sets of time-frequency cells associated with transient energy arriving from a consistent azimuthal sector are identified. Binary image processing techniques then link sets that share similar duration and bandwidth between different sensors, after which the algorithm triangulates the source location. Unlike most passive acoustic detectors, the threshold criterion for this algorithm is bandwidth instead of pressure magnitude. Data collected from shallow coral reef environments demonstrate the algorithm's ability to detect SCUBA bubble plumes and consistent spatial distributions of somniferous fish activity. Analytical estimates and direct evaluations both yield false transient localization rates from 3% to 6% in a coral reef environment. The SNR distribution of localized pulses off Hawaii has a median of 7.7 dB and interquartile range of 7.1 dB.

Research on Vision System Calibration Method of Forestry Mobile Robots

Forestry mobile robots can effectively solve the problems of low efficiency and poor safety in the forestry operation process. To realize the autonomous navigation of forestry mobile robots, a vision system consisting of a monocular camera and two-dimensional LiDAR and its calibration method are investigated. First, the adaptive algorithm is used to synchronize the data captured by the two in time. Second, a calibration board with a convex checkerboard is designed for the spatial calibration of the devices. The nonlinear least squares algorithm is employed to solve and optimize the external parameters. The experimental results show that the time synchronization precision of this calibration method is 0.0082s, the communication rate is 23Hz, and the gradient tolerance of spatial calibration is 8.55e−07. The calibration results satisfy the requirements of real-time operation and accuracy of the forestry mobile robot vision system. Furthermore, the engineering applications of the vision system are discussed herein. This study lays the foundation for further forestry mobile robots research, which is relevant to intelligent forest machines.

One Study on IEEE1588 clock synchronization algorithm based on Kalman filter

Clock synchronization is a major indicator of the distributed network test and control system. The precise time synchronization is also one of the core technologies of the network communication of test and control system. The paper illustrates the fundamental principle for clock synchronization under IEEE1588 protocol and concludes that its synchronized accuracy is influenced by the accuracy of time stamp and transmission delay jitter through analysis. An autoregressive clock drift model has been put forward in the paper in an effort to improve the time synchronization accuracy. It makes use of the Kalman filter to estimate the deviation and drift of the master and slave clock so as to offset it. The results of simulation indicate that the method can effectively ease the impact on time synchronization for time stamp and improve its accuracy of the test and control system.

Low latency information transfer based on precision time synchronization via wireless interferometry

Low latency information transfer based on precision time synchronization via wireless interferometry

Delay-Bounded Wireless Network Based on Precise Time Synchronization Using Wireless Two-Way Interferometry

The importance of reliable information transfer in wireless networks, especially regarding communication delay, is drastically increasing to fulfill safe and high-quality communication in the 5G and post-5G era. However, conventional media access control (MAC) protocol for wireless networks, notably Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), sometimes yields unexpectedly significant delay due to its complex arbitration mechanism assuming asynchronous communication among terminals. As the delay cannot be strictly bounded by a deterministic value, this causes a vulnerability of systems relying on wireless networks. This paper utilizes precise time synchronization achieved by Wireless Two-way Interferometry (Wi-Wi), enabling all terminals to be time-synchronized via wireless signals. We show that by an appropriate periodic assignment of each terminal's data transmission timing, named Arbitration Point (AP), a simple arbitration algorithm obtains a strictly bounded maximum value for the delay while ensuring equalities among all participants. Furthermore, we demonstrate that the total number of terminals manageable in a star-topology wireless network significantly increases by densely packing AP timings, taking into account the spatial geometry information of terminals, which is another feature delivered by Wi-Wi measurement. In the meantime, we experimentally constructed a star-topology wired network where all terminals are time-synchronized via Wi-Wi to confirm the fundamental properties identified in the proposed arbitration protocol. This study paves a new way for future wireless networks where the delay is strictly bounded and provides the basis for ultra-reliable and high-quality information transfer functionalities by utilizing precise time synchronization and space localization (space-time synchronization).

Speaker Identification for Business-Card-Type Sensors

Human collaboration has a great impact on the performance of multi-person activities. The analysis of speaker information and speech timing can be used to extract human collaboration data in detail. Some studies have extracted human collaboration data by identifying a speaker with business-card-type sensors. However, it is difficult to realize speaker identification for business-card-type sensors at low cost and high accuracy because of spikes in the measured sound pressure data, ambient noise in the non-speaker sensor, and synchronization errors across each sensor. This study proposes a novel sound pressure sensor and speaker identification algorithm to realize speaker identification for business-card-type sensors. The sensor extracts the user's speech at low cost and high accuracy by employing a peak hold circuit and time synchronization module for spike mitigation and precise time synchronization. The algorithm identifies a speaker with high accuracy by removing ambient noise. The evaluations show that the algorithm accurately identifies a speaker in a multi-person activity considering varying numbers of users, environmental noises, and reverberation conditions as well as long or short utterances. In addition, the peak hold circuit enables accurate extraction of speech and the synchronization error between the sensors is always within $\pm$ 30 $\boldsymbol\mu$ s, that is, negligible error.

