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The TianQin project is a space gravitational wave detection program proposed by China, imposing strict requirements on inertial sensors. This paper introduces the basic principle and development progress of the TianQin project inertial sensor. At present, key technologies such as test mass, capacitance displacement sensing, and charge management have been developed, with some performance exceeding requirements. Ground-based test systems utilizing torsion pendulums have initially investigated the patch effect, magnetic field effect, residual gas damping, and temperature gradient effect. The inertial sensor is currently undergoing system integration and ground performance tests, and will be tested on TQ-2. [ABSTRACT FROM AUTHOR]

The Sun-Earth L 2 libration point configuration is one of the options for space-based gravitational wave detection. Long-term configuration stability is crucial for high-precision measurements, challenged by the strong nonlinear dynamics of the Sun-Earth three-body system. This paper proposes an efficient design method and determines the feasible parameter domain for the libration point gravitational wave observatory. First, the dynamic model for the libration point configuration is established, and the stability indexes are defined. The sensitive parameters that affect the relative geometric configuration are discussed and the phase angle is found to be the key factor. Then, an efficient design method is proposed, and the procedure is divided into two steps. The phase angle of the Earth phase offset orbit and the libration point configuration are optimized successively. Finally, the proposed method is applied to the LAGRANGE mission concept. The results show that the three stability indexes decrease by 59%, 42% and 23%, respectively. Moreover, a mapping between configuration parameters and stability indexes is established. The feasible parameter domain for the stable libration point configuration is discussed. The feasible amplitudes domain in the x and z directions of the libration point orbit should be less than 6200 km and 42000 km, respectively, to guarantee configuration stability. This research could provide a reference for the stable design and implementation of gravitational wave detection missions utilizing libration point configuration in the future. • An efficient design method for the libration point GWs observatory is proposed. • Sensitive parameters of configuration are discussed; phase angle is the key factor. • Phase angle of Earth offset orbit and stability indexes are optimized successively. • A mapping between configuration parameters and stability indexes is established. [ABSTRACT FROM AUTHOR]

TianQin is expected to deploy three spacecraft in Earth orbit at the altitude of about 1 × 108 m to form an equilateral triangular constellation with arm lengths of about 1.7 × 108 m. It is aimed at detecting gravitational waves in the frequency range from 0.1 mHz to 1 Hz by means of high sensitive laser ranging interferometry with pathlength measurement noise below 1 pm/Hz1/2. The long-range propagation of beam in the intersatellite laser ranging interferometry between the three spacecraft results in a non-negligible time delay of 0.58 s. Therefore, there is an angular difference between the optimal outgoing and incoming laser beam of laser ranging interferometry on each spacecraft, which is denoted as the point-ahead angle. Due to the relative motion between the three spacecraft, the point-ahead angle of TianQin constellation is approximately 22.96 μrad with a dynamic variation range of ±24.71 nrad. Point-ahead-angle mechanism is a fine steering mirror used for point-ahead angle compensation to diminish the beam pointing error, thus to maintain the intersatellite laser link as well as to diminish the tilt-to-length coupling noise, which are vital to the gravitational wave detection. Both the static and dynamic point-ahead angle compensation strategies can be used in TianQin and the selection of the two is based on minimizing the total point-ahead-angle-related tilt-to-length coupling noise. In this paper, a criteria for selection of the two strategies are proposed by establishing a model of point-ahead-angle-related tilt-to-length coupling noise as well as a corresponding evaluation function for calculation and comparison. The results indicate that the dynamic compensation strategy is the optimal choice only when the first-order tilt-to-length coupling coefficient of point-ahead-angle mechanism is less than 6.4 × 10−9 m/rad, otherwise the static compensation strategy should be used. The proposed criteria, as well as the model and the requirement for point-ahead-angle mechanism, can serve as a common theoretical basis in laser ranging interferometry applications. • Criteria for selection of point-ahead angle compensation strategies. • Minimize tilt-to-length coupling noise in intersatellite laser interferometry. • Quantitative comparison of tilt-to-length noise in different compensation strategies. • Static compensation strategy is more appropriate in TianQin according to the criteria. [ABSTRACT FROM AUTHOR]

Addressing the challenge of horizontal tilt coupling is crucial for using inertial sensors in precise applications, such as seismology and seismic isolation, including gravitational wave detection. Researchers have proposed various design solutions, with the Double Link (DL) sensor standing out for its simplicity, precision, and effectiveness. This paper explores the use of the DL sensor in an active vibration isolation system. We evaluated different control algorithms, including Proportional-Integral-Derivative (PID), Linear Quadratic Regulator (LQR), Linear Quadratic Gaussian (LQG), and H-infinity. Simulations conducted in the Simscape environment showed that the H-infinity controller performed best, achieving a significant reduction in vibration. While the current study is based on simulations, future work will focus on experimental validation to confirm the system's practical applicability and robustness in real-world scenarios. Our results demonstrate the potential of the DL sensor and LQG controller to enhance vibration isolation in low-frequency applications. Additionally, we conducted a detailed literature review on various methods used in similar applications. This review highlights alternative approaches, such as other sensor designs and control strategies, and discusses their advantages and limitations. [ABSTRACT FROM AUTHOR]

