分类: 天文学 >> 天文仪器与技术 提交时间: 2025-06-03 合作期刊: 《Astronomical Techniques and Instruments》
摘要: We present analog clocks fitted to the Mars solar day. These clocks use the standard Earth-based second of the International System of Units (SI) as their operational unit of time, unlike current practice for Mars timekeeping. We discuss the importance of preserving the SI second. On this basis, we identify the two analog clocks most suitable for public use by a future Mars population. These are a 20-hour clock with a hand motion similar to that of the standard Earth clock, and a 24-hour clock with a novel “Martian” hand motion which strikes the hour when all 3 hands converge onto that hour mark on the dial. Both clocks have Earth-day equivalents to assist learning. We also present a 24-hour “SpaceClock”, similar to the Martian clock but with no favored reference plane, hence equally readable from any viewing orientation.
分类: 天文学 >> 天文仪器与技术 提交时间: 2024-09-30 合作期刊: 《天文技术与仪器(英文)》
摘要: A time and frequency system is a critical component of Very Long Baseline Interferometry (VLBI) stations, providing stable and reliable standards that directly impact data processing quality. At the Tianma 65 m radio telescope(TMRT), this system has been meticulously designed to ensure long-term reliability and high performance. It incorporates high-performance hydrogen atomic clocks, high-precision time standards, automatic signal switching, and robust system software. This comprehensive approach has enabled the system to achieve long-term reliable operation, successfully supporting both major national engineering tasks and daily scientific observations. The effectiveness of the system is evidenced by its consistent delivery of the precision and stability required for radio astronomy. This article provides an in-depth exploration of the design and operation of the time and frequency system at the Tianma 65 m telescope, examining various aspects of its architecture, implementation, and performance. By sharing these insights, we aim to contribute knowledge that could benefit similar systems at other VLBI stations, greatly advancing radio astronomy infrastructure.
分类: 生物学 >> 动物学 提交时间: 2019-06-11 合作期刊: 《古脊椎动物学报》
摘要: This paper provides an overview and a protocol of molecular clock dating using MrBayes. Two modern approaches, total-evidence dating and node dating, are demonstrated using a truncated dataset of Hymenoptera with molecular sequences and morphological characters. The similarity and difference of the two methods are compared and discussed. Besides, a non-clock analysis is performed on the same dataset to compare with the molecular clock dating analyses.
分类: 生物学 >> 动物学 提交时间: 2019-06-11 合作期刊: 《古脊椎动物学报》
摘要: This paper provides an overview and a protocol of molecular clock dating using MrBayes. Two modern approaches, total-evidence dating and node dating, are demonstrated using a truncated dataset of Hymenoptera with molecular sequences and morphological characters. The similarity and difference of the two methods are compared and discussed. Besides, a non-clock analysis is performed on the same dataset to compare with the molecular clock dating analyses.
分类: 地球科学 >> 空间物理学 提交时间: 2017-01-22
摘要: By use of the global PPMLR Magnetohydrodynamics(MHD) model, a serial of quasi-steady-state numerical simulations were conducted to examine the modulation property of the interplanetary magnetic field clock angle theta on the solar wind energy input into the magnetosphere. All the simulations can be divided into seven groups according to different criteria of solar wind conditions. For each group, 37 numerical examples are analyzed, with the clock angle varying from 0�to 360�with an interval of 10� keeping the other solar wind parameters(such as the solar wind number density, velocity, and the magnetic field magnitude) unchanged. As expected, the solar wind energy input into the magnetosphere is modulated by the IMF clock angle. The axisymmetrical bell-shaped curve peaks at the clock angle of 180� However, the modulation effect remains invariant with varying other solar wind conditions. The function form of such an invariant modulation is found to be sin(0/2)2.70 + 0.25.
分类: 物理学 >> 地球物理学、天文学和天体物理学 分类: 信息科学与系统科学 >> 信息科学与系统科学基础学科 提交时间: 2024-06-09
摘要: The precise estimation of the satellite clock bias (SCB) holds considerable importance in ensuring accurate timekeeping, navigation, and positioning. This studyintroduces a novel SCB prediction approach that integrates variational mode decomposition (VMD) and long short-term memory (LSTM) network techniques, combining signal decomposition with deep learning methodologies. Initially, the raw SCB data undergoespreprocessing, followed by decomposition using the VMD method to generate multiple intrinsic mode functions (IMFs). These decomposed IMFs serve as inputs for LSTM, where several independent LSTM models are established for training and prediction purposes. Subsequently, the predicted outcomes are aggregated and reconstructed to derive the finalSCB prediction. Experimental findings demonstrate notable advancements in clock bias prediction for the spaceborne hydrogen atomic clock for BDS, with prediction accuracies of 0.048 ns, 0.204 ns and 1.397 ns for 6 hours, 3 days and 15 days, respectively. These results exhibit significant enhancements compared to both the LSTM network and the Back Propagation (BP) neural network, with improvements of 56%, 84% and 83% for the aforementioned time intervals in comparison to LSTM, and enhancements of 59%, 82% and 83% relative to the BP neural network.
