High Precision Time Research Articles

Description of a Dimethylmercury Automatic Analyzer for the High-Resolution Measurement of Dissolved Gaseous Mercury Species in Surface Ocean Waters.

Our understanding of the significance of dimethylmercury (DMHg) to the mercury (Hg) global ocean biogeochemical cycle is unclear because of the lack of detailed DMHg measurements in the water column. To our knowledge, 30 years of published studies have generated no more than 200 DMHg data points in the ocean surface waters and marine boundary layer (MBL). To improve the precision and reduce the uncertainty in determining DMHg in surface seawater, we developed a simple and robust DMHg automatic analyzer (DAA). This DAA system couples the main sampling and analytic steps, including a continuous flow chamber, with dual Carbotrap preconcentration, a gas chromatographic column, a cold vapor atomic fluorescence spectrometry, and a data logger for signal integration. We compared the operation, performance, and reproducibility between our DAA and the traditional manual analytic method. Its advantages include the ease of operation, the high time resolution and precision (30 min sampling and <5% relative variation), and long-term stability (2 weeks). The DAA can determine DMHg in both the MBL and surface seawater. The estimated detection limits for DMHg with the DAA in the atmosphere and in surface seawater are 10 pg/m3 and 0.2 fM, respectively. The successful DAA field measurement in coastal waters indicates that it can help detect the low DMHg concentration in surface seawater, and the time series DMHg data helped our understanding of the DMHg behavior (sources and sinks) and its flux into the MBL. The comparison of DMHg concentration in various oceans also suggests that the coastal region had the lowest averaged DMHg, up to an order of magnitude lower than other ecosystems.

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.

Timing system based on customized frequency White Rabbit network in SHINE

Shanghai HIgh repetition rate XFEL aNd Extreme light facility (SHINE) is China's first hard X-ray FEL light source, which is expected to provide cutting-edge research methods for scientific research. To achieve the design target of SHINE, a high-precision timing system synchronized with the RF synchronization system of the accelerator is required, which could provide trigger signals and bunch ID for equipment on the accelerator, beamlines, and experimental stations so that those devices can be triggered in the correct sequence. This paper proposes a timing system based on WR technology that uses the RF signal from the synchronization system of SHINE as a reference and realizes the broadcasting of RF synchronization clock, including frequency and timescale, through an optical fiber network, achieving a timing accuracy of 8 picoseconds in the four-level network. Two enhanced timing applications, including the machine clock fanout and the random-event trigger distribution, are developed based on the RF timing benchmark. The scheme provides high-precision distribution of RF timing signal and general hardware interface, which can simplify the network structure and is expected to be extended to other large-scale physical accelerator facilities.

Continuous quantum clock with high precision and long recurrence time

Continuous clocks, i.e., clocks that measure time in a continuous manner, are regarded as an essential component of sensing technology. Precision and recurrence time are two basic features of continuous clocks. In this paper, in the framework of quantum estimation theory various models for continuous quantum clocks are proposed, and all tools of quantum estimation theory are employed to seek the characteristics of clocks with high precision and long recurrence time. Then, in a resource-based approach, the performances of the proposed models are compared. It is shown that quantum clocks based on an $n$ two-qubit system not only can have better precision than quantum clocks based on a $2n$ one-qubit system but also support long recurrence time. Finally, it is shown that while employing $n$ entangled qubits improves the precision of clocks by a factor of $1/\sqrt{n}$, it inevitably worsens the recurrence time of the clock.

In situ measurement of water vapor isotope ratios in air with a laser-based spectrometer

Simultaneous measurement of H217O/H216O, H218O/H216O, and HDO/H216O in air with a compact spectrometer based on a mid-infrared distributed feedback (DFB) laser was described. The obtained mixing ratios of H216O, H217O, and H218O agreed reasonably well with those measured by a hygrometer. The precision and repeatability of the spectrometer were analyzed. Indoor air tests demonstrated that its 220-s precision was 0.08 ‰, 0.06 ‰, and 0.14 ‰ for δ18O, δ17O, and δ2H respectively. The measured values of δ18O, δ17O, and δ2H in indoor air were highly correlated with the water vapor mixing ratios. The compact spectrometer provides in situ measurements of water vapor isotopes with high precision and fast time response, which opens new possibilities for its application in atmospheric and hydrological research in the future.