The noise caused by various temperature effects in the sensitive frequency band will cause errors in the detection results of space gravitational wave. Therefore, it is important to suppress the temperature noise of space-borne gravitational wave detectors. In this paper, a method is proposed to suppress temperature noise using a multi-layer composite structure consisting of low thermal conductivity material (R) and high specific heat capacity material (C). The arrangement in which heat flow passes through high specific heat capacity material first is "CR." The thermal simulation model is established to study the temperature noise transfer characteristics, and accuracy of the model is verified by experiments. The results show that the temperature noise of CRC is 90 % lower than that of RCR. The arrangement which heat flow passes through high specific heat capacity material first has an optimal high specific heat capacity material's proportion of 60 % to 70 %. When the number of composite layers is not less than 3 layers, the more the composite layers' number is, the better the suppression effect of multi-layer composite structure on temperature noise is. However, there is a limit to the way of obtaining noise reduction effect by increasing the number of layers. This paper provides a guidance for the suppression of temperature noise in gravitational wave detection. [ABSTRACT FROM AUTHOR]

With the advancement of space laser technology，laser systems are progressing towards shorter wavelengths，longer lifetimes，and higher orbits. It requires higher performance coatings that can withstand the damage caused by the harsh space environment，such as vacuum，high and low temperatures，proton radiation and Gamma-ray radiation. This paper reviews the development direction of the main space lasers at home and abroad. It also discusses the effects of the space environment on coatings during the development of space laser systems and the evolution mechanism of coating performance under service conditions. It presents the new advances in high-performance space laser coatings and provides insights into future development directions，such as gravitational wave detection in space and deep space exploration.

Tilt-to-length (TTL) coupling noise arises from angular misalignments of interfering beams in optical path length (OPL) measurements and significantly impacts the accuracy of interferometry measurement systems. This paper focuses on geometric TTL coupling in a test mass (TM) interferometer and examines how an imaging system influences TTL noise suppression. First, the analytical expression of the geometric TTL coupling in a TM interferometer with alignment errors is derived and confirmed through numerical simulation. Subsequently, an imaging system is incorporated into the geometric model and the corresponding analytical expressions are obtained under two common conjugate relationships. Nevertheless, the TTL coupling remains beyond the requirement of TM interferometer, as the residual TTL coupled with alignment errors persists even with the imaging system. Therefore, an optimal position of the imaging system capable of eliminating the second-order term of the TTL coupling is determined. Meanwhile, the first-order term can be mitigated through in-orbit calibrations. These findings offer valuable guidance for the design and adjustment of imaging systems in space-borne gravitational wave detection missions, which require high-precision laser interferometry. [ABSTRACT FROM AUTHOR]

The search for gravitational-wave (GW) signals is limited by non-Gaussian transient noises that mimic astrophysical signals. Temporal coincidence between two or more detectors is used to mitigate contamination by these instrumental glitches. However, when a single detector is in operation, coincidence is impossible, and other strategies have to be used. We explore the possibility of using neural network classifiers and present the results obtained with three types of architectures: convolutional neural network, temporal convolutional network, and inception time. The last two architectures are specifically designed to process time-series data. The classifiers are trained on a month of data from the LIGO Livingston detector during the first observing run (O1) to identify data segments that include the signature of a binary black hole merger. Their performances are assessed and compared. We then apply trained classifiers to the remaining three months of O1 data, focusing specifically on single-detector times. The most promising candidate from our search is 4 January 2016 12:24:17 UTC. Although we are not able to constrain the significance of this event to the level conventionally followed in GW searches, we show that the signal is compatible with the merger of two black holes with masses m 1 = 50.7 − 8.9 + 10.4 M ⊙ and m 2 = 24.4 − 9.3 + 20.2 M ⊙ at the luminosity distance of d L = 564 − 338 + 812 Mpc . [ABSTRACT FROM AUTHOR]

