BPM载频产生器的研制

介绍ＢＰＭ短波授时台新的载频产生器的设计与实现，该ＢＰＭ载频产生器以原子频标的５ＭＨｚ标频信号为参考，利用频率合成技术，产生出供发射机使用的２．５ＭＨｚ、５ＭＨｚ，１０ＭＨｚ、１５ＭＨｚ和２０ＭＨｚ五种高准确度的载频信号。这些信号既能单独输出，也可利用面板控制或遥控选择输出。为了保证系统的可靠性，采用了两冗余结构，并设置了必要的报警与指示。     

BPM短波授时信号模拟器的研制

介绍了BPM短波授时信号模拟器的主要功能和工作原理,阐述了该模拟器的系统组成、输入输出及软件流程。该模拟器以GPS信号作为时间源,以10 MHz的高稳定恒温晶振作为守时频率源,并在程序控制下产生载频与调制信号,放大输出6路BPM短波模拟信号。测试结果表明,该模拟器的功能与技术指标符合BPM短波授时信号的要求。     

从载频看我国的短波授时

本文分析了我国短波授时载波频率安排的历史和现状,剖析存在问题;根据短波时号传播特性及其在我国授时工作中的历史地位.建议同时发播2.5、5、10、15(MHz)载频信号,考虑载频偏置,改善短波授时服务.     

时号改正数

呼号BPL/6000 LC/8930 BPMc BPM1 载频100 100 2.5,5,10,15 2.5,5,10,15 kHz kHz MHz MHz 时刻(UTC)08:00:00 00:00:00 00:00:00 00:00:00 2005年6月MJD (μs) (μs) (μs) (μs) 1 53522 0.02 - -20.01 617.31 2 53523 0.11 - -20.01 618.07 3 53524 0.21 - -20.01 618.65 4 53525 0.32 - -20.01 618.97 5 53526 0.35 - -20.01 619.09     

一种时频信号相位补偿方法

为消除信号产生器内温度变化对信号相位的影响,提出了一种恒温控制和实时相位调整相结合的方法。在这种方法中,除恒温控制外,基于对信号产生器内部温度、相位测量值变化率、设备调相分辨率和相位波动指标等的分析研究,合理计算实时相位调整关键参数,进行实时相位调整,有效地减小信号产生器内部温度变化引起的相位波动,提高了输出端信号相位的一致性和稳定性。     

BPM时号在武昌

本文通过分析武昌时辰站5年接收BPM时号的资料,得出BPM时号的可用百分率和传播时延异常现象有随昼夜、季节和太阳活动周期而变化的规律,指出发射和接收工作载频高于收发路径上的最高可用率(MUF)是其主要原因。     

基于DSP的小型化高精度频标比对系统研制

针对传统双混频时差法测频的局限性,提出了一种差拍数字化精密频率测量方法,基于正弦差拍技术、同步采样技术和数字信号处理技术设计实现了一种新型频标比对系统,可实现对5MHz、10MHz等频标信号的比对测量。实验结果证明,测量10MHz频标信号时系统本底噪声约为1×10^-12／s。系统拥有测量精度高,体积小以及成本低的特点,在时频测量领域具有良好的推广和应用价值。     

高稳定性数字钟的分析与实现

介绍一种高稳定、低功耗、多功能、小型化的数字钟，由数字钟插入相位噪声引起的1pps输出信号相位抖动为5．3ps，前沿上升时间为4．6ns，指标测量通过2m长50Ω高频同轴电缆并接50Ω电阻性负载，输入信号为10MHz正弦波信号，移相精度为100ns，同步精度≤100ns，通过RS-232接口每秒一次的速率以ASCII码格式输出时间信息(年、月、日、时、分、秒)，在接数显钟面时其功耗为2W，不接数显钟面时为1．2W。     

钟信号分配放大器的研制(摘要)

钟信号分配放大器的主要用途是把原子钟的10MHz、5MHz、1MHz和1pps信号以高质量的缓冲分别以多路形式不失真地馈送给用户,其目的是:第一,隔离负载对原子钟的影响:第二,在同一原子钟情况下,能向负载提供5路与钟一样的信号。它是高精密时     

定时系统及时间统一和同步(摘要)

定时系统是航天通信测控网、靶场测控系统的一个重要组成部分,向各测控设备提供标准频率信号和标准时问信息,使靶场内的各测控设备在统一的时间尺度上工作。定时系统国内通常称为时间统一勤务系统(简称时统),主要由标准频率源(如原子频标、石英频标)、时间编码产生器、信号分配设备、定时校频、监测控制设备组成。定     