An internet of thing monitoring system with high-precision time reference for transmission lines and the fault recognition method

An internet of thing monitoring system with high-precision time reference for transmission lines and the fault recognition method

Study on PPP Time Comparison Based on BeiDou-3 New Signal

The Chinese BeiDou Navigation Satellite System (BDS-3) has completed the network construction and begun to provide high-precision Positioning, Navigation, and Timing (PNT) services for users all over the world. In this paper, we study the Precision Point Positioning (PPP) time comparison based on the BDS-3 new signal. The data presented in this paper includes the observation data from the National Time Service Center of Chinese Academy of the Sciences (NTSC) and the Electronics Academy of Sciences in Czech Republic (TP), and the precise product data released by the International GNSS Service (IGS), which includes the high-precision clock and orbit products. The results show that the multipath noise of the BDS-3 new signal (B1C and B2a) is less than that of the Asia-Pacific region BDS-2 (B1I and B2I), and the standard deviation (STD) of the multipath noise of B2a is the least. When the BDS-3 PPP is applied to the common clock short-baseline time comparison, the standard deviations of the BDS-3 and BDS-2 signals are 0.0296 ns and 0.0341 ns, respectively, the standard deviation of BDS-3 is smaller than that of BDS-2, and therefore the short-baseline time comparison performance of BDS-3 PPP is better than that of BDS-2, the standard deviation of BDS-3 PPP has increased by 13.19% compared with BDS-2. For long-baseline time comparisons between UTC (NTSC) and UTC (TP), the results based on the BDS-3 single system, or BDS-3 with other navigation system fusion PPP time comparison, follow the same trend as the results released by the International Bureau of Weights and Measures (BIPM). The residual standard deviations relative to the BIPM are 0.7039 ns and 0.2994 ns respectively. The stability of the BDS-3 new signal PPP and multi-GNSS fusion PPP time comparison has the same magnitude as the BIPM. As a result, with the completion of BDS-3, the high-precision time comparison based on BDS-3 expected to offer better services for users. The experimental results demonstrate that BDS-3 could be used to calculate the Coordinated Universal Time (UTC) in the future.

A 4.8ps root-mean-square resolution time-to-digital converter implemented in a 20nm Cyclone-10 GX field-programmable gate array.

It is difficult to improve the resolution and precision of a field-programmable gate array (FPGA)-based time-to-digital converter (TDC) in time interval measurement. In this study, we design a carry-look-ahead delay chain structure and integrate an interleaved sampling method with an online calibration and bin readjustment approach to implement a TDC. We take advantage of the adaptive logic module units applied in a Cyclone-10 GX (10CX220YF780E5G), which is a 20nm low-power consumption and low-cost FPGA. In this new generation FPGA, we implemented a high-precision time interval measurement, which exceeded all our previous works with a 4.8ps root-mean-square resolution and a 5.68ps least-significant-bit resolution.

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.

Stabilized Time Transfer via a 1000-km Optical Fiber Link Using High-Precision Delay Compensation System

Variations in optical fiber length and refractive index are induced by environmental perturbation, resulting in an additional dynamic propagation delay in fiber-based time synchronization systems, which deteriorate their transfer stability. This disadvantage can be significantly reduced by transmitting the time signal in both directions through fiber and constructing a feedback loop to compensate the propagation delay at the remote end of the link. This paper proposes an analog-digital hybrid proportional integral derivative (PID) control compensation system based on the time-frequency phase-locked loop (TF-PLL). The system is designed to keep the merits of wide servo bandwidth, servo accuracy, and a large dynamic delay compensation range up to 1 s, which is much greater than that reported in previous studies. For proving the validity of this proposed scheme, a self-developed optical fiber time synchronization equipment based on the delay compensation system is applied. The delay compensation system is used on a 1100-km long laboratory optical fiber, and the results show that the time synchronization stability in terms of time deviation (TDEV) is less than 5.92 ps/1 s and 2.56 ps/10,000 s. After successful laboratory evaluation, the proposed system is installed on a real 988.48-km line between the Xi’an Lintong branch of the National Time Service Center (NTSC) and Linfen City, Shanxi Province, realizing the time synchronization of 10 stations along the optical fiber link. The experimental results in the 988.48-km link illustrate that the measured time difference with a peak-to-peak value of 176 ps, the standard deviation of 19.3 ps, and a TDEV of less than 10.49 ps/1 s and 2.31 ps/40,000 s is achieved. The high-precision time delay compensation system proposed in this paper is simple, reliable, and accurate; has a wide range of compensation; and opens up a feasible scheme for providing synchronized time signals to multiple users over the long-distance field optical fiber networks.