This article explores novel in-orbit drag-free technology that can be utilized for deep space detection scientific missions. In this study, we considered a two-test-mass drag-free method and analyzed the design of the drag-free and attitude control system for the TianQin mission. The entire control system was comprehensively designed, including an actuator allocation design and controllers for two test masses and one spacecraft, with a total of 18 degrees of freedom. Furthermore, stability analysis was conducted. Based on our design, numerical analysis and simulations were performed assuming geocentric orbit conditions in the TianQin mission, confirming the feasibility of this aerospace engineering concept. The versatility of the design allows for its application to scientific observations across various disciplines by modifying the structure of the simulation environment, and consequently, the approach discussed in this study holds significant practical implications for effectively accomplishing deep space observation tasks. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Aeronautical Materials is the property of Editorial Board of Journal of Aeronautical Materials and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In future space-borne gravitational wave (GW) detectors, time delay interferometry (TDI) will be utilized to reduce the overwhelming noise, including the laser frequency noise and the clock noise etc., by time shifting and recombining the data streams in post-processing. The successful operation of TDI relies on absolute inter-satellite ranging with meter-level precision. In this work, we numerically and experimentally demonstrate a strategy for inter-satellite distance measurement. The distances can be coarsely determined using the technique of arm-locking ranging with a large non-ambiguity range, and subsequently TDI can be used for precise distance measurement (TDI ranging) by finding the minimum value of the power of the residual noises. The measurement principle is introduced. We carry out the numerical simulations, and the results show millimeter-level precision. Further, we perform the experimental verifications based on the fiber link, and the distances can be measured with better than 0.05 m uncertainty, which can well satisfy the requirement of time delay interferometry. [ABSTRACT FROM AUTHOR]

Gravitational wave (GW) astronomy is witnessing a transformative shift from terrestrial to space-based detection, with missions like Taiji at the forefront. While the transition brings unprecedented opportunities for exploring massive black hole binaries (MBHBs), it also imposes complex challenges in data analysis, particularly in parameter estimation amidst confusion noise. Addressing this gap, we utilize scalable normalizing flow models to achieve rapid and accurate inference within the Taiji environment. Innovatively, our approach simplifies the data's complexity, employs a transformation mapping to overcome the year-period time-dependent response function, and unveils additional multimodality in the arrival time parameter. Our method estimates MBHBs several orders of magnitude faster than conventional techniques, maintaining high accuracy even in complex backgrounds. These findings significantly enhance the efficiency of GW data analysis, paving the way for rapid detection and alerting systems and enriching our ability to explore the universe through space-based GW observation. [ABSTRACT FROM AUTHOR]

The stability of the spin of pulsars and the precision with which these spins can be determined, allows many unique tests of interest to physics and astrophysics. Perhaps the most challenging and revolutionary of these, is the detection of nanohertz gravitational waves. An increasing number of efforts to detect and study long-period gravitational waves by timing an array of pulsars have been ongoing for several decades and the field is moving ever closer to actual gravitational-wave science. In this review article, we summarize the state of this field by presenting the current sensitivity to gravitational waves and by reviewing recent progress along the multiple lines of research that are part of the continuous push towards greater sensitivity. We also briefly review some of the most recent efforts at astrophysical interpretation of the most recent GW estimates derived from pulsar timing.

Copyright of Chinese Journal of Applied Mechanics is the property of Chinese Journal of Applied Mechanics Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Tilt-to-length (TTL) coupling noise arises from angular misalignments of interfering beams in optical path length (OPL) measurements and significantly impacts the accuracy of interferometry measurement systems. This paper focuses on geometric TTL coupling in a test mass (TM) interferometer and examines how an imaging system influences TTL noise suppression. First, the analytical expression of the geometric TTL coupling in a TM interferometer with alignment errors is derived and confirmed through numerical simulation. Subsequently, an imaging system is incorporated into the geometric model and the corresponding analytical expressions are obtained under two common conjugate relationships. Nevertheless, the TTL coupling remains beyond the requirement of TM interferometer, as the residual TTL coupled with alignment errors persists even with the imaging system. Therefore, an optimal position of the imaging system capable of eliminating the second-order term of the TTL coupling is determined. Meanwhile, the first-order term can be mitigated through in-orbit calibrations. These findings offer valuable guidance for the design and adjustment of imaging systems in space-borne gravitational wave detection missions, which require high-precision laser interferometry.

This article explores novel in-orbit drag-free technology that can be utilized for deep space detection scientific missions. In this study, we considered a two-test-mass drag-free method and analyzed the design of the drag-free and attitude control system for the TianQin mission. The entire control system was comprehensively designed, including an actuator allocation design and controllers for two test masses and one spacecraft, with a total of 18 degrees of freedom. Furthermore, stability analysis was conducted. Based on our design, numerical analysis and simulations were performed assuming geocentric orbit conditions in the TianQin mission, confirming the feasibility of this aerospace engineering concept. The versatility of the design allows for its application to scientific observations across various disciplines by modifying the structure of the simulation environment, and consequently, the approach discussed in this study holds significant practical implications for effectively accomplishing deep space observation tasks.

