On-board Oscillator Research Articles

Clock noise reduction in geometric time delay interferometry combinations

Time-delay interferometry (TDI) is a critical data post-processing technique in space-borne laser interferometer gravitational wave detection, which is developed to suppress the overwhelming laser phase noise below the instrumental noise floor by time-shifting and synthesizing the interferometric data streams. There are virous kinds of TDI combinations, and geometric TDI, as a special means to search TDI combination, has been widely used because of its clear physical significance, i.e. constructing virtual equal-arm interferometer to realize an excellent differential performance of the laser phase noise. To achieve the desired noise level, it is necessary to further suppress the phase fluctuations of the onboard ultra-stable oscillators (USO) used as reference clocks. In this paper, we analyze the characteristics of geometric TDI clock noise residual, and formulate it into a general expression. We find the intrinsic equal-arm length constraint of geometric TDI is a sufficient condition for clock noise reduction, which makes the clock noise terms only appear in differential form. Based on this, we propose a clock noise reduction scheme. Compared with the existing algorithms, our method benefits from the sufficient condition to avoid the problem that the clock noise terms without any time-shift operators cannot be properly handled.

Trigger Timing Interface for the Read-Out Upgrade of the Belle II DAQ

To improve data throughput of the Belle II data acquisition we are upgrading the CPU-based COPPER system with a PCIe40 board carrying Arria 10 FPGA. Since one of the main functionalities of the new system is event building in FPGA, the read-out system must be synchronized with the front-end electronics. This task is performed by the bidirectional trigger timing distribution system. During system commissioning, we prepared several versions of the interface to this system. In the initial version of the interface, we ported the code from Xilinx FPGAs to Arria 10. This revision also introduces monitoring of the status for multiple channels and a ring buffer to distribute trigger information to all channels in parallel. To improve stability under external noise, we implemented a clock-data recovery using an independent on-board oscillator as a reference clock in the next revision of the interface. We are also developing a version utilizing a high-speed serial transceiver to replace CAT-7 RJ45 cables with optical fibers. The system commissioning started in 2021 with a few detectors and will be completed after the long shutdown 1 of SuperKEKB in 2023. In this paper, we present the architectures of the interface to the trigger timing system implemented in the PCIe40 board and the system performance in the experiment.

In-orbit Timing Calibration of the Insight-Hard X-Ray Modulation Telescope

Abstract We describe the timing system and the timing calibration results of the three payloads on board the Insight-Hard X-ray Modulation Telescope (Insight-HXMT). These three payloads are the High Energy X-ray telescope (HE; 20–250 keV), the Medium Energy X-ray telescope (ME; 5–30 keV), and the Low Energy X-ray telescope (LE; 1–10 keV). We present a method to correct the temperature-dependent period response and the long-term variation of the onboard crystal oscillator, especially for ME, which does not carry a temperature-compensated crystal oscillator. The times of arrival (ToAs) of the Crab pulsar are measured to evaluate the accuracy of the timing system. As the ephemeris of the Crab pulsar given by the Jodrell Bank Observatory has systematic errors around (Rots et al. 2004) 40 μs, we use the quasi-simultaneous observations of the X-ray Timing Instrument (XTI) on board the Neutron star Interior Composition Explorer (NICER) to produce the Crab ephemerides and to verify the timing system of Insight-HXMT. The energy-dependent ToAs’ offsets relative to the NICER measurements including the physical and instrumental origins are about 24.7 μs, 10.1 μs, and 864.7 μs, and the systematic errors of the timing system are determined to be 12.1 μs, 8.6 μs, and 15.8 μs, for HE, ME, and LE, respectively.

Refined clock-jitter reduction in the Sagnac-type time-delay interferometry combinations

