The development of techniques for the comparison of remote clocks plays an important role in metrology and fundamental science. Recent progress in the domain of ground-based and space-based atomic clocks requires important improvements in existing time and frequency transfer links. Here, we propose a satellite-based improved laser time transfer (ILTT) scheme based on existing satellite laser ranging (SLR) technique, which has been deployed on the Mengtian Laboratory Module of China’s Space Station (CSS). The ILTT payload comprises a single-photon avalanche diode (SPAD), a multi-level optimized optical system, an event timer, and a laser retroreflector array (LRA). A repetition rate of 10 kHz is adopted to improve normal-point precision. Laboratory experiments demonstrate that the time deviation (TDEV) of the entire ILTT system was better than 1 ps at 1 s and 0.6 ps at 100,000 s. In addition, we introduce a relativistic laser time transfer model and evaluate the impact of relativistic frequency shift terms on time transfer stability, meeting the requirements of the high-precision time–frequency rack (HTFR) mission aboard the CSS. In the on-orbit experiments, we have achieved state-of-the-art satellite-to-ground time synchronization with a precision of 2–5 ps at 1 s and a TDEV of 0.7 ps at 50 s. This approach is also sufficient for future time transfer links to geosynchronous and cislunar orbits.

Objective Current inter-satellite communication and navigation systems face critical technical challenges in single-direction laser ranging signal identification. Traditional bi-directional laser ranging methods suffer from severe energy attenuation, low ranging precision, and substantial signal-to-noise ratio degradation, seriously constraining space exploration technology development. This research systematically addresses the multifaceted challenges of noise interference and low echo rate in single-direction laser ranging signal identification. By proposing an innovative multi-stage signal identification method integrating fuzzy membership function and trend elimination techniques, the study aims to revolutionize current inter-satellite ranging signal processing paradigms. The primary research objectives encompass developing an autonomous signal identification framework for satellite platforms, breaking through technical bottlenecks in satellite payload autonomous processing, and providing robust technical solutions for inter-satellite one-way laser ranging applications. The proposed method's core value lies in its efficient deployment across satellite platforms, fully meeting the stringent space application requirements of autonomy, real-time performance, and reliability, and significantly improving the accuracy and reliability of inter-satellite one-way laser ranging signal identification. Methods The research designs an innovative multi-stage signal identification method for inter-satellite one-way laser ranging, specifically divided into three sophisticated and interconnected stages. The first stage involves initial fuzzy membership screening, representing a critical preprocessing approach to signal identification. By leveraging advanced statistical techniques and utilizing precise Gaussian distribution characteristics, the method constructs an adaptive membership function that enables unprecedented accuracy in laser pulse attribution. The approach ingeniously combines mode localization techniques with the rigorous 3 sigma principle, facilitating precise signal point identification. An innovative wide-threshold mechanism is strategically designed to maximize the retention of potential signal points, effectively preventing premature information loss and preserving critical signal integrity. This stage is crucial in establishing a robust foundation for subsequent signal processing steps. The second stage performs system bias trend elimination, implementing a sophisticated linear regression model to address systematic measurement uncertainties. This advanced approach precisely extracts and eliminates deterministic trend components arising from complex factors such as clock synchronization discrepancies and orbital prediction errors. By effectively separating random fluctuations from systematic biases, the method significantly enhances the statistical characteristics of the residual sequence, providing a more refined and reliable signal representation. The mathematical rigor of this stage ensures a comprehensive understanding of signal variations and removes potential sources of systematic error. The third stage implements membership re-calculation and interval threshold screening, representing a sophisticated refinement process. By recalculating membership using the mean of de-trended residuals and applying more stringent thresholds, the method dramatically improves signal identification reliability and noise suppression capabilities. This multi-layered approach ensures comprehensive signal quality assessment and optimization, ultimately delivering a robust and precise signal identification methodology. Results and Discussions Rigorous simulation experiments and comprehensive laboratory prototype data validation demonstrate the method's exceptional performance across various challenging scenarios. Under different echo rate conditions, the signal identification methodology consistently demonstrates remarkable capabilities. Signal identification accuracy remains consistently above 97 degrees o, with precision maintained at no less than 86 degrees o, and the overall false alarm rate reliably controlled below 0.98 degrees o. These results fully demonstrate the method's robustness and reliability across diverse operational conditions. Comparative analyses with traditional manual screening methods reveal significant performance improvements. Observation consistency (P0) and Kappa indicators reach 0.98 and 0.97 respectively, comprehensively validating the method's high reliability and practical value. Computational efficiency stands as another critical advantage, with the ability to efficiently process approximately 1000 data points within 0.1 s. This exceptional performance perfectly meets the strict real-time requirements characteristic of advanced space systems, highlighting the method ' s potential for practical implementation in complex space exploration scenarios. Conclusions The proposed inter-satellite one-way laser ranging signal identification method effectively resolves complex signal identification challenges by innovatively integrating fuzzy set theory and trend elimination techniques. The research constructs a novel signal identification framework for single-direction laser ranging, providing both theoretical insights and practical validation of its excellent performance. The method offers a forward-looking technical solution for space navigation, planetary exploration, and advanced communication systems, embodying significant scientific value and presenting far-reaching application prospects for advancing space information acquisition technology.