Neural network models have entered the realm of gravitational wave detection, proving their effectiveness in identifying synthetic gravitational waves. However, these models rely on learned parameters, which necessitates time-consuming computations and expensive hardware resources. To address this challenge, we propose a gravitational wave detection model tailored specifically for binary black hole mergers, inspired by the Random Convolutional Kernel Transform (ROCKET) family of models. We conduct a rigorous analysis by factoring in realistic signal-to-noise ratios in our datasets, demonstrating that conventional techniques lose predictive accuracy when applied to ground-based detector signals. In contrast, for space-based detectors with high signal-to-noise ratios, our method not only detects signals effectively but also enhances inference speed due to its streamlined complexity—a notable achievement. Compared to previous gravitational wave models, we observe a significant acceleration in training time while maintaining acceptable performance metrics for ground-based detector signals and achieving equal or even superior metrics for space-based detector signals. Our experiments on synthetic data yield impressive results, with the model achieving an AUC score of 96.1 % and a perfect recall rate of 100 % on a dataset with a 1:3 class imbalance for ground-based detectors. For high signal-to-noise ratio signals, we achieve flawless precision and recall of 100 % without losing precision on datasets with low-class ratios. Additionally, our approach reduces inference time by a factor of 1.88 . [ABSTRACT FROM AUTHOR]

High-precision inertial sensors or accelerometers can provide references for free-falling motion in gravitational fields in space. They serve as the key payloads for gravity recovery missions such as CHAMP, the GRACE-type missions, and the planned Next-Generation Gravity Missions. In this work, a systematic method for electrostatic inertial sensor calibration of gravity recovery satellites is suggested, which is applied to and verified with the Taiji-1 mission. With this method, the complete operating parameters including the scale factors, the center of mass offset vector, and the intrinsic biased acceleration can be precisely calibrated with only two sets of short-term in-orbit experiments. This could reduce the gaps in data that are caused by necessary in-orbit calibrations during the lifetime of related missions. Taiji-1 is the first technology-demonstration satellite of the "Taiji Program in Space", which, in its final extended phase in 2022, could be viewed as operating in the mode of a high–low satellite-to-satellite tracking gravity mission. Based on the principles of calibration, swing maneuvers with time spans of approximately 200 s and rolling maneuvers for 19 days were conducted by Taiji-1 in 2022. Given the data of the actuation voltages of the inertial sensor, satellite attitude variations, precision orbit determinations, the inertial sensor's operating parameters are precisely re-calibrated with Kalman filters and are relayed to the Taiji-1 science team. The relative errors of the calibrations are <1% for the linear scale factors, <3% for center of mass, and <0.1% for biased accelerations. Data from one of the sensitive axes are re-processed with the updated operating parameters, and the resulting performance is found to be slightly improved over the former results. This approach could be of high reference value for the accelerometer or inertial sensor calibrations of the GFO, the Chinese GRACE-type mission, and the Next-Generation Gravity Missions. This could also create some insight into the in-orbit calibrations of the ultra-precision inertial sensors for future GW space antennas because of the technological inheritance between these two generations of inertial sensors. [ABSTRACT FROM AUTHOR]

A critical consideration in the design of next-generation gravitational wave detectors is the understanding of the seismic environment that can introduce coherent and incoherent noise of seismic origin at different frequencies. We present detailed low-frequency ambient seismic noise characterisation (0.1–10 Hz) at the Gingin site in Western Australia. Unlike the microseism band (0.06–1 Hz) for which the power shows strong correlations with nearby buoy measurements in the Indian Ocean, the seismic spectrum above 1 Hz is a complex superposition of wind induced seismic noise and anthropogenic seismic noise which can be characterised using beamforming to distinguish between the effects of coherent and incoherent wind induced seismic noise combined with temporal variations in the spatio-spectral properties of seismic noise. This also helps characterise the anthropogenic seismic noise. We show that wind induced seismic noise can either increase or decrease the coherency of background seismic noise for wind speeds above 6 m s−1 due to the interaction of wind with various surface objects. In comparison to the seismic noise at the Virgo site, the secondary microseism (0.2 Hz) noise level is higher in Gingin, but the seismic noise level between 1 and 10 Hz is lower due to the sparse population and absence of nearby road traffic. [ABSTRACT FROM AUTHOR]

High precision temperature measurements are a transversal need in a wide area of physical experiments. Space-borne gravitational wave detectors are a particularly challenging case, requiring both high precision and high stability in temperature measurement. In this contribution, we present a design able to reach 1 μ K / Hz in most of the measuring band down to 1 mHz, and reaching 20 μ K / Hz at 0.1 mHz. The scheme is based on resistive sensors in a Wheatstone bridge configuration which is AC modulated to minimize the 1/f noise. As a part of our study, we include the design of a test bench able to guarantee the high stability environment required for measurements. We show experimental results characterising both the test bench and the read-out, and discuss potential noise sources that may limit our measurement. [ABSTRACT FROM AUTHOR]