分类: 生物学 >> 植物学 >> 植物生理学 提交时间: 2016-05-03
摘要: Arabidopsis thaliana plants are less resistant to attack by the generalist lepidopteran herbivore Trichoplusia ni when plants and herbivores are entrained to opposite, versus identical diurnal cycles and tested under constant conditions. This effect is associated with circadian fluctuations in levels of jasmonic acid, the transcription factor MYC2, and glucosinolate contents in leaves. We tested whether a similar effect could be observed in a different plant-herbivore system: the wild tobacco Nicotiana attenuata and its co-evolved specialist herbivore, Manduca sexta. We measured larval growth on plants under both constant and diurnal conditions following identical or opposite entrainment, profiled the metabolome of attacked leaf tissue, quantified specific metabolites known to reduce M. sexta growth, and monitored M. sexta feeding activity under all experimental conditions. Entrainment did not consistently affect M. sexta growth or plant defense induction. However, both were reduced under constant dark conditions, as was M. sexta feeding activity. Our data indicate that the response induced by M. sexta in N. attenuata is robust to diurnal cues and independent of plant or herbivore entrainment. We propose that while the patterns of constitutive or general damage-induced defense may undergo circadian fluctuation, the orchestration of specific induced responses is more complex.
分类: 地球科学 >> 空间物理学 提交时间: 2016-05-03
摘要: HY-2 satellite, the first ocean dynamic satellite of China, was launched in August 2011, carrying a dual frequency (Ku band and C band) altimeter, to measure the global mean sea level [1]. To provide truly sea level measurements, in-orbit calibration is essential. This paper concerns the transponder in-orbit calibration method. A signal reconstructive transponder for HY-2 altimeter in-orbit calibration is presented. Different from the existing bent-pipe ones, this kind of transponder can capture and track altimeter pulses, record them, and reconstruct the chirp pulses, transmit them back to the altimeter. It is carried on a SUV, reduces requirement for a permanent installation site. With the characteristics of capture and record altimeter signals, the HY-2 altimeter inner path delay and clock drift could be derived from the calibration data.
分类: 生物学 >> 动物学 提交时间: 2021-05-20 合作期刊: 《古脊椎动物学报》
摘要: 贝叶斯支端定年法是近些年开发的推断类群分异时间和演化速率的方法。它克服了传统分步计算的缺陷,但涉及到的统计学知识也更多。本文从贝叶斯统计计算的角度分层剖析了支端定年法的原理和计算过程,按照分异时间的先验分布、演化速率的先验分布、特征状态变化的模型和马氏链蒙特卡罗算法几个部分,叙述并讨论了定年计算中的主要模型和算法。旨在一定程度上为古生物学家分析实际数据提供参考。
分类: 生物学 >> 生物进化论 提交时间: 2025-02-13
摘要: In his recent paper, Zhang attempts to refute the genetic equidistance phenomenon (GEP) while defending the molecular clock and neutral theory. However, he overlooks a fundamental contradiction: the neutral theory is based on the molecular clock, which in turn relies on the GEP. While Zhang demonstrates a superficial understanding of common methods in molecular phylogeny, he fails to grasp several critical facts and concepts relevant to the topic. The methods and results presented are largely irrelevant to the GEP, and his criticisms of the maximum genetic diversity (MGD) hypothesis largely amount to a strawman argument. Notably, Zhang neglects the multiple lines of evidence supporting the existence of an upper limit on genetic distance or diversity. He also overlooks the inconvenient fact that the implicit assumption of the molecular clock and neutral theory—that genetic distance has no upper limit—has yet to be validated by any direct tests. As a result, Zhang’s paper cannot be considered a meaningful contribution.
分类: 天文学 >> 天文仪器与技术 提交时间: 2024-04-18 合作期刊: 《天文技术与仪器(英文)》
摘要:Optical frequency combs, as powerful tools for precision spectroscopy and research into optical frequency standards, have driven continuous progress and significant breakthroughs in applications such as time-frequency transfer, measurement of fundamental physical constants, and high-precision ranging, achieving a series of milestone results in ground-based environments. With the continuous maturation and evolution of femtosecond lasers and related technologies, optical frequency combs are moving from ground-based applications to astronomical and space-based applications, playing an increasingly important role in atomic clocks, exoplanet observations, gravitational wave measurements, and other areas. This paper, focusing on astronomical and space-based applications, reviews research progress on astronomical frequency combs, optical clock time-frequency networks, gravitational waves, dark matter measurement, dual-comb large-scale absolute ranging, and high-resolution atmospheric spectroscopy. With enhanced performance and their gradual application in the field of space-based research, optical frequency combs will undoubtedly provide more powerful support for astronomical science and cosmic exploration in the future.