Precise Time Signals Research Articles

Transferring time and frequency signal through phase compensated telecommunication fibres

Precise time and frequency signal transfer over White Rabbit Precision Time Protocol (WRPTP) based underground optical fibre link has been experimentally demonstrated for its potential enhancement and better signal integrity. In order to prove its advantages in practical applications, the time synchronization performance of WRPTP-based underground telecommunication optical fibre links have been examined in controlled environment conditions as well as in varying ambient conditions. Under controlled environment conditions, time synchronization has been achieved within ± 100 ps uncertainty but variation in ambient conditions degrades the stability as well accuracy of the time transfer link. The introduction of active phase variation compensation enhances the accuracy by ∼ 66 % of the WRPTP-based underground time transfer link. The link’s instability in terms of Modified Allan deviation reaches to 2.0 E -15 from 7.5 E -15 at 4096 s of integration time which indicate that dynamic phase variation compensation improves the stability of WRPTP-based time transfer link under varying ambient conditions.

The emergence of time light signals for rating of marine chronometers

Precise visual time signals for marine chronometer rating began with a shuttered oil lamp in about 1823, but dropping time balls became the favoured visual signal for chronometer calibration. Electric time light signals became practicable in the twentieth century and replaced or complemented time balls as principal signals at various locations from 1909 onwards, with the first known implementations in Mozambique, China and Argentina. They were listed by the British Admiralty for 46 locations worldwide between 1911 and 1947. Not all existed together. There were 29 listed locations in Europe but none in the British Isles or North America. Their locations and types are described in this article, with specific details of signals outside Europe. They existed in extraordinary variety, ranging from coloured lights that replaced time balls in New Zealand to steady or flashing white lights at many other locations. All used the moment of extinction as the time signal.

Обобщенная модель действий злоумышленника при манипулировании сообщениями, содержащими сигналы точного времени

This article gives the place and role of the common time system in the telecommunications system, gives an overview of the process of functioning of the common time system, indicating the main means of transmitting accurate time signals. Based on the analysis of the process of functioning of the common time system, the main risks of failures in technological and automated systems in case of violation of the accuracy of the transmission of common time signals are identified. As a result of the analysis of emerging risks, attention is focused on the main types of threats to the single time system when attacks are carried out by organized malefactors. A special place among the threats is occupied by the manipulation of messages containing precise time signals. To analyze this threat, a semi-Markov model of an attacker's actions when manipulating messages containing precise time signals is formed. The main stages of the process of carrying out the attack are singled out. Using the method of minors, the main stationary characteristics of the attack process are calculated, such as the probability of finding the attack process at various stages of its implementation, as well as the average time of the attack duration and the average recovery time of the unified time system from the consequences of the attack. This information also serves as the basis for determining the coefficient of correct operation of the unified time system. The results obtained can be reflected in the formation of strategies for protecting the unified time system from attacks by organized intruders.

Songbird subthalamic neurons project to dopaminergic midbrain and exhibit singing-related activity.

Skill learning requires motor output to be evaluated against internal performance benchmarks. In songbirds, ventral tegmental area (VTA) dopamine neurons (DA) signal performance errors important for learning, but it remains unclear which brain regions project to VTA and how these inputs may contribute to DA error signaling. Here, we find that the songbird subthalamic nucleus (STN) projects to VTA and that STN microstimulation can excite VTA neurons. We also discover that STN receives inputs from motor cortical, auditory cortical, and ventral pallidal brain regions previously implicated in song evaluation. In the first neural recordings from songbird STN, we discover that the activity of most STN neurons is associated with body movements and not singing, but a small fraction of neurons exhibits precise song timing and performance error signals. Our results place the STN in a pathway important for song learning, but not song production, and expand the territories of songbird brain potentially associated with song learning.NEW & NOTEWORTHY Songbird subthalamic (STN) neurons exhibit singing-related signals and are interconnected with the motor cortical nucleus, auditory pallium, ventral pallidum, and ventral tegmental area, areas important for song generation and learning.