Space-based gravitational wave (GW) detection at low frequencies is of great scientific significance and has received extensive attention in recent years. This work designs and optimizes the low-energy transfer of the heliocentric formation of GW detectors, which starts from a geosynchronous transfer orbit and targets an Earth-like orbit. Based on the example of the Laser Interferometer Space Antenna (LISA), the transfer is first designed in two-body dynamical models and then refined in simplified high-fidelity dynamical models that only consider the major orbital perturbations evaluated here. The main contributions of this work are to present an adaptive model continuation technique and to exploit the lunar swingby technique to reduce the problem-solving difficulty and velocity increment of orbital transfer, respectively. The adaptive model continuation technique fully reveals the effect of perturbations and rapidly iterates the solutions to the simplified models. The simulation results show that the lunar swingby does reduce the energy needed to escape the Earth's sphere of influence. It is found that the gravitation of the Earth–Moon system has a significant contribution to reducing the velocity increment. The solution of low-energy transfer in the simplified models is that the duration is 360.6615 days and the total velocity increment is 0.8468 km/s. [ABSTRACT FROM AUTHOR]

In space gravitational-wave-detection missions, inertial sensors are used as the core loads, and their acceleration noise needs to reach 3 × 10 − 15   ms − 2 / Hz at a frequency of 0.1   mHz , which corresponds to the capacitive sensing system; the capacitive sensing noise on the sensitive axis needs to reach 1   aF / Hz . Unlike traditional circuit noise evaluation, the noise in the mHz frequency band is dominated by the thermal noise and the 1 / f noise of the device, which is a challenging technical goal. In this paper, a low-frequency, high-precision resonant capacitor bridge method based on a planar transformer is used. Compared with the traditional winding transformer, the developed planar transformer has the advantages of low temperature drift and low 1 / f noise. For closed-loop measurements of capacitive sensing circuits and sensitive structures, the minimum capacitive resolution in the time domain is about 3   aF , which is far lower than the scientific measurement resolution requirement of 5.8   fF for gravitational wave detection. The capacitive sensing noise is converted to 1.095   aF / Hz in the frequency band of 10   mHz – 1   Hz . Although there is a gap between the closed-loop measurement results and the final index, the measurement environment is an experimental condition without temperature control on the ground; additionally, in China, the measurement integrity and actual measurement results of the capacitive sensing function have reached a domestic leading level. This is the realization of China's future space gravitational wave exploration. [ABSTRACT FROM AUTHOR]

The space gravitational wave detection mission has put forward multiple performance requirements for micro-newton thrusters, such as "fast response, high resolution, and low noise". In order to solve the problem that the traditional open-loop propulsion system can hardly meet the high-precision performance, the microwave ion thruster with high accuracy and controllability of thrust estimation is adopted as the research object, and the precision feedback control method combining analog circuit and digital control technology is adopted to achieve a resolution of less than 0.1 μN, fast response of 20 ms, and 10−3~1 Hz frequency band noise less than 0.05 μN/Hz1/2 of the thrust performance. Experimental results show that, compared with open-loop regulation, the feedback control can effectively suppress various noises and disturbances caused by device temperature drift and complex operating characteristics of the thruster, and the anti-aliasing filter scheme adopted in this paper can suppress the folding of high-frequency data in the low- and medium-frequency bands during the digital control process, while it can effectively improve the system performance and reduce the impact of thrust noise. The feedback control strategy proposed in this paper verifies the possibility of a microwave ion thruster as an actuator for super "static and precise" space experiment satellites and provides a feasible solution for the application of a microwave ion thruster in deep space science experiment missions such as gravitational wave detection. [ABSTRACT FROM AUTHOR]

The establishment of a laser link between satellites, i.e., the acquisition phase, is a key technology for space-based gravitational detection missions, and it becomes extremely complicated when the long distance between satellites, the inherent limits of the sensor accuracy, the narrow laser beam divergence and the complex space environment are considered. In this paper, we investigate the laser acquisition problem of a new type of satellite equipped with two two-degree-of-freedom telescopes. A predefined-time controller law for the acquisition phase is proposed. Finally, a numerical simulation was conducted to demonstrate the effectiveness of the proposed controller. The results showed that the new strategy has a higher efficiency and the control performance can meet the requirements of the gravitational detection mission. [ABSTRACT FROM AUTHOR]

Taiji-1 is the first technology demonstration satellite of the Taiji program of China’s space-borne gravitational wave antenna. After the demonstration of the key individual technologies, Taiji-1 continues collecting the data of the precision orbit determinations, satellite attitudes, and non-conservative forces exerted on the S/C. Therefore, during its free-fall, Taiji-1 can be viewed as operating in the high-low satellite-to-satellite tracking mode of a gravity recovery mission. In this work, we have selected and analyzed the one month data from Taiji-1’s observations, and developed the techniques to resolve the long term interruptions and disturbances in the data due to the scheduled technology demonstration experiments. The first global gravity model TJGM-r1911, that independently derived from China’s own satellite mission, is successfully built from Taiji-1’s observations. Compared with gravity models from CHAMP and other satellite gravity missions, the accuracy discrepancies exist, which is mainly caused by the data discontinuity problem. As the extended free-falling phase been approved, Taiji-1 could serve as a gravity recovery mission for China since 2022 and it will provide us the independent measurement of both the static and the monthly time-variable global gravity field. [ABSTRACT FROM AUTHOR]