High-precision and high-stability measurement is an essential approach for situational awareness of space targets,such as high-precision orbit determination,attitude determination,etc.Satellite Laser Ranging(SLR)technology has been developed for more than 60 years,and its accuracy and stability have been continuously im-proving,the laser pulse width and the jitter of single-photon detectors in the SLR systems are important factors limiting the improvement of ranging accuracy.In this study,a high-stability industrial-grade 532 nm picosecond laser and a high-quantum efficiency MPD single-photon detector(SPAD)are used in the SLR system of the Shang-hai Astronomical Observatory.The pulse width of the laser is～15 ps;the jitter of SPAD is less than 35 ps and the quantum efficiency is about 50%,a matched aspheric len is applied for the SPAD.The measurement accuracy of ground targets is improved from 6-8 mm to 2-3 mm,and the satellite measurement accuracy is enhanced from the optimal 6-10 mm to 2-4 mm,meeting the 24-hour measurement requirements of low-orbit,high-orbit,and geosyn-chronous orbit satellites.Specifically,the measurement accuracy of the beidou compassg8 satellite in the geosyn-chronous orbit(36 000 km)reaches 1.9 mm,which shows significant improvements in system ranging accuracy and long-term operational stability.Continuous day-and-night routine SLR observations are carried out at a repetition rate of 5 kHz,and the data are sent to the International Laser Ranging Service(ILRS).Based on the one-year mea-surement data from July 2024 to June 2025,the annual average ranging accuracy of ground targets reaches 2.4 mm;the normal point accuracy of the lageos satellite reaches 0.9 mm;the short-term stability of orbit evaluation data for the lageos satellite is 5.8 mm;and the long-term stability is 2.1 mm.These results indicate that the quality of SLR ranging data has been improved,ranking among the top in international SLR stations and achieving the best level among domestic SLR systems.Thus it provides an effective approach for high-precision measurement of space tar-gets.