The ongoing development of the space-based laser interferometer missions is aiming at unprecedented gravitational wave detections in the millihertz frequency band. The spaceborne nature of the experimental setups leads to a degree of subtlety regarding the otherwise overwhelming laser frequency noise. The cancellation of the latter is accomplished through the time-delay interferometry technique. Moreover, to eventually achieve the desired noise level, the phase fluctuations of the onboard ultra-stable oscillator must also be suppressed. This can be fulfilled by introducing sideband signals which, in turn, give rise to an improved cancellation scheme accounting for the clock-jitter noise. Nonetheless, for certain Sagnac-type interferometry layouts, it can be shown that resultant residual clock noise found in the literature can be further improved. In this regard, we propose refined cancellation combinations for two specific clock noise patterns. This is achieved by employing the so-called geometric time-delay interferometry interpretation. It is shown that for specific Sagnac combinations, the residual noise diminishes significantly to attain the experimentally acceptable sensitivity level. Moreover, we argue that the derived combination, in addition to the existing ones in the literature, furnishes a general-purpose cancellation scheme that serves for arbitrary time-delay interferometry combinations. The subsequential residual noise will only involve factors proportional to the commutators between the delay operators. Our arguments reside in the form of the clock noise expressed in terms of the coefficients of the generating set of the first module of syzygies, the linear combination of which originally constitutes the very solution for laser noise reduction.

Absolute frequency readout derived from ULE cavity for next generation geodesy missions.

The next generation of Gravity Recovery and Climate Experiment (GRACE)-like dual-satellite geodesy missions proposals will rely on inter-spacecraft laser interferometry as the primary instrument to recover geodesy signals. Laser frequency stability is one of the main limits of this measurement and is important at two distinct timescales: short timescales over 10-1000 seconds to measure the local gravity below the satellites, and at the month to year timescales, where the subsequent gravity measurements are compared to indicate loss or gain of mass (or water and ice) over that period. This paper demonstrates a simple phase modulation scheme to directly measure laser frequency change over long timescales by comparing an on-board Ultra-Stable Oscillator (USO) clocked frequency reference to the Free Spectral Range (FSR) of the on-board optical cavity. By recording the fractional frequency variations the scale correction factor may be computed for a laser locked to a known longitudinal mode of the optical cavity. The experimental results demonstrate a fractional absolute laser frequency stability at the 10 ppb level (10-8) at time scales greater than 10 000 seconds, likely sufficient for next generation mission requirements.

Clock-jitter reduction in LISA time-delay interferometry combinations

The Laser Interferometer Space Antenna (LISA) is a European Space Agency mission that aims to measure gravitational waves in the millihertz range. The three-spacecraft constellation forms a nearly-equilateral triangle, which experiences flexing along its orbit around the Sun. These time-varying and unequal armlengths require to process measurements with time-delay interferometry (TDI) to synthesize virtual equal-arm interferometers, and reduce the otherwise overwhelming laser frequency noise. Algorithms compatible with such TDI combinations have recently been proposed in order to suppress the phase fluctuations of the onboard ultra-stable oscillators (USO) used as reference clocks. In this paper, we propose a new method to cancel USO noise in TDI combinations. This method has comparable performance to existing algorithms, but is more general as it can be applied to most TDI combinations found in the literature. We compute analytical expressions for the residual clock noise before and after correction, accounting for the effect of time-varying beatnote frequencies, previously neglected. We present results of numerical simulations that are in agreement with our models, and show that clock noise can be suppressed below required levels. The suppression algorithm introduces a new modulation noise, for which we propose a partial mitigation. This modulation noise remains the limiting effect for clock-noise suppression, setting strict timing requirements on the sideband generation.

Testing the product quality of Galileo and GPS on-board oscillators

The final clock product provided by the IGS (International GNSS Service) with a 30/300 s sampling interval for satellites and stations respectively is characterized by accuracy of 75 ± 20 ps RMS (Root Mean Square). Since mid-2012, the MGEX (Multi-GNSS Experiment) experiment provides full products containing all available GNSS (Global Navigation Satellite Systems) satellite signals. In this paper, the phase, frequency and stability of Galileo and GPS (Global Positioning System) on-board oscillators based on the MGEX data are analysed. Moreover, the authors also determined the noise type for specific on-board oscillators depending on GNSS type and frequency standard. The analysis showed different characteristics of clock correction phase and frequency for both satellite systems. Also, each system and clock type has different noise for each averaging time. In order to distinguish more types of noises, three different variants of stability analysis were conducted: Allan deviation (ADEV), modified Allan deviation (MDEV), and Hadamard deviation (HDEV).

Improving the GRACE Kinematic Precise Orbit Determination Through Modified Clock Estimating.