Thermal noise sources are relevant for future gravitational wave detectors due to the foreseen increase in sensitivity, especially at frequencies below ▪. As most thermal noise sources scale with the square root of the temperature, cooling critical optical components and their suspension system is essential. This also requires a much wider range of temperature compatibility from all technology deployed in the last suspension stages, including displacement and inertial sensors. We demonstrate and characterize a setup for stable light sources and light intensity sensing for temperatures from 300 to ▪. Commercial collimators and fibers were tested to use light from stabilized laser sources in the cryogenic environment. We also investigated multiple semiconductor compositions of photodiodes and identified a solution with high and stable responsivity at ▪. • Study of the cryogenic compatibility of commercial optical components for 1550 nm. • Identification of collimators with stable light throughput from 8 to 300 K. • Investigation of different InGaAs photodiode semiconductor compositions. • Characterization of responsivity and dark current of photodiodes from 12 to 300 K. [ABSTRACT FROM AUTHOR]

Capacitive transducers are the core of electrostatic space accelerometers and have been applied in several space science missions. However, the noise of the front-end circuit in a capacitive transducer is a major limitation to capacitance resolution. The current theoretical models cannot accurately and comprehensively describe the performance of the front-end circuit in an ultra-high precision capacitive transducer. In this study, we modified the sensitivity and noise models of the front-end circuit and verified them by precision experiments. Under the guidance of the corrected models, we optimized the total stray capacitance by thickening the insulating layer between the shield and signal from 123 pF to 38 pF, thereby reducing the noise to 0.08 aF/Hz $^{\text {1/2}}$. We believe that this work will support the development of an ultrasensitive electrostatic accelerometer for application in space science missions. [ABSTRACT FROM AUTHOR]

This paper is concerned with a state-space approach to deep Gaussian process (DGP) regression. We construct the DGP by hierarchically putting transformed Gaussian process (GP) priors on the length scales and magnitudes of the next level of Gaussian processes in the hierarchy. The idea of the state-space approach is to represent the DGP as a non-linear hierarchical system of linear stochastic differential equations (SDEs), where each SDE corresponds to a conditional GP. The DGP regression problem then becomes a state estimation problem, and we can estimate the state efficiently with sequential methods by using the Markov property of the state-space DGP. The computational complexity scales linearly with respect to the number of measurements. Based on this, we formulate state-space MAP as well as Bayesian filtering and smoothing solutions to the DGP regression problem. We demonstrate the performance of the proposed models and methods on synthetic non-stationary signals and apply the state-space DGP to detection of the gravitational waves from LIGO measurements. [ABSTRACT FROM AUTHOR]

Space-based gravitational wave (GW) detection at low frequencies is of great scientific significance and has received extensive attention in recent years. This work designs and optimizes the low-energy transfer of the heliocentric formation of GW detectors, which starts from a geosynchronous transfer orbit and targets an Earth-like orbit. Based on the example of the Laser Interferometer Space Antenna (LISA), the transfer is first designed in two-body dynamical models and then refined in simplified high-fidelity dynamical models that only consider the major orbital perturbations evaluated here. The main contributions of this work are to present an adaptive model continuation technique and to exploit the lunar swingby technique to reduce the problem-solving difficulty and velocity increment of orbital transfer, respectively. The adaptive model continuation technique fully reveals the effect of perturbations and rapidly iterates the solutions to the simplified models. The simulation results show that the lunar swingby does reduce the energy needed to escape the Earth’s sphere of influence. It is found that the gravitation of the Earth–Moon system has a significant contribution to reducing the velocity increment. The solution of low-energy transfer in the simplified models is that the duration is 360.6615 days and the total velocity increment is 0.8468 km/s.

The space gravitational wave detection mission has put forward multiple performance requirements for micro-newton thrusters, such as “fast response, high resolution, and low noise”. In order to solve the problem that the traditional open-loop propulsion system can hardly meet the high-precision performance, the microwave ion thruster with high accuracy and controllability of thrust estimation is adopted as the research object, and the precision feedback control method combining analog circuit and digital control technology is adopted to achieve a resolution of less than 0.1 μN, fast response of 20 ms, and 10−3~1 Hz frequency band noise less than 0.05 μN/Hz1/2 of the thrust performance. Experimental results show that, compared with open-loop regulation, the feedback control can effectively suppress various noises and disturbances caused by device temperature drift and complex operating characteristics of the thruster, and the anti-aliasing filter scheme adopted in this paper can suppress the folding of high-frequency data in the low- and medium-frequency bands during the digital control process, while it can effectively improve the system performance and reduce the impact of thrust noise. The feedback control strategy proposed in this paper verifies the possibility of a microwave ion thruster as an actuator for super “static and precise” space experiment satellites and provides a feasible solution for the application of a microwave ion thruster in deep space science experiment missions such as gravitational wave detection.