Research on Urban Traffic Incident Detection Based on Vehicle Cameras

Situational detection in the traffic system is of great significance to traffic management and even urban management. Traditional detection methods are generally based on roadside equipment monitoring roads, and it is difficult to support large-scale and fine-grained traffic incident detection. In this study, we propose a detection method applied to the mobile edge, which detects traffic incidents based on the video captured by vehicle cameras, so as to overcome the limitations of roadside terminal perception. For swarm intelligence detection, we propose an improved YOLOv5s object detection network, adding an atrous pyramid pooling layer to the network and introducing a fusion attention mechanism to improve the model accuracy. Compared with the raw YOLOv5s, the mAP metrics of our improved model are increased by 3.3% to 84.2%, enabling it to detect vehicles, pedestrians, traffic accidents, and fire traffic incidents on the road with high precision in real time. This provides information for city managers to help them grasp the abnormal operation status of roads and cities in a timely and effective manner.

High Accuracy Software-Based Clock Synchronization Over CAN

In distributed real-time communication systems, common knowledge of the global time is crucial. It prevents message violations on the bus and allows independent components to collaborate within a real-time system on a timely basis. Systems with hard real-time requirements need to have high precision and accuracy of time. This is achieved by hardware-supported frame time-stamping mechanisms as found in dedicated protocols, such as time-triggered CAN (TTCAN), Flexray, and time-sensitive networking (TSN)-enabled Ethernet. However, many microcontroller units are not specifically designed to provide such a hardware-based solution at the communication interface. Therefore, a software-based implementation of the time synchronization algorithm is needed. Nevertheless, some commercial off-the-shelf (COTS) microcontroller units already provide an IEEE 1588-enabled Ethernet interface, including a high-precision timer module with rate correction. This module can be used for time synchronization purposes to align a set of distributed clocks via various communication interfaces. This article investigates the accuracy of software-based and hardware-supported time synchronization algorithm over the controller area network (CAN) protocol using a COTS microcontroller. As a result, we present identified jitter and delay sources as well as the achieved time accuracy. We show that using an advanced timer module combined with additional system knowledge allows submicrosecond precision and accuracies.

A High-Precision Transfer of Time and RF Frequency via the Fiber-Optic Link Based on Secure Encryption

This paper presents a high-precision transfer system of time and RF frequency via the fiber optic link based on secure encryption. On the basis of the two-way time transfer of optical fiber, a security strategy composed of an SM2 encryption algorithm is introduced, which can resist the security risk of time information being tampered with. The experimental results show that the developed picosecond-precision fiber-optic time transfer equipment can ensure high stability while realizing the encryption function. Time synchronization stability in terms of time deviation (TDEV) of 1 PPS can reach around 10.7 ps at 1 s and 7.1 ps at 10 s averaging time. The stability of the 10 MHz frequency can reach around 4.7 × 10−12 at 1 s and 1.1 × 10−12 at 10 s averaging time. There is no significant difference in time transfer accuracy, compared with unencrypted conditions. Furthermore, this paper realizes a ring time transfer network via a 150 km fiber-optic link with three nodes using three devices. The TDEV of 1PPS can reach around 20.8 ps at 1s averaging time. This paper provides a reference to establish a high-precision, safe, and stable time synchronization fiber network in the future.

