Stable Clock Research Articles

Evolution of orbit and clock quality for real-time multi-GNSS solutions

High-quality satellite orbits and clocks are necessary for multi-GNSS precise point positioning and timing. In undifferenced GNSS solutions, the quality of orbit and clock products significantly influences the resulting position accuracy; therefore, for precise positioning in real time, the corrections for orbits and clocks are generated and distributed to users. In this research, we assess the quality and the availability of real-time CNES orbits and clocks for GPS, GLONASS, Galileo, and BeiDou-2 separated by satellite blocks and types, as well as the product quality changes over time. We calculate the signal-in-space ranging error (SISRE) as the main orbit and clock quality indicator. Moreover, we employ independent orbit validation based on satellite laser ranging. We found that the most accurate orbits are currently available for GPS. However, Galileo utmost stable atomic clocks compensate for systematic errors in Galileo orbits. As a result, the SISRE for Galileo is lower than that for GPS, equaling 1.6 and 2.3 cm for Galileo and GPS, respectively. The GLONASS satellites, despite the high quality of their orbits, are characterized by poor quality of clocks, and together with BeiDou-2 in medium and geosynchronous inclined orbits, are characterized by SISRE of 4–6 cm. BeiDou-2 in geostationary orbits is characterized by large orbital errors and the lowest availability of real-time orbit and clock corrections due to a large number of satellite maneuvers. The quality of GNSS orbit and clock corrections changes over time and depends on satellite type, block, orbit characteristics, onboard atomic clock, and the sun elevation above the orbital plane.

Frequency stability characterization: DFB laser and Raman pump performance on a distributed clock signal over 24.69 km fiber.

Distribution of timing and frequency signals in a fast-changing world requires unprecedented levels of stability for characterization. Transfer of frequency references over long distance without introducing any additional instability is of urgent concern for optical clock development. Choice of optical transmitter is critical to achieving accurate and stable RF clocks for end users. In this paper, we report the stability performance of the distributed feedback (DFB) laser and the Raman pump for transmitted clock signals. The DFB laser and the Raman pump were modulated with 2, 4, and 6 GHz RF clock signals from a signal generator and transmitted over 24.69 km SMF-Reach fiber. At 10 kHz offset frequency, we measured lowest phase noise of -121.22dBc/Hz and highest spectra power of -5.38dBm at 2 GHz for the DFB laser. Transfer stabilities of 1.366×10-12 and 1.626×10-12 for 2 and 4 GHz, respectively, at 1000 s averaging time were achieved. This technique does not require additional amplifiers for long-distance frequency distribution, making it simple and economical, and hence satisfying the requirements for next-generation optical fiber networks.

Light-resetting impact on behavior and the central circadian clock in two vole species (genus: Lasiopodomys).

The behavioral circadian rhythms of subterranean rodents show intra- and interspecies diversity in terms of adaptation to dark underground environments, but the endogenous molecular mechanism of rhythm regulation in the suprachiasmatic nuclei (SCN) is stable to many species. In this study, we sought to determine the rhythms of behavior and central molecular regulatory mechanisms in the SCN of the subterranean Mandarin voles (Lasiopodomys mandarinus) compared with a related aboveground species, Brandt's voles (Lasiopodomys brandtii). Both species were reared under a 12L:12 D cycle or in continuous darkness for 4weeks. The pattern of wheel-running activity was similar in both species and had a periodicity of almost 24h regardless of rearing conditions. However, the intensity of daily activity in Brandt's voles decreased markedly in darkness, while there was no significant difference in activity intensity in mandarin voles under different light regimes. In both vole species, all tested genes in the SCN showed significant time-dependent expression regardless of rearing conditions, and the expression levels of most genes did not differ significantly between different species and conditions. However, the peak phase shift in gene expression differed between the two species. In conclusion, behavioral patterns in mandarin and Brandt's voles were regulated by a stable molecular endogenous biological clock. The observed differences in activity intensity and phase shift suggest that different mechanisms regulate circadian rhythms in different living environments.