The stability of the spin of pulsars and the precision with which these spins can be determined, allows many unique tests of interest to physics and astrophysics. Perhaps the most challenging and revolutionary of these, is the detection of nanohertz gravitational waves. An increasing number of efforts to detect and study long-period gravitational waves by timing an array of pulsars have been ongoing for several decades and the field is moving ever closer to actual gravitational-wave science. In this review article, we summarize the state of this field by presenting the current sensitivity to gravitational waves and by reviewing recent progress along the multiple lines of research that are part of the continuous push towards greater sensitivity. We also briefly review some of the most recent efforts at astrophysical interpretation of the most recent GW estimates derived from pulsar timing. • Pulsar timing Arrays are expected to detect nanohertz gravitational waves soon. • International collaboration is key in this endeavor. • Likely sources are supermassive black hole binaries or a background of these. • Evolution scenarios for supermassive black hole binaries are already being informed. • Challenges are mostly related to modeling noise, both intrinsic and interstellar. [ABSTRACT FROM AUTHOR]

The micro capacitive sensing and electrostatic drive control system (front-end electronics, FEE) is the core component of inertial sensor in space gravitational wave detection. The FEE requires high-precision displacement detection, high-stability electrostatic drive, and stable system control to achieve an acceleration resolution of 10-15 m/s2/Hz1/2 in the low-frequency range of 0.1 mHz-1 Hz. Based on the requirements of the future Chinese space gravitational wave detection task (Taiji Program), this paper conducted key technical research of the FEE using differential capacitance detections and electrostatic drives. The structure and working principle of the FEE were also introduced. The structural parameters of the entire system, working parameters, and electrostatic control system model were provided, and the performance of the PID controller was analyzed. Finally, using the torsion pendulum to overcome the influence of gravity on the earth, the FEE multi-degree of freedom control function was verified on the vibration isolation marble platform, the measurement range and power conversion coefficient were calibrated, and the noise level under current conditions was measured. Experimental results show that the FEE developed in this paper can achieve stable control in multiple degrees of freedom, the acceleration range is larger than 10-3 m/s2, the electric force conversion factor is 4.8 × 10-3 m/s2/V, and the measured acceleration resolution is 9.6 × 10-6 m/s2/Hz1/2. After optimizing the sensitive structure parameters, the acceleration resolution can be estimated at 3.3 × 10-15 m/s2/Hz1/2. These results satisfy the Taiji Program requirements. This paper provides a solid foundation for the future exploration of space gravitational waves in China and clears the optimization direction for the next step.

Due to their excellent noise performance and technical maturity, ultra-stable continuous-wave neodymium-doped yttrium aluminum garnet lasers are one of the major light sources in a series of precision measurements such as ground and space-based gravitational wave detection, inter-satellite laser ranging and coherent optical communication. As the first step of developing an ultra-stable spaceborne laser, we carried out the design, development and environmental test of an all-solid-state Nd:YAG NPRO spaceborne laser. The laser deliveries 11.48 mW of optical power at 1064.405 nm, exhibits intensity and frequency noises less than 1 × 10 − 2 / √ Hz and 1 MHz/ √ Hz (Fourier frequencies > 10 mHz), respectively, passed environmental tests and has been onboard the first satellite of the Taiji program for one year. The work laid a solid foundation for realizing ultra-stable lasers for the Taiji program as well as being used in many space applications. [ABSTRACT FROM AUTHOR]

Taiji-1 is the first technology demonstration satellite of the Taiji Program in Space, which, served as the pre-PathFinder mission, had finished its nominal science operational phase and successfully accomplished the mission goal. The gravitational reference sensor (GRS) on-board Taiji-1 is one of the key science payloads that coupled strongly to other instruments, sub-systems and also the satellite platform itself. Fluctuations of the physical environment inside the satellite and mechanical disturbances of the platform generate important noises in the GRS measurements, therefore their science data can also be used to evaluate the performance of the μ N -thrusters and the stability of the platform. In this work, we report on the methods employed in Taiji-1 GRS data processing in the systematical modelings of the spacecraft orbit and attitude perturbations, mechanical disturbances, and internal environment changes. The modeled noises are then removed carefully from the GRS science data to improve the data quality and the GRS in-orbit performance estimations. [ABSTRACT FROM AUTHOR]

Due to the temperature fluctuation, both the self-gravity disturbance level of the gravitational wave detection vehicle and the laser interference measurement system are affected, which brings great noise to the gravitational wave measurement, and the temperature field fluctuation has become one of the most important noise sources of the inertial sensor. In order to ensure the noise level of the inertial sensor to meet the needs of detection, this paper uses the AC bridge to reduce the self-noise of the temperature measurement system, improve the resolution of the temperature measurement system, and realize a high-resolution and low-noise temperature measurement system. The experimental results show that the effective value of self-noise of the temperature measurement system is about 3.65 × 10 − 5 ∘ C , and the long-period noise is less than 1 × 10 − 5 ∘ C. It can be used in the space gravitational wave detection test and control system. [ABSTRACT FROM AUTHOR]