Key technology of high-precision time frequency transfer via 200 km desert urban fiber link

The precise time and frequency signal dissemination has significant applications in scientific research such as baseline interferometry, deep space network and metrology. Aside from satellite based systems, optical fiber has become an attractive alternative medium for transferring time and frequency signals, offering much improved accuracy. For the urban fiber link in the desert environment, there are many complex noise sources, such as temperature change, outdoor wind and ground vibration. Therefore, a systematical study on the noise source and on the noise reduction method in the dessert environment have practical significance. In this paper, we demonstrate a time (1 pps) and frequency signal dissemination and time synchronization system through a 200 km urban fiber in dessert environment. The noise source of the urban fiber under dessert environment is analyzed and studied in detail; the results show that the vibration and temperature shift are the major influencing factors. The vibration of urban fiber can induce the noise in the high Fourier frequency, and the temperature shift of urban fiber can induce the noise at a low Fourier frequency. An optical compensation setup is used, including the optical delay line with temperature controlled and piezoelectric ceramics driving. The phase fluctuation of frequency signal is detected and used to control the feedback of the optical compensating setup. In order to compensate for the fiber loss in a long range, a special bi-directional erbium-doped fiber amplifier is used to regenerate optical signals to achieve the long distance transmission. Then, we study the effective link noise suppression technology under different feedback compensation parameters. The systematic feedback parameters are optimized through using the different system feedback bandwidths, feedback intensities, optical power and other key parameters. The optimized systematic feedback parameters are obtained via the careful experimental observation and discussion. With the optimized systematic feedback parameters, experimental results show that the frequency stabilities are up to 8 × 10<sup>–14</sup> at 1 s and 1 × 10<sup>–16</sup> at 1000 s, and time stabilities are up to 1.2 ps in an average time of 10<sup>3</sup> s. The phase stabilized transmission of hydrogen clock signal in the 200 km level desert environment urban fiber link is realized. The verification experiment plays an important role in measuring the satellite orbit based on a connected elements’ interferometry. The relevant study result is of significance for improving the precision of time and frequency signal dissemination in the dessert environmental urban fiber.

Simultaneous time and frequency transfer over 100 km optical fiber based on sub-carrier modulation

In this paper a novel scheme is proposed to realize the simultaneous transfer of precise time and frequency signals. By performing sub-carrier modulation to the time signal, the time and frequency signals can be directly transferred in the same optical channel without causing mutual interference. The remote user does not need a slave oscillator to clear up the frequency signal and therefore can maintain a simple structure. We test the scheme in the proof-of-concept experiment, in which the 50 MHz modulated time signal and 400 MHz frequency signal are transferred together over 100 km optical fiber link. Good signal compatibility and system performance is observed in the experimental results.

Application of VCSELs in next-generation telescope array networks such as the Square Kilometre array

The distribution of precise and accurate optical clock signals across fibre links has a significant impact in various fields. Many applications require the transfer of stable and precise timing signals from a single central location to multiple remote users via a fibre network. One such application is the big data instrument, the Square Kilometre Array (SKA). An increase in demand for more bandwidth by broadband providers has prompted the expansion and development of attractive and intelligent, modern communication networks. Vertical cavity surface-emitting lasers (VCSELs) are an attractive solution and a potential candidate for realizing low cost and high bandwidth data transmission. In this paper a novel and unique, all-optical technique for measuring the one-way transmission time delay of the propagating signal along an optical fibre is presented. This has been successfully achieved by optically injecting a pulse-per-second (PPS) signal into the secondary mode of a 1550 nm VCSEL located at the client end. A round-trip latency time of 113.2 μs was experimentally measured over a 22 km G.652 single mode fibre (SMF). A novel VCSEL capacity upgrade technique is further demonstrate. This was experimentally achieved by simultaneously modulating a single 1310 nm VCSEL with a 10 Gbps 27 − 1 pseudorandom binary sequence (PRBS) and a polarization based PPS clock signal. An error free transmission over 11 km of G.652 SMF was reported, with a measured transmission penalty of 0.52 dB when the VCSEL was simultaneously modulated with the 27 − 1 PRBS and the polarization based PPS signal.