The NTSC VLBI System and its application in UT1 measurement

In order to measure the Universal Time (UT1) in real time, National Time Service Center (NTSC) has built a VGOS-like (VLBI Global Observing System) broadband VLBI network, which includes three 13-m radio telescopes located in Jilin, Sanya and Kashi, and a data analysis center in Xi’an. Each station is equipped with a highly stable hydrogen atomic clock and a self-developed VLBI backend, and is colocated with two GPS receivers. This VGOS-like VLBI network may play an important role in improving the Chinese broadband VLBI technology and making valuable contributions to domestic VLBI measurements of UT1. In this paper, we introduce the specifications of this VLBI network, and present the UT1 measurements at C-band conducted in 2018 using the Jilin-Kashi baseline of this network. The comparisons between our UT1 estimates and those provided by IERS suggest that the NTSC VLBI network is capable to determine UT1 accurate at the level of 58.8 μs.

Improved Strategies for BeiDou Ultrarapid Satellites’ Clock Bias Prediction Using BDS-2 and BDS-3 Integrated Processing

GNSS ultrarapid clock biases are key inputs of rapid high-accuracy applications, especially for its prediction parts. With the fast development of the BeiDou system (BDS), the system performances are mainly represented by orbit and clock products. However, it is suggested that the BDS-predicted clock biases cannot meet the requirement of real-time or near real-time services. In this research, the BDS satellite-predicted ultrarapid clock bias products are optimized with three methods, namely, one-step strategy, intersatellite correlation, and variogram model, using the combined estimation of BDS-2 and BDS-3 satellites. Firstly, considering the traditional two-step strategy for modelling clock bias prediction, we take all terms (including trend and periodic terms) into one-step solution of model estimation based on the sparse modelling in machine learning. Secondly, because of the much more stable on-board atomic clock of BDS-3 satellites, the intersatellite correlations between BDS-2 and BDS-3 are utilized to enhance the solution of model coefficients. Thirdly, to further improve the model, the temporal correlations in model residuals are used to reconstruct the stochastic function obtained by variogram. In addition, to verify the proposed improved strategies, 12 schemes of BDS clock bias prediction experiments are designed and analyzed with different conditions. According to the results of predicted clock biases, it is indicted that (1) the stability of BDS-3 on-board clocks is more optimal compared with BDS-2, which can be used to strengthen the solution of the clock bias prediction model; (2) the one-step estimation of the clock bias model by sparse modelling can slightly increase the accuracy of prediction results; (3) both BDS-2- and BDS-3-predicted clock biases benefited each other by inserting the intersatellite correlations into the weight matrix, in which the accuracy of 18-hour period with one-step strategy can be improved by 28.6% and 27.2% for BDS-2 and BDS-3, respectively; and (4) after the introduction of the variogram model in updating the weight matrix, the clock bias prediction model is further corrected by 8.0% and 11.1% for BDS-2 and BDS-3. In summary, improved strategies for BDS ultrarapid satellites’ clock bias prediction using BDS-2 and BDS-3 integrated processing are meaningful for the current BDS ultrarapid satellites’ clock bias prediction products.

Clock and Data Recovery-Free Data Communications Enabled by Multi-Core Fiber With Low Thermal Sensitivity of Skew

Optical switching has the potential to scale the capacity of data center networks (DCN) with a simultaneously reduction in latency and power consumption. One of the main challenges of optically-switched DCNs is the need for fast clock and data recovery (CDR). Because the DCN traffic is dominated by small packets, the CDR locking time is required to be less than one nanosecond for achieving high network throughput. This need for sub-nanosecond CDR locking time has motivated research on optical clock synchronization techniques, which deliver synchronized clock signals through optical fibers such that the CDR modules in each transceiver only need to track the slow change of clock phase, due to change of the time of flight as temperature varies. It is desired to remove the need for clock phase tracking (and thereby the CDR modules) if the temperature-induced clock phase drift can be significantly reduced, which would reduce the power consumption and the cost of transceivers. Previous studies have shown that the temperature-induced skew change between multi-core fiber (MCF) cores can be forty times lower than that of standard single mode fibers. Thus, clock-synchronized transmission maybe possible by using two different MCF cores for clock and data transmission, respectively, enabling the sharing of an optical clock with stable clock phase. To investigate the potential of MCF for CDR-free short-reach communications, we first improve the measurement method of the temperature dependent inter-core skew change by using a modified delay interferometer, achieving a resolution of 3.8 femtoseconds for accurate inter-core skew measurements. Building on the MCF measurement results, we carried out an MCF-based clock-synchronized transmission experiment, demonstrating the feasibility of CDR-free data communications over a temperature range of 43 $^{\circ }$ C that meets DCN requirements.