Objective In recent years, atomic clocks have made spectacular progress, with ground-based optical lattice atomic clocks reaching stabilities of less than 10-18 and accuracies of 1x10-18. High-performance atomic-clock satellite-satellite, satellite-ground, and ground-ground interconnections can be achieved by establishing time-frequency transfer links. This advancement provides insights into various critical technological and scientific domains, including global satellite navigation systems, deep space exploration, verification of general relativity, measurement of gravitational waves, gravity field assessment of the earth, and fundamental physical constant measurements. This year, the Chinese Academy of Sciences plans to launch a lunar orbiting spacecraft equipped with a laser time- frequency transfer payload to assess the performance of onboard hydrogen atomic clocks and to conduct a comparison of clocks in remote observatories. For high-precision time-frequency transfer data processing, establishing a computational model that meets the requirements of the mission within the framework of general relativity is necessary. Methods Based on the existing relativistic theory for time and frequency transfer, in this study, we derived a relativistic model of one- and two-way satellite-ground laser time-frequency transfer on a distant retrograde orbit (DRO), which can be directly used in the data processing of the DRO laser time-frequency transfer. Using the simulated DRO orbit, we analyzed the magnitudes and distribution patterns of various error correction terms in the laser time-frequency transfer. These terms include light-time correction, atmospheric refraction, relativistic rate shifts, and position correction between the detector and reflector. In addition, Monte Carlo methods were employed to simulate and compute the uncertainties of link corrections, considering the DRO orbit and attitude determinations and the probe payload calibration parameters. The effects of these uncertainties on the stability and accuracy of the time-frequency transfer measurements were investigated for both one- and two-way satellite-ground modes and for one-way laser time- frequency transfer in the common-view mode. Results and Discussions In the two-way mode, the dependence on the satellite orbit accuracy is relatively weak. With a radial distance error in orbit determination of 10 m, we anticipate that the accuracy of link error correction will exceed 1.5 ps, with corresponding link stability (modified Allan deviation) of better than 2x10-17@10000 s. However, the comparative performance of the one-way mode is highly dependent on the accuracy of the satellite orbit determination. The measurement clock error is coupled with the orbit errors. In scenario 1, the link correction accuracy is expected to be within 38 ns, with corresponding link stabilities of approximately 8x10-15@1000 s and 7x10-14@10000 s. This may affect the assessment of the long-term stability of onboard hydrogen clocks. Correction of the detector- reflector positioning relationship is a major factor that affects the laser time-frequency transfer in low- earth-orbit satellites. In lunar laser time-frequency transfer, assuming ground measurement errors of 1 cm and attitude measurement errors of 360 arcsec, the correction uncertainty is approximately 1x10-13. Both one- and two-way modes require correction for atmospheric refraction, with the one-way mode possibly requiring more accurate meteorological parameters than the two-way mode. When two observation stations each use one-way mode to conduct measurements on a DRO probe and achieve one-way laser time-frequency transfer in the common-view mode, the link is expected to achieve an accuracy of better than 1.7 ns and a stability of 1x10-14@1000 s. Conclusions The computational model developed in this research enables high-precision processing of one-way and two-way laser time-frequency transfer measurement data between the earth and the moon, thereby providing a theoretical foundation for the performance evaluation of atomic clocks and time synchronization in deep space exploration missions.

Objective The laboratory module II of the China Space Station (CSS), known as the Mengtian lab experiment module, has been part of the CSS since its launch in October 2022. It carries a Sr optical clock, an H-maser, and a laser-cooled microwave clock, along with a microwave link and a pulsed laser link. The CSS mission's pulsed laser time-frequency transfer (CLT) system is led by the Shanghai Astronomical Observatory (SHAO). In this paper, we aim to present the development and performance evaluation of the CLT system. Methods The CLT payload unit measures 230 mm x 190 mm x 169 mm, with a mass of 6 kg and power consumption of approximately 25 W, subject to fluctuations depending on the operating mode. The onboard hardware includes a laser retro-reflector, a single-photon detection package, and an event timer. The CLT detector utilizes an avalanche photodiode operating in Geiger mode, featuring the K14 SPAD chip with a 100 mu m detection area and a timing precision of 20 ps. The detection optics system is equipped with snowflake attenuators, polytetrafluoroethylene (PTFE) scatterers, pinholes, and an optical filter. For high-precision event timing, an FPGA and the THS788 timing chip are employed, achieving a timing accuracy of 8 ps and supporting a maximum measurement frequency of 20 kHz. To meet the stringent requirements of space-to-ground laser time-frequency transfer for CSS, several technical challenges are addressed. These include enhancements in large-field optical intensity stability detection, compensation for temperature drift-induced delays in the detector, and high-repetition-rate measurements at 10 kHz to improve overall stability. In addition, a real-time calibration channel for compensating delay drift is developed to mitigate the influence of temperature fluctuations and aging effects in the CLT event timer. Results and Discussions The temperature-induced delay drift of the CLT detector is mitigated through optimization of the comparator configuration and bias voltage, including the adjustment of the feedback coefficient. Experimental results demonstrate that, with a turning point at 21 degrees C, the CLT detector achieves temperature drift compensation of 0.14 ps/degrees C when operating above 21 degrees C. The detection optics maintain a 25% relative photon change across varying incident optical angles. Ground-based laboratory tests have confirmed that the CLT payload achieves a timing precision of 23 ps, with an instability of less than 0.5 ps over the course of one day and 0.09 ps over 300 s. Ranging experiments using the CLT laser retro-reflector array (LRA) are conducted by ground-based satellite laser ranging (SLR) systems located in Shanghai, Xi'an, and Beijing. Moreover, dedicated CLT ground stations in Xi'an and Beijing conduct satellite-based CLT measurements. The results indicate that the ranging precision of the Xi'an and Beijing ground stations is approximately 4 mm, with a clock bias measurement precision of 22 ps. Conclusions Our research marks a breakthrough in the engineering development of the pulsed laser time-frequency transfer system. As ground stations connect to high-performance atomic clock signals and sufficient measurement data is collected, the system offers profound insights into the fields of time-frequency metrology, space geodesy, and fundamental physics research. It enables the calibration and validation of microwave systems, a better understanding of clock behavior, comparison of clocks across remote observatories, and testing of Einstein's gravitational redshift effect.