Utilizing global positioning system (GPS) to determine the precise kinematic orbits for the twin satellites of the Gravity Recovery and Climate Experiment (GRACE) plays a very important role in the earth’s gravitational and other scientific fields. However, the orbit quality is highly depended on the geometry of observed GPS satellites. In this study, we propose a kinematic orbit determination method for improving the GRACE orbit quality especially when the geometry of observed GPS satellites is weak, where an appropriate random walk clock constraint between adjacent epochs is recommended according to the stability of on-board GPS receiver clocks. GRACE data over one month were adopted in the experimental validation. Results show that the proposed method could improve the root mean square (RMS) by 20–40% in radial component and 5–20% in along and cross components. For those epochs with position dilution of precision (PDOP) larger than 4, the orbits were improved by 50–70% in radial component and 17–50% in along and cross components. Meanwhile, the Allan deviation of clock estimates in the proposed method was much closer to the reported Allan deviation of GRACE on-board oscillator. All the results confirmed the improvement of the proposed method.

Monitoring of Galileo on-board oscillators variations, disturbances & noises

In Global Navigation Satellite Systems (GNSSs), the application of the satellite clocks is essential and pertains to the ground of the positioning operation. Any error in the time measure, and hence distance, in a simple way leads to the positioning error. Accordingly, for precise positioning, the on-board clock behaviour needs to be continuously monitored. Currently, several GNSS systems are in use and their on-board clocks are equipped with different technologies. In this paper the author performed 120 days (001–120 DOY [Day of Year] 2018) analysis of the European GNSS system – Galileo clock corrections. Data – acquired from CODE MGEX (the Centre for Orbit Determination in Europe, the Multi-GNSS Experiment and Pilot Project) products – were processed, combined and finally analysed using Allan variance for measure of frequency clocks stability. In addition, the author focused on the anomalous trends and frequency behaviour in the clock characteristics. The performed analysis of the 16 available Galileo satellites signals reveal some clocks anomalies in a part of them.

A Novel Method for Phasor Measurement Unit Sampling Time Error Compensation

This paper focuses on a novel method for successive approximation register analog to digital converter control in synchronized phasor measurement units (PMUs). To compensate for the sampling time error caused by the division remainder between the desirable sampling rate and the oscillator frequency, a variable sampling interval control method is presented by interlacing two integers under a proposed criterion. The frequency of the onboard oscillator is monitored in real-time using the pulse per second (PPS) timing reference from global positioning system. The sampling control is adaptively adjusted each second according to the latest estimated oscillator frequency. The “saw tooth” in phasor angle error and DC offset and spikes in frequency error can be effectively eliminated by applying the proposed method. The simulation and experiment results validate the effectiveness and accuracy of the proposed method, which is believed to be able to greatly improve the performance of PPS-disciplined synchronous sampling methods in PMUs.

Initial performance of the radio occultation experiment in the Venus orbiter mission Akatsuki

After the arrival of Akatsuki spacecraft of Japan Aerospace Exploration Agency at Venus in December 2015, the radio occultation experiment, termed RS (Radio Science), obtained 19 vertical profiles of the Venusian atmosphere by April 2017. An onboard ultra-stable oscillator is used to generate stable X-band downlink signals needed for the experiment. The quantities to be retrieved are the atmospheric pressure, the temperature, the sulfuric acid vapor mixing ratio, and the electron density. Temperature profiles were successfully obtained down to ~ 38 km altitude and show distinct atmospheric structures depending on the altitude. The overall structure is close to the previous observations, suggesting a remarkable stability of the thermal structure. Local time-dependent features are seen within and above the clouds, which is located around 48–70 km altitude. The H2SO4 vapor density roughly follows the saturation curve at cloud heights, suggesting equilibrium with cloud particles. The ionospheric electron density profiles are also successfully retrieved, showing distinct local time dependence. Akatsuki RS mainly probes the low and middle latitude regions thanks to the near-equatorial orbit in contrast to the previous radio occultation experiments using polar orbiters. Studies based on combined analyses of RS and optical imaging data are ongoing.Graphical abstract.