Simulation of motion of many ions in a linear Paul trap

The quadrupole linear Paul trap is one of the key instruments in building highly stable atomic clocks. However, a frequency reference based on a single trapped ion is limited in stability due to the time needed for the interrogation cycle which cannot be further shortened. A promising strategy is the utilization of multiple trapped ions. The ions of the same kind then repulse each other with the Coulomb force, which is countered by the ponderomotive force of the time depended field in the trap. A few ions form a chain along the axis of a linear Paul trap. Adding more ions (a few tens or hundreds) gives rise to Coulomb crystals. We created an efficient simulation code which calculates the motion of such collections of ions in quasi-static radiofrequency fields of real linear quadrupole traps (including the micromotion). We include a model for laser cooling of the ions. The simulation tool can be used to study the formation and the dynamics of Coulomb crystals under conditions corresponding to various experimental setups.

A flexible strategy for handling the datum and initial bias in real-time GNSS satellite clock estimation

Investigations have been focused on improving the precision of real-time GNSS satellite clock corrections. There is a little literature analyzing the characteristics and impacts of the clock datum and initial clock biases (ICBs). We illustrated the clock anomaly caused by the two factors and proposed a new strategy flexible for global and regional clock estimation based on the mixed-difference observation method. First, we analyzed the clock datum of the final and real-time clocks. Results show that the variation in the clock datum can be up to 500 ns per day depending on the datum definition strategy. The variations can lead to large inconsistencies in the datum for the re-convergence of estimated satellite clocks when using the mixed-difference clock estimation method. To resolve the problem, we propose a new method for the epoch-by-epoch datum definition, which can screen out any outliers in broadcast ephemeris in real time, and particularly suitable for the real-time clock estimation. Second, we analyzed ICB jumps in real-time clock corrections, which can degrade the accuracy of precise point positioning. We propose a short-term prediction of clock corrections for bridging ICBs with the precision better than 0.1 ns for stable satellite clocks within a gap of 1–3 min. The prediction can decrease the magnitude of ICB jumps and thus improve a user solution. Considering the prediction of clocks, high precision cannot be guaranteed for all conditions, we thus recommend broadcasting a discontinuity flag for real-time service, so the re-convergence of ICB can be identified by users for resetting carrier-phase ambiguities.

An Atomic-Array Optical Clock with Single-Atom Readout

Currently, the most accurate and stable clocks use optical interrogation of either a single ion or an ensemble of neutral atoms confined in an optical lattice. Here, we demonstrate a new optical clock system based on an array of individually trapped neutral atoms with single-atom readout, merging many of the benefits of ion and lattice clocks as well as creating a bridge to recently developed techniques in quantum simulation and computing with neutral atoms. We evaluate single-site resolved frequency shifts and short-term stability via self-comparison. Atom-by-atom feedback control enables direct experimental estimation of laser noise contributions. Results agree well with an ab initio Monte Carlo simulation that incorporates finite temperature, projective read-out, laser noise, and feedback dynamics. Our approach, based on a tweezer array, also suppresses interaction shifts while retaining a short dead time, all in a comparatively simple experimental setup suited for transportable operation. These results establish the foundations for a third optical clock platform and provide a novel starting point for entanglement-enhanced metrology, quantum clock networks, and applications in quantum computing and communication with individual neutral atoms that require optical clock state control.