A distributed feedback picosecond (DFB) pulse laser has been designed with a single pulse energy of 5 pJ and a pulse width of 200 ps, resulting in a spectrum width of 0.12 nm. By controlling the time sequence of the DFB picosecond pulse laser and regenerative amplification (RA) at a repetition rate of 5 kHz and pulse width of 200 ps, the single-pulse energy has been amplified to 320 mu J, an amplification factor of 6.4 x 107is achieved. After frequency doubling, an average power of 0.6 W for the green picosecond laser is obtained, the synchronization accuracy of root mean square (RMS) between the pulse picosecond laser and the external signal is 18.1 ps. Based on the principle of satellite laser ranging (SLR), an external trigger signal has been used to replace the locally collected laser pulse as the local main wave. This new approach has been implemented in the SLR system at the Shanghai Astronomical Observatory, allowing for measurements of low-earth orbit, medium-earth orbit, high-altitude orbit, and geosynchronous (GEO) orbit (-36000 km) satellites with a measurement accuracy ranging from 6 mm to 20 mm. It is the first successful integration of a DFB + RA-based low jitter picosecond laser with an externally triggered signal as the primary waveform for SLR of multiple satellites,it provides a new and innovative method for laser ranging.

Frequency-modulated continuous wave(FMCW)laser measurement exhibits the advantages of high precision,wide range,good sensitivity,and strong anti-interference ability,enabling the noncontact measurements of high dynamic targets.This measurement technique has an important value in the large-scale precision measurement and machining and manufacturing fields and has become a hot spot in the laser precision and absolute measurements.Herein,the principle of FMCW laser measurement is introduced,current application requirements of long-distance measurements are analyzed,and key points of FMCW laser measurement at home and abroad are highlighted.In this study,an FMCW laser measurement system is developed based on high-sensitive coherent detection technology,which realizes the meter-level ranging of diffuse reflection targets at 10 km.Results provide an important reference for the future long-distance,high-precision FMCW laser measurement at home.

Located at the highest point on the Antarctic Plateau’s ice sheet, Dome A is generally believed to be one of the best places on Earth for nighttime astronomy in the optical and near-infrared (NIR) bands. Daytime optical/NIR site characteristics are yet to be quantified, however. Here we report the first daytime observations of bright stars at the J band during the austral summer of 2023/2024. The experiments were conducted using a 150 mm telescope with a field of view of 0 . ° 87 × 0 . ° 69 and a pixel size of 2 . ″ 5. The sky brightness at zenith was measured to be ∼5.2 mag arcsec −2 at noon when the solar elevation was ∼27°, and it slightly darkened to ∼5.8 mag arcsec −2 at midnight with a solar elevation angle of ∼10°. Stars as faint as J = 10.06 mag were significantly detected at 5 σ levels with an effective exposure time of 175 s around midnight. The pathfinding experiments indicate that a sensitivity ∼2 mag deeper can be reached by the planned 1 m class telescopes, taking advantage of the small free atmosphere seeing. Considering the high latitude and the extremely high fraction of clear days at this site, valuable bright transients with J ≲ 12 mag, such as (super)novae in the local universe and space debris at low orbits, within ∼1/4 of the whole sky around the south celestial pole can be timely discovered and continuously monitored throughout the year.

To address the challenge of accurately recognizing signals in real time for space debris laser ranging, this paper proposes a deep learning-based method for real-time recognition of space debris laser ranging signals. Utilizing the long shortterm memory (LSTM) network of recurrent neural networks in deep learning, the proposed method enables real-time recognition of signals that are difficult for traditional methods to detect. The LSTM network excels at capturing and maintaining long-term dependencies in time series data, effectively handling missing, noisy, or irregular sequences, while also demonstrating strong generalization capabilities. Validation using actual observational data shows that the proposed method improves the recognition rate of space debris. Compared to the echo signal correlation method, the average running time is reduced by similar to 8%, and the F1 score for signal recognition increases from 0.2144 to 0.6068, representing an improvement of nearly twofold. This method will play a significant role in the detection and identification of space debris and provide valuable insights for the application of deep learning techniques in space target laser ranging.