The electrostatic suspension control system (ESCS) is the core component of the inertial sensor in space gravitational wave detection. To adapt to the changes in orbit environment and satellite platform vibration, the test mass stable is kept in the center of the electrode cage, the ESCS requires high-precision parameters calibration and high-stability verification of the control system. This paper studied the ESCS method, an adaptive controller with variable preload was proposed, and the stability conditions of the control system are given to provide a theoretical basis for the design of the controller. The structure and working principle of the system were also introduced. The electrostatic control system model was derived, and the performance of adaptive preload controller was analyzed. Finally, a simulation was conducted under different disturbance conditions. The results show that the control method proposed in this paper can achieve stable control in difference disturbance, and the preload voltage will change with the conditions. Compared with the traditional fixed preload method, we can see that our method has significantly improved the stability margin and the control quality during the process of dynamic response. This paper provides a solid foundation for the future exploration of space gravitational waves in China and clears the optimization direction for the next step. [ABSTRACT FROM AUTHOR]

Space-borne gravitational wave detection imposes a demanding requirement on the sensitivity of the laser interferometer. Among all disturbances that affect the measurement accuracy of the laser interferometer, temperature fluctuations contribute significantly. In this paper, the structure model and the interference path design of Taiji-1 laser interferometer have been used to conduct a preliminary simulation analysis of the temperature fluctuation noise through the finite element method. The temperature, the displacement and the optical path difference fluctuations have been obtained and theoretically analyzed. The preliminary simulation results are consistent with the theoretical analysis, which shows that the thermal–structural–optical simulation scheme adopted in this paper is reasonable. With the preliminary simulation results and the actual temperature control of Taiji-1 laser interferometer, we estimate that in Taiji-1 laser interferometer system, the temperature fluctuation is below the order of mK, the node displacement is within 1 0 0 pm, and the interference arm length difference fluctuation amplitude of the laser interferometer is also within 1 0 0 pm. [ABSTRACT FROM AUTHOR]

The space-based gravitational wave detectors at low frequencies has received great attention recently. Due to lack of considering the relative motion between spacecrafts, the accuracy of current sensitivity models is inadequate for space-based gravitational wave detectors. In this paper, an accurate sensitivity model of the space-based gravitational wave detector is established by taking into account the relative motion between two spacecrafts. The accurate sensitivity model is derived by employing a gravitational wave perturbed metric tensor. Then, to minimize the sensitivity loss due to the solar radiation, an enhancement strategy is proposed. Finally, simulation studies are presented with two drag-free spacecrafts in Earth's orbit. Simulation results validate the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

Copyright of Acta Scientiarum Naturalium Universitatis Sunyatseni / Zhongshan Daxue Xuebao is the property of Sun-Yat-Sen University and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Acta Scientiarum Naturalium Universitatis Sunyatseni / Zhongshan Daxue Xuebao is the property of Sun-Yat-Sen University and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The TianQin space gravitational waves (GW) observatory will contain three geocentric and circularly orbiting spacecraft with an orbital radius of 105 km, to detect the GW in the milli‐hertz frequency band. Each spacecraft pair will establish a 1.7 × 105 km‐long laser interferometer immersed in the solar wind and the magnetospheric plasmas to measure the phase deviations induced by the GW. GW detection requires a high‐precision measurement of the laser phase. The cumulative effects of the long distance and the periodic oscillations of the plasma density may induce an additional phase noise. This study aims to model the plasma induced phase deviation of the interspacecraft laser signals, using a realistic orbit simulator and the Space Weather Modeling Framework model. Preliminary results show that the plasma density oscillation can induce the phase deviations close to 2 × 10−6 rad/Hz1/2 or 0.3 pm/Hz1/2 in the milli‐hertz frequency band and it is within the error budget assigned to the displacement noise of the interferometry. The amplitude spectrum density of phases along three arms become more separated when the orbital plane is parallel to the Sun‐Earth line or during a magnetic storm. Finally, the dependence of the phase deviations on the orbital radius is examined.Key Points: Additional phase deviation of the laser beams induced by the space plasmas is estimated for TianQin gravitational wave observatoryThe oscillating plasma density could cause the phase deviation close to 2 × 10−6 rad/Hz1/2 or 0.3 pm/Hz1/2 in the mHz frequency bandThe plasma influence becomes more moderate when the constellation plane is perpendicular to the Sun‐Earth line and with low‐orbital radius [ABSTRACT FROM AUTHOR]

Copyright of Journal of Harbin Institute of Technology. Social Sciences Edition / Haerbin Gongye Daxue Xuebao. Shehui Kexue Ban is the property of Harbin Institute of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)