Phase-locked loops in acoustic analysis

Tracking of signal frequency and/or phase is made much more challenging by the presence of “rapid” frequency or phase changes in a signal—occurring in a time short compared to the amount of frequency shift: Δf×Δt << 1. In these scenarios, the usual Fourier decomposition does not provide detail sufficient to tell what is going on. Although there are time-frequency tricks one can use (such as the reassigned spectrogram, or wavelet analysis), these do not provide simple answers to the question of what is the phase as a function of time, and they do not track the instantaneous frequency as effectively as is wanted. An interesting solution from the realm of electrical engineering is provided by the phase-locked loop (PLL) detector: a construct that takes the input signal and tries to phase lock an on-board oscillator to it. It turns out that this process is surprisingly effective at tracking instantaneous frequency/phase while discarding noise if an initial estimate of the target signal frequency is known. We present examples of the superior frequency and phase tracking provided by a PLL approach, including the extremely brief chirps produced by electric fish, and the forensic detection of editing cuts in audio samples.

FPGA-Based High-Frequency Digital Pulse Width Modulator Architecture for DC-DC Converters

Digital pulse width modulator is an integral part in digitally controlled Direct Current to Direct Current (DC-DC) converter utilized in modern portable devices. This paper presents a new Digital Pulse Width Modulator (DPWM) architecture for DC-DC converter using mealy finite state machine with gray code encoding scheme and one hot encoding method to derive the variable duty cycle Pulse Width Modulation (PWM) signal without varying the clock frequency. To verify the proposed DPWM technique, the architecture with control input of six, five and four bits are implemented and the maximum operating frequency along with power consumption results is obtained for different Field Programmable Gate Array (FPGA) devices. The post layout timing results are presented showing that architecture can work with maximum frequency of 326 MHz and derive PWM signal of 3.59 MHz. Experimental results show the implementation of the proposed architecture in low-cost FPGA (Spartan 3A) with on-board oscillator clock frequency of 12 MHz which is multiplied internally by two with Digital Clock Manager (DCM) and derive the PWM signal of 1.5 MHz with a time resolution of 1 ps.

Time Transfer by Laser Link: Data analysis and validation to the ps level

The Time Transfer by Laser Link (T2L2) is a very high resolution time transfer technique based on the recording of arrival times of laser pulses at the satellite. T2L2 was designed to achieve time stability in the range of 1ps over 1000s and an accuracy better than 100ps. The project is in operation onboard the Jason-2 satellite since June 2008. The principle is based on the Satellite Laser Ranging (SLR) technology; it uses the input of 20–25 SLR stations of the international laser network which participate in the tracking. This paper focuses on the data reduction process which was developed specifically to transform the raw information given by both space instrument and ground network: first to identify the triplets (ground and onboard epochs and time of flight of the laser pulse), second to estimate a usable product in terms of ground-to-space time transfer (including instrumental corrections), and thirdly to produce synchronization between any pair of remote ground clocks. In describing the validation of time synchronizations, the paper opens a way for monitoring the time difference between ultra-stable clocks thanks to a laser link at a few ps level for Common View passes. It highlights however that without accurately characterizing the onboard oscillator of Jason-2 and knowing the unavailability of time calibrations of SLR stations generally, time transfer over intercontinental distances remain difficult to be accurately estimated.

Demonstration Experiments of a Remote Synchronization System of an Onboard Crystal Oscillator Using “MICHIBIKI”

Two kinds of demonstration experiments have been performed for the Remote Synchronization System of the Onboard Crystal Oscillator (RESSOX) for the first Japanese Quasi-Zenith Satellite System (QZSS) “MICHIBIKI.” In Experiment One, a RESSOX control signal that includes information of the standard time is sent from ground stations and the onboard crystal oscillators (VCOCXO) are controlled to synchronize the arrival of the RESSOX control signal. The maximum synchronization error is 50 ns. In Experiment Two, on the basis of the results of the time comparison between VCOCXO and ground standard time, the voltage applied to VCOCXO is calculated at the ground station and transmitted. The synchronization error of the time comparison is less than 0.3 ns for six days. Moreover, switchover experiments between Okinawa and Tokyo stations were successfully conducted. Although the QZSS is loaded with two rubidium atomic standards, RESSOX is used in future QZSSs. Copyright © 2013 Institute of Navigation.