Lunar laser ranging (LLR) has the highest precision in measuring the earth-moon distance. The lunar laser ranging retro-reflector (LRRR) is an essential piece of equipment for realizing lunar laser ranging. China has plans to deploy a manually emplaced new generation LRRR. For the LRRR to work effectively, the azimuth and altitude angle of the LRRR must be adjusted such that the LRRR is aligned with the mean Earth direction. We established a method for calculating the adjustment angle of the LRRR and analyzed the misalignment caused by the biases of emplacement time and emplacement site. Results show that the aiming of error of the LRRR can be estimated at approximately 2. 7 degrees, and the maximum value is less than 5. 0 degrees, which satisfies the requirements for an aiming accuracy of 5. 5 degrees. The calculation method and analysis presented in this study can act as guidelines for future China LRRR emplacement missions.

High-precision space debris measurements can provide more accurate real-time information on debris targets and enhance the effectiveness of satellite avoidance warnings for space debris. Through the modification of the 1. 2 m aperture quantum communication telescope (altitude 3200 m) in Qinghai Province, the satellite laser ranging (SLR ) and space debris laser ranging (DLR ) experiments were carried out by using a single pulse energy of 1. 2 mJ and a repetition rate of 1 kHz picosecond laser, in which the detection range of cooperative satellites has been extended from Low Earth Orbit to Geosynchronous Eearth Orbit, and the ranging accuracy was better than 2 cm. The maximum distance of space debris target measurement is 1620. 5 km, the radar cross section (RCS ) is 2. 41 m(2), and the ranging accuracy reaches 10. 64 cm. A single laser system has been realized, which can not only carry out centimeter-level high-precision ranging of cooperative targets, but also realize space debris observation. This is the first time in the world to use high repetition frequency and low power laser ranging system to achieve high precision measurement of space debris targets, reflecting the advantages of picosecond laser and high-altitude large-aperture telescope measurement, providing reference for developing space target laser ranging in western China, and providing an effective way for space debris laser ranging system site selection and space debris monitoring capability enhancement.

High -precision Time -Frequency Rack (HTFR) mission, a part of experiment module II of the China Space Station (CSS), was equipped with a Sr optical clock, H-maser, and laser-cooled microwave clock. To compare time and frequency across widely separated clocks, a microwave link, and a pulsed laser link were both established, which probably enables new possibilities for fundamental science, geodesy, and metrology. Based on the existing relativistic theory for time and frequency transfer, we derive a relativistic model of satellite-ground laser time transfer on CSS (CLT), which can be of direct use in CLT data processing. In addition to first and second-order Sagnac effects, it also includes relativistic rate shifts, atmospheric refraction, and turbulence, laser retroreflector array (LRA) range correction, and position correction between detector and reflector, satisfying the stability evaluation of CSS atomic clocks with fractional frequency stability of 3 x 10-17/10000 s. Additionally, we numerically investigate the link error magnitude and use Monte Carlo simulations to assess the impact of correcting for link errors on the stability of the CLT, taking into account the CSS orbit and attitude determinations as well as the CLT payload calibration parameters. It is shown that the CLT link can be considered to reach a TDEV of approximately 0.1 ps/300 s while meeting the CSS design objectives. The primary factors that influence the stability of the CLT are the position correction between detector and reflector, LRA range correction, and the coupling relation of these two error distributions. (c) 2023 COSPAR. Published by Elsevier B.V. All rights reserved.

Picosecond pulse laser is the main light source for satellite laser ranging. In this paper, a 10 kHz repetition rate picosecond green laser with an average output power of 5.3 W is demonstrated. The laser generates a pulse width of 18.6 ps at a center wavelength of 532.20 nm with a spectral width of .066 nm. The beam quality is well preserved with M 2 of 1.1 with the beam divergence measured to be .62 mrad and pointing stability of 7 μrad over 30 min of operation. The laser system was then applied to measure the BeiDou satellite (Compass-I3) and generated a single range accuracy of 3.2 mm, which is the highest reported range accuracy for synchronous orbit satellite laser ranging.