Coincidence of Homeostasis and Bifurcation in Feedforward Networks

Homeostasis is an important and common biological phenomenon wherein an output variable does not change very much as an input parameter is varied over an interval. It can be studied by restricting attention to homeostasis points — points where the output variable has a vanishing derivative with respect to the input parameter. In a feedforward network, if a node has a homeostasis point then downstream nodes will inherit it. This is the case except when the downstream node has a bifurcation point coinciding with the homeostasis point. We apply singularity theory to study the behavior of the downstream node near these homeostasis-bifurcation points. The unfoldings of low codimension homeostasis-bifurcation points are found. In the case of steady-state bifurcation, the behavior includes multiple homeostatic plateaus separated by hysteretic switches. In the case of Hopf bifurcation, the downstream node may have limit cycles with a wide range of near-constant amplitudes and periods. Homeostasis-bifurcation is therefore a mechanism by which binary, switch-like responses or stable clock rhythms could arise in biological systems.

Modulated Raman pump for integrated VCSEL-based reach enhancement and clock tone dissemination in optical communication

The fast growth of data traffic in access networks calls for high speed and low-cost optical links. To extend reach for users in metro areas requires optical amplification of the signal carrying data. In this paper we experimentally demonstrate a cost effective technique of extending reach by exploiting Raman pump to both amplify data signal and transmit clock signals. A 1550 nm VCSEL is modulated with a 10 Gbps data and coupled into a 50.7 km fibre to co-propagate with a 4 GHz modulated Raman pump. Error free and enhanced data transmission was achieved with minimum receiver sensitivity of −14.8 dBm and distributed stable clock signal over 50.7 km SMF-Reach. A maximum phase noise of −100.6 dBc/Hz at 10 kHz offset frequency is experimentally measured for 50.7 km fibre transmission with backward Raman amplification. This work presents a real-time technique of transmitting accurate data and stable clock signal for long-reach optical networks.

Measuring phase scintillation at different frequencies with conventional GNSS receivers operating at 1\xa0Hz

Ionospheric scintillation causes rapid fluctuations of measurements from Global Navigation Satellite Systems (GNSSs), thus threatening space-based communication and geolocation services. The phenomenon is most intense in equatorial regions, around the equinoxes and in maximum solar cycle conditions. Currently, ionospheric scintillation monitoring receivers (ISMRs) measure scintillation with high-pass filter algorithms involving high sampling rates, e.g. 50 Hz, and highly stable clocks, e.g. an ultra-low-noise Oven-Controlled Crystal Oscillator. The present paper evolves phase scintillation indices implemented in conventional geodetic receivers with sampling rates of 1 Hz and rapidly fluctuating clocks. The method is capable to mitigate ISMR artefacts that contaminate the readings of the state-of-the-art phase scintillation index. Our results agree in more than 99.9% within ± 0.05 rad (2 mm) of the ISMRs, with a data set of 8 days which include periods of moderate and strong scintillation. The discrepancies are clearly identified, being associated with data gaps and to cycle-slips in the carrier-phase tracking of ISMR that occur simultaneously with ionospheric scintillation. The technique opens the door to use huge databases available from the International GNSS Service and other centres for scintillation studies. This involves GNSS measurements from hundreds of worldwide-distributed geodetic receivers over more than one Solar Cycle. This overcomes the current limitations of scintillation studies using ISMRs, as only a few tens of ISMRs are available and their data are provided just for short periods of time.