Remote Synchronization Experiments for Quasi-Zenith Satellite System Using Multiple Navigation Signals as Feedback Control

The remote synchronization system for the onboard crystal oscillator (RESSOX) is a remote control method that permits synchronization between a ground station atomic clock and Japanese quasi-zenith satellite system (QZSS) crystal oscillators. To realize the RESSOX of the QZSS, the utilization of navigation signals of QZSS for feedback control is an important issue. Since QZSS transmits seven navigation signals (L1C/A, L1CP, L1CD, L2CM, L2CL, L5Q, and L5I), all combinations of these signals should be evaluated. First, the RESSOX algorithm will be introduced. Next, experimental performance will be demonstrated. If only a single signal is available, ionospheric delay should be input from external measurements. If multiple frequency signals are available, any combination, except for L2 and L5, gives good performance with synchronization error being within two nanoseconds that of RESSOX. The combination of L1CD and L5Q gives the best synchronization performance (synchronization error within 1.14 ns). Finally, in the discussion, comparisons of long-duration performance, computer simulation, and sampling number used in feedback control are considered. Although experimental results do not correspond to the simulation results, the tendencies are similar. For the overlapping Allan deviation of long duration, the stability of 1.23×10−14 at 100,160 s is obtained.

Autonomous low-power glare sensing chip

A custom-designed CMOS glare sensing chip is presented that can operate without external control. The chip is a 64×32 pixel focal-plane array that checks periodically for the presence of glare—a light intensity greater than a predefined threshold. If glare is present, the chip wakes up an external microcontroller, which takes control of the chip and reads out the precise glare location. When the microcontroller is done, it returns the chip to autonomous mode and goes back to sleep to save power. Autonomous operation is accomplished using an onboard ring oscillator to generate the control signals. Its small size and very low power consumption in idle and readout modes make it ideally suited to wearable devices.

Remote Synchronization Experiments for Quasi-Senith Satellite System Using Current Geostationary Satellites

The remote synchronization system for the onboard crystal oscillator (RESSOX) realizes accurate synchronization between an atomic clock at a ground station and the QZSS onboard crystal oscillator, reduces overall cost and satellite power consumption, as well as onboard weight and volume, and is expected to have a longer lifetime than a system with onboard atomic clocks. Since a QZSS does not yet exist, we have been conducting synchronization experiments using geostationary earth orbit satellites (JCSAT-1B or Intelsat-4) to confirm that RESSOX is an excellent system for timing synchronization. JCSAT-1B, the elevation angle of which is 46.5 degrees at our institute, is little affected by tropospheric delay, whereas Intelsat-4, the elevation angle of which is 7.9 degrees, is significantly affected. The experimental setup and the results of uplink experiments and feedback experiments using mainly Intelsat-4 are presented. The results show that synchronization within 10 ns is realized.

Long-term frequency aging for unpowered space-class oscillators

Very little data exists regarding the long-term aging performance of space-rated oscillators in the non-operating mode. This paper provides empirical evidence that may be used to estimate the performance of unpowered oscillators for long-term space missions, as well as an aid in validating the worst-case analyses that are routinely performed on these oscillators. The proper operation of any space vehicle is dependent on the performance of the onboard master oscillator. For maximum reliability, the onboard clock is often provided as a dual-or triple-redundant ensemble, with one active oscillator. The backup oscillator(s) are usually powered off and may be off for a significant percentage of the mission. A significant parameter of importance for oscillator performance is the aging rate. Papers have been presented on the aging performance of active oscillators in space, but very little data exists regarding the long-term aging performance of oscillators in the nonoperating mode. The aging rate data are extremely important for predicting the expected performance of these backup oscillators after 10 or 15 years in of non-operation in space. This paper presents performance data derived from temperature controlled crystal oscillators and oven controlled crystal oscillators that have been dormant for decades.

EOM sideband phase characteristics for the spaceborne gravitational wave detector LISA

The Laser Interferometer Space Antenna (LISA) is a joint ESA/NASA mission proposed to observe gravitational waves. One important noise source in the LISA phase measurement will be on-board reference oscillators. An inter-spacecraft clock tone transfer chain will be necessary to remove this non-negligible phase noise in post processing. One of the primary components of this chain are electro-optic modulators (EOMs). At modulation frequencies of 2 GHz, we characterise the excess phase noise of a fibre-coupled integrated EOM in the LISA measurement band (0.1 mHz to 1 Hz). The upper phase noise limit was found to be almost an order of magnitude better than required by the LISA mission. In addition, the EOM’s phase dependence on temperature and optical power was determined. The measured coefficients are within a few milliradians per kelvin and per watt respectively and thereby negligible with the expected on-board temperature and laser power stability.