Long-Distance Free-Space Measurement-Device-Independent Quantum Key Distribution.

Measurement-device-independent quantum key distribution (MDI-QKD), based on two-photon interference, is immune to all attacks against the detection system and allows a QKD network with untrusted relays. Since the MDI-QKD protocol was proposed, fiber-based implementations aimed at longer distance, higher key rates, and network verification have been rapidly developed. However, owing to the effect of atmospheric turbulence, MDI-QKD over a free-space channel remains experimentally challenging. Herein, by developing a robust adaptive optics system, high-precision time synchronization and frequency locking between independent photon sources located far apart, we realized the first free-space MDI-QKD over a 19.2-km urban atmospheric channel, which well exceeds the effective atmospheric thickness. Our experiment takes the first step toward satellite-based MDI-QKD. Moreover, the technology developed herein opens the way to quantum experiments in free space involving long-distance interference of independent single photons.

Real-Time DAS VSP Acquisition and Processing on Single- and Multi-Mode Fibers

Distributed acoustic sensing (DAS) is a rapidly evolving fiber-optic sensing technology that converts any standard fiber-optic cable into a distributed sensor capable of measuring acoustic events every meter over large (100 km+) distances. We describe a conveyance-agnostic DAS system for vertical seismic profiling (VSP) that has the capability to sense either single- or multi-mode sensing fibers, and encode all auxiliary seismic source signals directly onto the optical data stream using a series of piezo-electric fiber stretchers in-line with the sensing fiber. The DAS system can operate with latency-free synchronization via wireless telemetry between the seismic source controller and the DAS unit in either master or slave modes. Similar to geophone-based VSP data acquisition systems, the time-break signal is used to trigger data acquisition to ensure precise time synchronization at sub-millisecond accuracy between the DAS system and the seismic source. The system enables effective data management and the real-time generation of 1 millisecond sampled seismic and navigation data that conforms to the industry-standard seismic data formats, thus eliminating any latency between acquisition and field data deliverables.

Edge Computing-Based ERBS Time Synchronization Algorithm in WSNs

In the practical application of large-scale photovoltaic module monitoring, adopting wireless sensor network (WSN) technology is a method worth researching. With increasing nodes in the wireless sensor network, widely existing clock skew, increased geometrically, is bringing about greater energy consumption. Due to the random distribution of nodes, in order to improve the transmission efficiency and reduce the computational load of the coordinator, the node processor needs to the use edge computing for preliminary analysis. This paper puts forward an improved energy-efficient reference broadcast synchronization algorithm (ERBS). This algorithm firstly calculates the average phase offset of nonadjacent nodes in the network after receiving a message. It then uses the least square method to solve the clock skew to achieve high-precision synchronization of the whole network. Simulation results show that compared with RBS, the time synchronization precision of ERBS is greatly improved and synchronization times are greatly reduced, decreasing energy consumption significantly.

Analysis of meteor trails providing combined forward and backward scattering of radio waves

In this paper, the authors perform the first attempt on examining the possible existence of ionized meteor trails that provide combined forward and backward scattering of radio waves at radio links larger 300 ​km. The authors call such meteor trails as “Forward-Backward Scattering” or, simply, FBS-trails and propose a model for their computer simulation. The authors believe that the FBS-trails might find use in a variety of practical applications, including precise time synchronization of passive remote stations. Based on simulation results, it was found that the most reliable distinctive feature of the FBS-trails is their nearly transverse orientation relative to axis of meteor-scatter radio link. The longer the radio link, the less probable FBS-trails become, and almost all detections occur from meteor trails that provide only a single-direction (the forward) scattering. Depending on the link length, the percentage of FBS-trails in total number of detected meteor events varies between 1% (at meteor-scatter links longer 1500 ​km) to about 10% (at meteor-scatter links shorter 150 ​km).

The Issues on Time Synchronization Technology in Time Triggered Ethernet

Based on the idea of time-triggered, global unified high precision time synchronization is the foundation of network deterministic communication. In the process of time synchronization, clock synchronization algorithm, transmission delay, clock drift and so on will affect the accuracy of clock synchronization. It is a great technical challenge to realize high precision and reliable clock synchronization. This paper collates and analyses the relevant literature, first introduces the time-triggered Ethernet, then analyses the time synchronization protocol that can be used for high-precision clock synchronization, reviews the development status of time synchronization technology and makes an overall analysis. Finally summarizes the clock synchronization difficulty and prospects the development trend of the time synchronization technology.