RHBD Charge Pump PLL based 2.4 GHz Frequency Synthesizer

Frequency Synthesizer forms the heart of electronic communication system. Phase Locked Loop (PLL) based Frequency Synthesizers over the years has become the ubiquitous solution for generation of stable clock source. But it is a challenging task to design and develop PLL to be used in radiation environment such as in satellites, space systems and military electronics. Since impact of radiation strike on PLL is said to introduce transient faults resulting in increased timing jitter, distortion in phase, and bit flips. One or more of the above said effects can initiate false triggering which may result in incorrect data to be latched, loss of synchronization in data processing and networking. This may lead to catastrophic effect. Hence, as the stability of frequency synthesizer is of vital importance, there is a stressful need for design of radiation hard, fault tolerant frequency synthesizer. With this motivation, in this paper, a radiation hard CMOS Charge Pump PLL is designed to synthesize a 2.4GHz frequency source using 20MHz reference input frequency. The proposed radiation hard PLL design uses a hybrid Radiation Hardening By Design (RHBD) fault tolerant technique combined with redundancy, hence offering a twofold level of fortification from radiation spikes. Cadence tool was used for simulation. The PLL designed has exhibited satisfactory performance. The RHBD Charge Pump PLL in presence of radiation strike resulted in rms jitter of 128.9ps, phase noise of -94.03dbc/Hz and settling time of 159ns against the IEEE 802.11b/g standard requirement of 250ps jitter, -110dbc/Hz phase noise and 10us setting time.

Clock Skew Based Computer Identification on Different Types of Area Networks

The increase of client devices along with the growth of internet access currently affects to security threats at the user's identity. Identifiers that commonly used today, such as SSID, IP address, MAC address, cookies, and session IDs have a weakness, which is easy to duplicate. Computer identification based on clock skew is an identification method that is not easily duplicated because it is based on the hardware characteristics of the device. Clock skew is the deviation of the clock to the true time which causes each clock to run at a slightly different speed. This study aims to determine the effect of network types to the clock skew stability as a reliable device identification method. This research was conducted on five client computers which running windows and linux operating systems. The measurement was conducted based on three different types of area networks, i.e., LAN, MAN, and WAN. The skew estimation was done using two linear methods i.e., linear programming and linear regression. The measurement results show that the most stable clock skew is found on the LAN measurement because it meets the threshold tolerance limit i.e., ±1 ppm. Skew estimation using linear programming method has better accuracy than linear regression method.

Design of a passive CMOS implantable continuous monitoring biosensors transponder front-end

This paper presents the design of an implantable passive transponder front-end that can power implantable biosensors and also provide a power-efficient communication means for it with an extracorporeal reader, enabling continuous real-time monitoring of internal body parameters. The implantable passive transponder front-end is composed of power management circuity, demodulator, oscillator, and load-shift keying modulator. To control and power the implantable biosensors we used a 13.56 MHz Pulse Width Modulated signal. A stable DC voltage, data and clock are extracted from this modulated signal. The proposed transponder was designed, simulated and layout in a 40-nm 1P9M TSMC's CMOS technology. The simulation shows that the proposed implantable passive transponder front-end has a total average power consumption of 17.14 μW, with receiving data rate of 142.85 kB/s and transmitting data rate of 50 kB/s.

G132-12: A one-mode ZZ Ceti star located at the blue edge of instability strip

G132-12 is a ZZ Ceti star close to the blue edge of the instability strip, where the stars show few pulsation modes with short periods and low amplitudes in general. We obtained nearly 90 h of time-series photometric data from single site observations in both September and October of 2017. After carrying out aperture photometry, the light curves were acquired and analyzed. Only one mode with the period of 212.69 s and the amplitude of about 3 mmag is detected. G132-12 is hence confirmed to be a one-mode ZZ Ceti star with short period and low amplitude near the blue edge of the instability strip. In this sense, it might be another stable white dwarf clock, providing a good opportunity for exploring the pulsation characteristics of members located near the blue edge of instability strip. In addition, we note that G132-12 is included in the list of TESS candidate target. Hence, our research provides a useful reference for future work on this star based on the TESS data.