High-Precision Depth Domain Migration Method in Imaging of 3D Seismic Data in Coalfield

Fault structures developed in coal seams, which are often associated with roof collapse, water inrush, gas outburst, and other accidents, are common geological hazards in coal exploration and development. The accurate detection of micro-structures such as small faults has always been a research focus to ensure safety in coalfields. Three-dimensional (3D) seismic research is one of the most efficient methods for obtaining the structural characteristics of coal areas and identify small faults in coal seams, but it is difficult for traditional seismic data imaging technologies to meet the high-precision demand of current coal exploration. Aiming at the characteristics of 3D seismic data in coalfields, we calculated the difference coefficients based on the optimized inversion algorithm and proposed a variable-density acoustic equation optimized with a temporal–spatial staggered-grid finite difference forward algorithm. On this basis, by combining normalized cross-correlation imaging conditions and GPU/CPU collaborative parallel processing technology, we developed an efficient and high-precision 3D reverse time migration method suitable for 3D seismic data in coalfields. Numerical tests verified the accuracy and efficiency of the proposed migration method for the imaging of coal-measure strata with small fault structures and could effectively identify 5 m small faults in the coal seam. The migration test of 3D seismic data in real coalfields showed that our 3D reverse time migration method has good practicability for high-precision imaging of 3D seismic data in coalfields and is an effective method for the precise imaging of small faults in coal measures.

Deriving 3-D Surface Deformation Time Series with Strain Model and Kalman Filter from GNSS and InSAR Data

This study proposes a new set of processing procedures based on the strain model and the Kalman filter (SM-Kalman) to obtain high-precision three-dimensional surface deformation time series from interferometric synthetic aperture radar (InSAR) and global navigation satellite system (GNSS) data. Implementing the Kalman filter requires the establishment of state and observation equations. In the time domain, the state equation is generated by fitting the pre-existing deformation time series based on a deformation model containing linear and seasonal terms. In the space domain, the observation equation is established with the assistance of the strain model to realize the spatial combination of InSAR and GNSS observation data at each moment. Benefiting from the application of the Kalman filter, InSAR and GNSS data at different moments can be synchronized. The time and measurement update steps are performed dynamically to generate a 3-D deformation time series with high precision and a high resolution in the temporal and spatial domains. Sentinel-1 SAR and GNSS datasets in the Los Angeles area are used to verify the effectiveness of the proposed method. The datasets include twenty-seven ascending track SAR images, thirty-four descending track SAR images and the daily time series of forty-eight GNSS stations from January 2016 to November 2018. The experimental result demonstrates that the proposed SM-Kalman method can produce high-precision deformation results at the millimeter level and provide two types of 3-D deformation time series with the same temporal resolution as InSAR or GNSS observations according to the needs of users. The new method achieves a high degree of temporal and spatial fusion of GNSS and InSAR data.

A Continuous Change Tracker Model for Remote Sensing Time Series Reconstruction

It is hard for current time series reconstruction methods to achieve the balance of high-precision time series reconstruction and explanation of the model mechanism. The goal of this paper is to improve the reconstruction accuracy with a well-explained time series model. Thus, we developed a function-based model, the CCTM (Continuous Change Tracker Model) model, that can achieve high precision in time series reconstruction by tracking the time series variation rate. The goal of this paper is to provide a new solution for high-precision time series reconstruction and related applications. To test the reconstruction effects, the model was applied to four types of datasets: normalized difference vegetation index (NDVI), gross primary productivity (GPP), leaf area index (LAI), and MODIS surface reflectance (MSR). Several new observations are as follows. First, the CCTM model is well explained and based on the second-order derivative theorem, which divides the yearly time series into four variation types including uniform variations, decelerated variations, accelerated variations, and short-periodical variations, and each variation type is represented by a designed function. Second, the CCTM model provides much better reconstruction results than the Harmonic model on the NDVI, GPP, MSR, and LAI datasets for the seasonal segment reconstruction. The combined use of the Savitzky–Golay filter and the CCTM model is better than the combinations of the Savitzky–Golay filter with other models. Third, the Harmonic model has the best trend-fitting ability on the yearly time series dataset, with the highest R-Square and the lowest RMSE among the four function fitting models. However, with seasonal piecewise fitting, the four models all achieved high accuracy, and the CCTM performs the best. Fourth, the CCTM model should also be applied to time series image compression, two compression patterns with 24 coefficients and 6 coefficients respectively are proposed. The daily MSR dataset can achieve a compression ratio of 15 by using the 6-coefficients method. Finally, the CCTM model also has the potential to be applied to change detection, trend analysis, and phenology and seasonal characteristics extractions.