SDN/NFV Enhanced Time Synchronization in Packet Networks

The need for distribution of accurate and reliable time synchronization is more and more urgent, especially with the development of the fifth-generation (5G) network and the rise of high precision communication. However, the vast majority of the current packet networks is the best effort and cannot offer precision time synchronization. To enable the precise time feature in packet networks, a novel time synchronization scheme is proposed based on software-defined networking/network functions virtualization enabled network architecture, exploring the programmability of network time synchronization as a fundamental concept to define the function model for core components. Time synchronization is built as a virtual network function (VNF), and a synchronization controller is designed to manage the synchronization processing and precision compensation together with the network controller and the proposed VNF. More specifically, the time synchronization model and compensation algorithms are introduced to reveal the mechanism of the synchronization process. To thoroughly study the performance of the proposed scheme, an analytical model is developed that covers the primary influencing factors of synchronization accuracy, including the link length, delay coefficient, timestamping granularity, and local oscillator's characteristic of the network element. Further, experimental and analytical results are presented to demonstrate the effectiveness of the proposed scheme.

Automated two-dimensional localization of underwater acoustic transient impulses using vector sensor image processing

An algorithm is presented for automatically localizing transient impulsive sounds collected on several autonomous underwater vector sensors, spaced 15 to 23 m apart. The procedure, which does not require precise time synchronization, exploits transient signals of interest arriving from different azimuthal directions on each sensor. For each sensor the method first constructs time-frequency representations of both the squared acoustic pressure (spectrogram) and dominant directionality of the active intensity (azigram). Within each azigram sets of time-frequency cells associated with transient energy arriving from a consistent azimuthal sector are identified. Standard image processing techniques then link sets that share similar duration and bandwidth between different sensors, after which the algorithm triangulates the source location using the azimuths associated with the detection set. Data collected from shallow coral reef environments demonstrate the algorithm's ability to detect SCUBA bubble plumes and humpback whale song, and reveal consistent spatial distributions of somniferous fish activity. Analytical estimates and direct evaluations both yield false transient localization rates of 3%–6% in the coral reef environment. Many localized pulses have low signal-to-noise ratios, whose distribution has a median of 7.7 dB and an IQR of 7.1 dB. [Work sponsored by DARPA PALS.]

A Design of a Time Synchronization Protocol Based on Dynamic Route and Forwarding Certification

Time synchronization is a key technique in large-scale wireless sensor network applications. In order to tackle the problems of multi-hop synchronization error accumulation, clock frequency skew swinging, and network topology changes, a time synchronization protocol based on dynamic routing and forwarding certification (DRFC-TSP) is proposed in this paper. During the time synchronization process, a reference node with fewer synchronization hops and a more stable clock frequency is selected for every single hop, in order to obtain the best synchronization route. In this way, synchronization error accumulation can be restrained and the impact of clock frequency skew swinging on the time synchronization precision can be reduced. Furthermore, changes of the network topology can be well adapted by dynamic routing, in which the reference node is updated in every synchronization round. In the forwarding certification process, the status of nodes forwarding synchronous information outwards is authored by information exchange between neighboring nodes. Only synchronous information of the certificated nodes with a better performance can be forwarded. The network traffic can be decreased and the time synchronization precision can also be ensured, even with less energy consumption. Feasibility testing in large-scale wireless sensor networks is verified on NS2 simulation and more performances are evaluated on an embedded Linux platform.

Sysplex time synchronization using IEEE 1588 Precision Time Protocol (PTP)

Timekeeping and highly accurate, precise time synchronization are key requirements for modern information technology systems. While true for several industries, this is especially true for industries involved in transaction processing such as the financial industry. As such, the IBM Z Sysplex needs highly accurate timing/timekeeping and synchronization technology to ensure data integrity, and to also provide the ability to reconstruct a database based upon logs. Recently enacted changes and new regulatory requirements, both in Europe and the United States, have brought increasing attention to time synchronization accuracy. These regulations spurred an interest, both from IBM Z and from our IBM Z clients, in the IEEE 1588 Precision Time Protocol (PTP) being implemented in IBM Z. This article explains the history of PTP, PTP technology, the regulations that led IBM to introduce PTP to IBM Z, PTP's implementation on IBM Z, and IBM's involvement and leadership in the development of the PTP technology and standards going forward.