Measurement of Ionospheric Total Electron Content Using Single‐Frequency Geostationary Satellite Observations

AbstractThe ionized upper portion of the atmosphere, the ionosphere, affects radio signals traveling between satellites and the ground. This degrades the performance of satellite navigation, surveillance, and communication systems. Techniques to measure and mitigate ionospheric effects and in particular to measure the total electron content (TEC) are therefore required. TEC is usually determined by analyzing the differential delay experienced by dual‐frequency signals. Here we demonstrate a technique which enables TEC to be derived using single‐frequency signals passing between geostationary satellites and terrestrial Global Positioning System (GPS) receivers. Geostationary satellites offer the key advantage that the raypaths are not moving and hence are easier to interpret than standard GPS TEC. Daily TEC time series are derived for three ground receivers from Europe over the year 2015. The technique is validated by correlation analysis both between pairs of ground receiver observations and between ground receivers and independent ionosonde observations. The correlation between pairs of receivers over a year shows good agreement. Good agreement was also seen between the TEC time series and ionosonde data, suggesting the technique is reliable and routinely produces realistic ionospheric information. The technique is not suitable for use on every GPS receiver type because drift in derived TEC values was observed for profiles calculated using receivers without links to highly stable clocks. The demonstrated technique has the potential to become a routine method to derive TEC, helping to map the ionosphere in real time and to mitigate ionospheric effects on radio systems.

Fast start crystal oscillator design with negative resistance control

Clock is an essential part of most of the integrated circuits as time base reference. It must be very accurate, highly reliable and readily available as soon as it is enabled. In all types of oscillator architectures, crystal oscillator is the most accurate and stable clock generator. But usually the crystal oscillator circuit suffers from slow startup. Therefore, it is essential to improve the startup time with optimally controlled crystal drive such that crystal drive power rating is not compromised. We propose a method that discusses about increasing the negative resistance during startup, using a startup circuit for fast start. Once the reliable startup is achieved, the negative resistance is decreased and the startup circuit is disconnected. The reduction in negative resistance is done with current steps and there are two ways in which it is achieved, the Digital control method and Analog control method. In digital control method, the current steps are timed at regular intervals and in analog control method, oscillator output amplitude is given as feedback to the startup circuit there by reducing the negative resistance. In the 32768 Hz real time clock generating oscillator, the startup time can be improved from 330 ms to 220 ms using the conventional startup method. With the proposed digital control method, lesser startup time of 160 ms is achieved and in analog control method it is further reduced to 120 ms.

A ± 1.55ppm Stable FBAR Reference Clock with Oven-Controlled Temperature Compensation

We present an oven-controlled FBAR oscillator that achieves a frequency stability of +/-1.55ppm from -5°C to 85°C. The highly integrated system includes a 0.64mm2 FBAR chip with integrated heater and sensor resistors and a 3 mm2 CMOS chip with the control electronics. The oscillator achieves an Allen deviation of 4ppb enabled by a temperature-to-digital converter (TDC) with a 150uK resolution. It corresponds to a 1.68JK2 FOM. The heater consumes a power of 14mW at -5°C and the oscillator consumes only 0.25mW. The ovenized oscillator meets the stringent frequency stability requirements (2 ppm) of GPS applications.

GPS/GLONASS Combined Precise Point Positioning With the Modeling of Highly Stable Receiver Clock in the Application of Monitoring Active Seismic Deformation

AbstractThe high‐rate kinematic Precise Point Positioning (PPP) of the Global Navigation Satellite System has become an effective method for monitoring crustal deformation caused by earthquakes. In this contribution, the method of GPS/GLONASS PPP with the receiver clock modeling is applied in active seismic deformation monitoring for the first time. With the modeling method, the short‐term vertical positioning accuracy of 2–4 mm that usually cannot be obtained by standard PPP is achieved. Our PPP results confirm that the positioning accuracy is improved due to the increase of GLONASS observations compared to the GPS‐only solution. Based on the external seismic data and the high‐rate GPS/GLONASS data for the 2011 Japan earthquake and 2010 and 2015 Chile earthquakes, comparative analyses concerning receiver clock modeling are carried out. The results show that a high degree of decorrelation between the height position estimates and receiver clock offsets can be obtained by using the receiver clock modeling. The short‐term accuracy of the GPS‐based vertical displacements is improved to the level of about 4.4 mm, and the short‐term accuracy of better than 4 mm for the GPS/GLONASS‐combined vertical displacements is achievable. Furthermore, the weak vertical signals that are not detected by standard PPP can be captured with the modeling of highly stable receiver clock.