Shrinking Atomic Clocks

Physics Atomic clocks housed in national laboratories around the world provide a precise time signal on which our present high-technology lives depend. Precise synchronization allows for colossal amounts of data to be reliably transferred around networks of optic fibers at ever-increasing rates, as well as providing accurate navigation systems. With increasing demand for mobile devices, there is a requirement for these atom-based time pieces to shrink. Present chip-scale atomic-clock (CSAC) devices based on alkali atoms in a buffer gas are lightweight (<50 g) and consume little power (<150 mW) but tend to show frequency drift over time that requires frequent recalibration. Clocks based on trapped single atoms or ions should be more stable because the atoms are vacuum-packed and do not interact. However, these tend to be relatively large and power-hungry. Jau et al. have developed a miniature atomic clock based on trapped Yb ions that has size and power requirements similar to those of existing CSAC technology but also offers to match the long-term stability expected of the much larger trapping systems. Appl. Phys. Lett. 101 , 253518 (2012).

GPS Galileo Time Offset (GGTO)의 항법해 영향 분석

미국의 GPS와 유럽연합의 Galileo 시스템과 같은 위성항법시스템(Global Navigation Satellite System, GNSS)은 정확한 시각과 주파수로 동기화된 항법신호를 지상의 사용자에게 제공하는 전 지구 측위시스템으로 이를 기반으로 넓은 범위에 걸쳐 항법 및 시각동기 서비스를 제공하고 있다. 이와 더불어 다중위성항법신호를 활용하여 측위를 할 경우 단독위성항법신호를 사용할 때 보다 가용 항법신호의 수가 증가하고 이에 따라 측위 정확도를 향상시킬 수 있다는 장점이 있는 것으로 알려져 있다. 하지만 현재의 GPS와 Galileo 시스템은 각기 다른 기준 시각체계를 사용하고 있기 때문에 항법해 계산 시 시각 편이가 발생하며 이를 적절히 보정하지 못할 경우 통합 항법 측위 성능을 저하시킬 수 있다. 본 논문에서는 이러한 미국의 GPS와 유럽의 Galileo 시스템에 대한 기준 시각체계에 대한 차이점을 분석하고 시각체계 불일치에 따른 항법해 영향을 시뮬레이션을 통하여 분석하였다. 또한, 현재 알려진 대표적 보정기법을 적용하였을 경우의 측위 정확도의 향상과 기법별 문제점에 대해서도 분석하였으며 기법별 문제점을 보완할 수 있는 새로운 기법에 대한 개념을 함께 제시한다. The Global Navigation Satellite System (GNSS) like US Global Positioning System (GPS) and EU Galileo are based on providing precise time and frequency synchronized ranging signals. Because of the exploitation of very precise timing signals these GNSS are used to provide both navigation and time distribution services. Moreover, because the positioning accuracy will improve as more satellites become available, we should expect that a combination of Galileo and GPS will provide better performance than those of both systems separately. However, Galileo will not use the same time reference as GPS and thus, a time difference arises - the GPS-Galileo Time Offset (GGTO). The navigation solution calculated by receivers using signals from both navigation systems will consequently contain a supplementary error if the GGTO is not accounted for. In this paper, we compared GPS Time (GPST) with Galileo Sytem Time (GST) and analyzed the effects of GGTO on positioning accuracy by simulation test. And then we also analyzed the characteristics of two representative GGTO correction methods such as the navigation message based method at system level and the estimation method at user level and propose the conceptual design of the novel correction method being capable of preventing previous method's problems.

Book Review: Measuring Longitude by Telegraphy: Finding North America: Longitude by Wire