A novel differential time-of-flight algorithm for high-precision ultrasonic gas flow measurement

The use of ultrasonic gas flow meters has expanded in industrial and household fields owing to their unique advantages. The transit-time method is applied to measure the gas flow rate by determining the absolute time-of-flight (TOF). However, it is inevitable that there is a contradiction between long time scale and high time precision of absolute TOF. In this paper, a novel differential TOF (ΔT) algorithm is proposed for high-precision ultrasonic gas flow measurements. The experimental results proved that ΔT plays a decisive role in the actual flow rate calculation. Therefore, when the absolute TOF is obtained, furthermore, the high-precision ΔT is calculated by the cross-power spectrum method separately. The two echo signals are aligned and compared to obtain ΔT, instead of subtracting the absolute TOFs. Either of the two echo signals is the reference signal of each other, which also avoids the problem of finding reference signals. The results of the calibration experiment indicate that the algorithm improves the stability of zero flow drift under static conditions and the accuracy of the dynamic flow rates. The algorithm allows the adjustment of parameters to be applied to pipes of different diameters. The parameters of the algorithm can be adjusted for application to ultrasonic flow metering systems of different diameters.

Detection of Frequency-dependent Dispersion Measure toward the Millisecond Pulsar J2241–5236 from Contemporaneous Wideband Observations

Making precise measurements of pulsar dispersion measures (DMs) and applying suitable corrections for them is among the major challenges in high-precision timing programs such as pulsar timing arrays (PTAs). While the advent of wideband pulsar instrumentation can enable more precise DM measurements and thence improved timing precision, it also necessitates doing careful assessments of frequency-dependent (chromatic) DMs that were theorized by Cordes et al (2016). Here we report the detection of such an effect in broadband observations of the millisecond pulsar PSR J2241−5236, a high-priority target for current and future PTAs. The observations were made contemporaneously using the wideband receivers and capabilities now available at the Murchison Widefield Array, the upgraded Giant Metrewave Radio Telescope, and the Parkes telescopes, thus providing an unprecedentedly large frequency coverage from 80 MHz to 4 GHz. Our analysis shows the measurable changes in DM that scale with the observing frequency (ν) as δDM ∝ ν 2.5±0.1. We discuss the potential implications of such a frequency dependence in the measured DMs and the likely impact on the timing noise budget and comment on the usefulness of low-frequency observations in advancing PTA efforts.

TAPv2: An Approach Towards Sub-Microsecond Level Timing Accuracy Over Air Interface

Absolute time synchronization is a basic issue in wireless communication. With the advent of 5G, many emerging vertical industry applications have put forward demands for timing accuracy over air interface. High Precision Timing method over Air Interface based on PHY Layer Signals (TAP) is proposed for mobile networks. It is designed to provide microsecond level timing accuracy at a relatively low cost compared to the existing timing methods, such as GNSS, NTP, PTP, etc. The purpose of this paper is to comprehensively improve TAP, including timing accuracy, stability and compatibility, so as to promote its practical application. This paper proposes improvements to TAP from four aspects: timing algorithm, timing pilot signal, terminal timing process and timing parameters determination. We improve the TAP’s deficiencies in delay estimation, delay compensation accuracy and real-time matching of timing information. We implement the workflow of TAPv2 for UE based on FPGA. Finally, we settle the problem of TAP parameters determination by applying a DQN method. The simulations and tests not only show that the DQN model can effectively determine the TAP parameters, but also show TAPv2 can achieve higher accuracy and stability than TAP in multiple scenarios based on the interaction with baseband.

Clock Skew Compensation Algorithm Immune to Floating-Point Precision Loss

We propose a novel clock skew compensation algorithm based on Bresenham's line drawing algorithm. The proposed algorithm can avoid the effect of limited floating-point precision (e.g., 32-bit single precision) on clock skew compensation and thereby provide high-precision time synchronization even with resource-constrained sensor nodes in wireless sensor networks.