The origin and spatial?temporal variation of the Earth's magnetic field (EMF) is one of the important scientific problems that has long been unsolved. The Macau Science Satellite-1 (MSS-1) under construction is China's first high-precision EMF measurement satellite. To satisfy the highly precise requirements of the MSS-1 orbit measurement, a light, high-precision, four-prism laser retro- reflector array was designed. It weighs approximately 285 g, its effective reflection area is greater than 1.77 cm2, and its size is 100 × 100 × 41 mm. The laser retro-reflector array has excellent performance, and it can achieve a ranging precision at the subcentimeter level for satellite laser ranging. It will be developed and installed on the MSS-1 as a power-free load for high-precision orbit measurement and accurate orbit calibration. The MSS-1 is planned to be brought into the International Laser Ranging Service observations. More than 31 satellite laser ranging stations in the International Laser Ranging Service around the world will be able to measure the MSS-1 with long arcs, which will support the scientific mission of high-precision EMF exploration.

Objective Satellite Laser Ranging (SLR) has contributed a new prospect to clock comparison and monitoring with higher precision and accuracy. Several laser time transfer projects, such as Time Transfer by Laser Link (T2L2), and Laser Time Transfer in China's Space Station (CLT), have been conducted over the past 20 years. However, the traditional laser time transfer is limited because of incident light intensity and ambient temperature on onboard hardware. Alternatively, Forward Laser Time Transfer (FLTT) based on curved mirrors is a new type of laser time transfer technology with great application potential, which has the advantages of zero -time delay in the forward process, high reliability, long service life, and high measurement accuracy. However, due to the lack of suitable satellite experimental platforms, experimental data and performance evaluation results of the FLTT have not been obtained. The current research is only limited to theoretical analysis. In this study, we investigated the FLTT with two adjacent SLR systems as the test station and the retroreflectors on the cooperative target satellite as the forward payload. Our research should provide some technical reserves for the curved mirror based FLTT and evaluate the feasibility and performance of FLTT technology. Methods First, we briefly introduced the basic principles and difficulties of FLTT, including one-way and two-way FLTT modes. Furthermore, the clock difference extraction algorithm of a one-way FLTT was improved to make it suitable for laser time transfer scenarios with high precision and accuracy. Then, two sets of FLTT test systems were built through the adaptive transformation of the SLR systems of the Shanghai Astronomical Observatory (SHAO). Among them, we added the precise control circuit of the laser emission epoch, the calibration device of the FLTT system time delay, and the clock difference extraction module. Depending on the two FLTT systems, we carried out one-way and two-way FLTT experiments, homologous clock FLTT experiments for BDS45 and BDS22 satellites, and non-homologous clock FLTT experiments for BDS21 and Galileo206 satellites. In addition, we obtained the real clock difference of the two FLTT systems with a DG645 delay signal generator, which is the clock difference obtained by the electrical signals, and then the reliability of the clock difference of one-way and two-way FLTT experiments was demonstrated. Finally, we analyzed the FLTT results and studied the main factors that affect the performance of this technology. Results and Discussions Based on BDS45, BDS22, and other satellites, the FLTT experiments in one-way and two-way modes were carried out, and both obtained a precision better than 100 ps (Fig. 8, Fig. 9, Fig. 10, Fig. 11). There are two critical issues to realize FLTT using curved mirrors. One is the limited hitting probability at the right time and another issue is the weak strength of reflected signals. Therefore, the one-way FLTT mode will have better applicability in the future. The results of the one-way FLTT were similar to those of the two-way FLTT (Fig. 10 vs Fig. 11, Fig. 11 vs Fig. 12), which verified the reliability of the improved clock difference extraction algorithm in this study. Compared the FLTT experiment results and the clock difference obtained by the DG645 electrical signals, the absolute accuracy was better than 2 ns, demonstrating the reliability of the FLTT experiments (Fig. 12). However, there was plenty of room for improvement. In the FLTT with the homologous clock, the clock difference jump phenomenon was caused by the synchronization of the event timer and clock. In the FLTT with non -homologous clocks, the clock difference fluctuated greatly due to the poor stability of the End-run clock. Conclusions The curved mirror-based FLTT has been considered a more accurate and stable time transfer technique than existing methods, which is a popular scheme for the next generation of laser time transfer. However, there is a lack of suitable test platforms for validation purposes, and the current research interests in FLTT remain in theoretical simulation. In this study, we selected satellite reflectors instead of curved mirrors as a forwarding payload for the FLTT experiments, demonstrating the feasibility and performance of this technique. We improved the clock difference extraction algorithm of the one-way FLTT and proposed a high-precision interpolation model. It was shown from the results that the precision was better than 100 ps with an absolute accuracy of 2 ns, and the one-way FLTT was feasible. Of course, the FLTT will probably face other challenges in practical application, such as weak laser echo signal recognition, and the probability of success is affected by the attitude and rotation period of satellites. This study provides significant support for FLTT development, such as in laser emission timing control, data processing, and more.