MEASURING LONGITUDE BY TELEGRAPHY Finding North America: Longitude by Wire. Richard Stachurski (University of South Carolina Press, Columbia, 2009). Pp. ? + 239. $29.95. ISBN 978-1-57003-801-3.This book tells a story. Focusing on the U.S. Coast Survey from the early to midnineteenth century, Stachurski narrates the development of a telegraphic method for measuring longitude. It is a competently told story, in the main, and Stachurski provides a readable overview of this important technological development for a general audience. Based largely on published sources, embellished with some imagined scene painting, Longitude by wire aims to bring a vivid set of characters to life in a drama of public science writ large.The opening chapters centre on Ferdinand Hassler, who first led the Coast Survey, following the travails and (partial) triumphs of the Swiss-born savant as he struggled to bring European scientific precision and rigour to a public project meant to serve American pragmatic ends. The remainder of the book focuses on the Coast Survey under Alexander Dallas Bache, one of the most prominent American scientists of the mid-nineteenth century. It was during Bache's superintendency that the innovative method of using telegraphs to coordinate precise time signals - and therefore to measure longitude - came to fruition. One gets a strong sense of the details of problem-solving at the ground level, the challenges and their resolutions.In one sense, it is impressive that Stachurski managed to coax such a textured narrative out of published sources alone. Based primarily on government reports and secondary sources, though perhaps a bit more lightly footnoted than most historians would like, the book reveals well-grounded insights from straightforward readings of those publications. Yet one wonders if an even deeper and richer story could be told through archival materials, especially relating to the practice of the Coast Survey. This could potentially reveal even more about the roles of various subordinate workers, interactions with locals, and further challenges that were smoothed over or ignored in the published reports. There is already enough of a taste of these matters in the book - and their importance to the practice of the Coast Survey - that the reader is left wanting to know still more about what was going on behind the scenes.The orientation of the book tilts toward a general, rather than scholarly, audience. Many basic methods and technologies are presented with textbook-type diagrams and illustrations. …

Power grid fault traveling wave network protection scheme

Based on study on propagation characteristics of traveling wave in transmission lines, a novel power grid fault traveling wave network protection scheme was proposed in this paper. When a line fault occurs, all traveling wave protection relays (TWPR) record arrival time of initial traveling wave using the precise timing signals from Global Positioning System (GPS). Then the ineffective arrival time of initial traveling wave is eliminated and modified, while the effective arrival time of initial traveling wave is utilized to detect the fault line. Different from the traditional protection schemes, which are only based on fault information of one line, the proposed scheme not only solves the problems of traveling wave protection, which are susceptible to the factors (such as the structure of the lines), but also can make reliable tripping action when one or more TWPRs break down, start-up failure or record wrong arrival time of initial traveling wave. Both theoretical analysis and simulations indicate that the protection method has high reliability, and redounds to further enhancing the practicability of traveling wave protection.

Using bifurcations in the determination of lock-in ranges for third-order phase-locked loops

Transmission and switching in digital telecommunication networks require distribution of precise time signals among the nodes. Commercial systems usually adopt a master-slave (MS) clock distribution strategy building slave nodes with phase-locked loop (PLL) circuits. PLLs are responsible for synchronizing their local oscillations with signals from master nodes, providing reliable clocks in all nodes. The dynamics of a PLL is described by an ordinary nonlinear differential equation, with order one plus the order of its internal linear low-pass filter. Second-order loops are commonly used because their synchronous state is asymptotically stable and the lock-in range and design parameters are expressed by a linear equivalent system [Gardner FM. Phaselock techniques. New York: John Wiley & Sons; 1979]. In spite of being simple and robust, second-order PLLs frequently present double-frequency terms in PD output and it is very difficult to adapt a first-order filter in order to cut off these components [Piqueira JRC, Monteiro LHA. Considering second-harmonic terms in the operation of the phase detector for second order phase-locked loop. IEEE Trans Circuits Syst I 2003;50(6):805–9; Piqueira JRC, Monteiro LHA. All-pole phase-locked loops: calculating lock-in range by using Evan’s root-locus. Int J Control 2006;79(7):822–9]. Consequently, higher-order filters are used, resulting in nonlinear loops with order greater than 2. Such systems, due to high order and nonlinear terms, depending on parameters combinations, can present some undesirable behaviors, resulting from bifurcations, as error oscillation and chaos, decreasing synchronization ranges. In this work, we consider a second-order Sallen-Key loop filter [van Valkenburg ME. Analog filter design. New York: Holt, Rinehart & Winston; 1982] implying a third order PLL. The resulting lock-in range of the third-order PLL is determined by two bifurcation conditions: a saddle-node and a Hopf.