卫星激光测距(Satellite Laser Ranging,SLR)以脉冲激光为媒介获取卫星的精确距离,是空间大地测量技术中准确度最高的手段.在传统卫星激光测距系统中,通过测量已知距离的固定靶目标实现系统总时延的标定,对获取单向发射或接收时延的研究较少,这制约了卫星激光测距在激光时间比对、多台站协同测距及行星际激光测距等方面的应用.文中开展皮秒准确度时延标定方法的研究,首先,分析了卫星激光测距系统的时延组成及影响因素;其次,以中国科学院上海天文台卫星激光测距系统为平台,开展电学、光学和光电转换等时延的高精度测量,并将各部分时延组合完成收发时延的标定;最后,分析发射和接收时延标定的准确度,并将时延标定方法应用于地靶距离偏差的校验,验证时延标定方法的可行性.结果表明,发射和接收时延标定的准确度分别优于11 ps和13 ps,地靶距离偏差与国际激光测距组织(ILRS)反馈值相差仅11 ps.

With the development of aerospace and space scientific research, satellite laser ranging (SLR) has put forward higher requirements for response speed, data density, and measurement accuracy. In coaxial common optical path laser ranging, the emitted laser and the received laser echoes pass through the same optical system. Due to the reversibility of the optical path, the laser emission, monitoring, and laser echoes’ optical path all pass through the same optical system structure, and the response speed and ranging ability of the laser ranging system have been greatly improved. Based on the SLR system of the Shanghai Astronomical Observatory (SHAO), the laser transmitting telescope with an aperture of 21 cm was used to build a polarized coaxial SLR system. It uses a picosecond pulsed laser with a pulse repetition frequency of 2 kHz and a single-pulse energy of 2 mJ. Also, a 4f system was applied to shrink the laser echo beam and filter out noise, the measurements of low-Earth orbit and long-distance high-orbit satellites were realized, and the ranging accuracy was ∼2 cm. As far as we know, this is currently the smallest aperture telescope for SLR globally, which is conducive to the miniaturization and integrated development of SLR systems.

卫星激光测距(SLR)是卫星轨道测量中精度最高的一种技术,广泛应用于卫星精密定轨、卫星轨道精度检核、激光时间比对等,是导航系统卫星轨道精确测定的重要组成部分.本文结合中国科学院上海天文台SLR系统及北斗卫星国际激光联测,取得全球激光台站观测数据,为实施全球导航卫星系统联合研究提供了条件;发展了 100 kHz高重复频率SLR技术并应用于北斗卫星激光观测,标准点数据精度达百微米水平,有效提升了导航卫星激光观测数据质量;同时描述卫星激光测距技术在北斗导航系统其他应用情况,包括卫星独立定轨、广播星历精度检核、星地激光时间比对等.

Remote sensing satellite SiCH-2 from Ukraine was launched in 2011. At present, it has stopped working and is out of control, and it has been forecasted to become space debris by a two-line root (TLE) from the North American Air Defense Command (NORAD). Here, the "cat's eye" effect of the optical system is analyzed, and the satellite laser ranging (SLR) system at Shanghai Observatory is used to measure the echo intensity of satellite SiCH-2, which is very strong, and the ranging accuracy, which is better than 10 cm. The analysis of the measurement capability results shows that the laser reflection echo from satellite SiCH-2 reaches the laser ranging level of the satellite with the reflector, and is consistent with the detection ability of the photoelectric detection equipment "cat's eye" effect analysis. The measurement results also show that satellite SiCH-2 is in a state of rotation, with a period of 4.3 s. In this study, the laser ranging of on-orbit targets based on the "cat's eye" effect is realized for the first time, which provides a new method for analyzing long-distance space targets, promotes the application and development of high-precision laser ranging technology, and is beneficial to monitor the photoelectric detection series of satellites that fail or have abnormal orbits.