Two-way master-slave double-chain networks: limitations imposed by linear master drift for second order PLLs as slave nodes

Distribution of precise time signals among the nodes of a network is a fundamental requirement for digital transmission and switching systems in telecommunication and control. Cideciyan et al., (1987) conjectured that two-way master-slave (TWMS) networks present, in the general case, a better performance than one-way master-slave (OWMS) considering the long term linear master frequency drift. In this work we study the TWMS case using dynamical system theory showing that, due to the effects of long-term clock instabilities, the steady-state frequency-error is unstable for a number of slaves higher or equal than four, limiting the use of this kind of architecture.

GPS travelling wave fault locator systems: investigation into the anomalous measurements related to lightning strikes

A new fault location system based on the travelling wave principle and capable of locating faults on power lines to within /spl plusmn/one tower span (300 m) has been successfully developed and applied to BC Hydro's extensive 500 kV network. Unlike earlier schemes which are based on impedance measurements, its accuracy is not affected by load conditions, high grounding resistance and most notably series capacitor banks. This system measures the time of arrival of a fault-generated travelling wave at the line terminals using the precise timing signals from the Global Positioning System (GPS). Operating experience with the fault locator on lightning related faults indicated highly accurate results were obtained for the majority of the cases. In a few of the lightning-caused disturbances, the system gave anomalous measurements. This paper describes the operation of the system, summarizes the operating experience and explains the observed anomalous measurements.

A time digitizer CMOS gate-array with a 250 ps time resolution

A pipelined time digitizer CMOS gate-array has been developed using 0.5 /spl mu/m Sea-of-Gate technology. Precise timing signals which are used to sample input signals are generated from 32 taps of an asymmetric ring oscillator. The frequency of the oscillator is controlled by a phase-locked loop (PLL) circuit which runs in the 10-50 MHz frequency range. A test chip has been developed and tested; a time resolution of 250 ps rms at 40 MHz clock was measured. The chip has 4 channels and encoding circuits for both the rising and the falling edges of the input signals. The chip has 128-word dual-port memories, allowing the histories of the input signals to be stored and causing no deadtime for the conversion.

A review of satellite time transfer technology: Accomplishments and future applications

A brief review of the research accomplishments by NASA in meeting the needs of the space program for precise time in satellite tracking is presented. As a major user of precise time signals for clock synchronization of NASA's worldwide satellite tracking networks, the agency provided much of the necessary impetus for the development of stable frequency sources and time synchronization technology. The precision in time required for both satellite tracking and space science experiments has increased at a rate of about 1 order of magnitude per decade from 1 ms in the 1950's to 100 μs during the Apollo era in the 1960's to 10 μs in the 1970's. In the 1980's, when the Tracking and Data Relay Satellite System (TDRSS) comes into operation, satellite timing requirements will be extended to 1 μs and below. These requirements are needed for spacecraft autonomy and data packeting which are now in active planning stages.

Precise real‐time signal dissemination over the TV broadcasting satellite

The problem of dissemination of precise time signals over the TV broadcasting satellite is analyzed. The method described is designed to combine the existing system of time and standard frequency (T/ F) signal dissemination over the TV network and the broadcasting possibilities of the satellite, thus forming a new T/F distribution system which could retain the existing format of the T/F signals. Hence the user could utilize the same equipment for both systems. The high precision of the T/F signal dissemination over the ground TV system is retained in the new system, which uses the TV satellite for broader coverage, by possible methods designed to greatly reduce propagation time variations. The method is based on the Time and TV Synchro system previously developed at Studio Belgrade in Yugoslavia.

Transformation to a new time scale and a new system of transmitting standard frequencies and precise time signals

Transformation to a new time scale and a new system of transmitting standard frequencies and precise time signals

Checking chronometers by means of the precise-time signals

Checking chronometers by means of the precise-time signals

Instrument for receiving automatically precise-time radio signals

The device for the automatic locking-in of precise time signals uses a method based on a new principle for fixing these signals by their coincidence area with a rectangular gating pulse. The advantages of this method of forming input pulses in the automatic device as compared with recording the time signal at a given level of its leading edge has been shown [3].