A thermodynamic model of high temperature lava vaporization on Io

We modified the MAGMA chemical equilibrium code developed by Fegley and Cameron (1987, Earth Planet. Sci. Lett. 82, 207-222) and used it to model vaporization of high temperature silicate lavas on Io. The MAGMA code computes chemical equilibria in a melt, between melt and its equilibrium vapor, and in the gas phase. The good agreement of MAGMA code results with experimental data and with other computer codes is demonstrated. The temperature-dependent pressure and composition of vapor in equilibrium with lava is calculated from 1700 to 2400 K for 109 different silicate lavas in the ONaKFeSiMgCaAlTi system. Results for five lavas (tholeiitic basalt, alkali basalt, Barberton komatiite, dunite, and a molten type B1 Ca, Al-rich inclusion) are discussed in detail. The effects of continuous fractional vaporization on chemistry of these lavas and their equilibrium vapor are presented. The predicted abundances (relative to Na) of K, Fe, Si, Al, Ca, and Ti in the vapor equilibrated with lavas at 1900 K are lower than published upper limits for Io's atmosphere (which do not include Mg). We predict evaporative loss of alkalis, Fe, and Si during volcanic eruptions. Sodium is more volatile than K, and the Na/K ratio in the gas is decreased by fractional vaporization. This process can match Io's atmospheric Na/K ratio of 10±3 reported by Brown (2001, Icarus 151, 190-195). Silicon monoxide is an abundant species in the vapor above lavas. Spectroscopic searches are recommended for SiO at IR and mm wavelengths. Reactions of metallic vapors with S- and Cl-bearing volcanic gases may form other unusual gases including MgCl₂, MgS, MgCl, FeCl₂, FeS, FeCl, and SiS.     

Instructions for authors

正Research in Astronomy and Astrophysics publishes original research papers and reviews on all branches of astronomy and astrophysics.Reviews are by invitation only.Important new results that require rapid publication can be submitted as a Letter(Letters must be restricted in length to 6 printed pages).Authors who submit a paper are expected to be able to certify that the paper is original work,     

Research in Astronomy and Astrophysics CONTENTS

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Alkali and halogen chemistry in volcanic gases on Io

We use chemical equilibrium calculations to model the speciation of alkalis and halogens in volcanic gases emitted on Io. The calculations cover wide temperature (500-2000 K) and pressure (10⁻⁶ to 10⁺¹ bars) ranges, which overlap the nominal conditions at Pele (T=1760 K, P=0.01 bars). About 230 compounds of 11 elements (O, S, Li, Na, K, Rb, Cs, F, Cl, Br, I) are considered. The elemental abundances for O, S, Na, K, and Cl are based upon observations. CI chondritic elemental abundances relative to sulfur are used for the other alkalis and halogens (as yet unobserved on Io). We predict the major alkali species in Pele-like volcanic gases and the percentage distribution of each alkali are LiCl (73%), LiF (27%); NaCl (81%), Na (16%), NaF (3%); KCl (91%), K (5%), KF (4%); RbCl (93%), Rb (4%), RbF (3%); CsCl (92%), CsF (6%), Cs (2%). Likewise the major halogen species and the percentage distribution of each halogen are NaF (88%), KF (10%), LiF (2%); NaCl (89%), KCl (11%); NaBr (89%), KBr (10%), Br (1%); NaI (61%), I (30%), KI (9%). We predict the major halogen condensates and their condensation temperatures at P=0.01 bar are NaF (1115 K), LiF (970 K); NaCl (1050 K), KCl (950 K); KBr (750 K), RbBr (730 K), CsBr (645 K); and solid I₂ (200 K). We also model disequilibrium chemistry of the alkalis and halogens in the volcanic plume. Based on this work and our prior modeling for Na, K, and Cl in a volcanic plume, we predict the major loss processes for the alkali halide gases are photolysis and/or condensation onto grains. Their estimated photochemical lifetimes range from a few minutes for alkali iodides to a few hours for alkali fluorides. Condensation is apparently the only loss process for elemental iodine. On the basis of elemental abundances and photochemical lifetimes, we recommend searching for gaseous KCl, NaF, LiF, LiCl, RbF, RbCl, CsF, and CsCl around volcanic vents during eruptions. Based on abundance considerations and observations of brown dwarfs we also recommend a search of Io's extended atmosphere and the Io plasma torus for neutral and ionized Li, Cs, Rb, and F.     

Study on the Stressed Mirror Polishing with a Continuous Polishing Machine for Large Aperture Off-axis Aspheric Mirrors

A special stressed annular polishing technique is proposed to mill the off-axis aspheric sub-mirrors of a large segmented mirror with an annular polishing machine. Based on the basic principle of stressed annular polishing technique, a set of special stressing mechanisms are designed to convert milling the aspheric surfaces of sub-mirrors with different off-axis distances into milling the spherical surfaces with identical radii of curvature, so that they can be pol- ished simultaneously on a continuous polishing machine. It took about contin- uous 40 hours to polish a scaled-down mirror of the planning Chinese Future Giant Telescope (CFGT) using this technique. This mirror has the 330 mm di- ameter, 3.6 m off-axis distance, and the 21.6 m radius of curvature, and its max- imum asphericity is 16 micron. The experiment shows that this method has a high effciency, suits batch manufacturing, especially the batch manufacturing of aspheric sub-mirrors of the segmented primary mirror of an extremely large aperture telescope.     

Born–Infeld type phantom model in the ω–ω′ plane

In this paper, we investigate the dynamics of Born–Infeld (B–I) phantom model in the ω–ω′ plane, which is defined by the equation of state parameter for the dark energy and its derivative with respect to N (the logarithm of the scale factor a). We find the scalar field equation of motion in ω–ω′ plane, and show mathematically the property of attractor solutions which correspond to ω _φ ∼−1, Ω _φ =1, which avoid the “Big rip” problem and meets the current observations well.      

A Comparison of Several Image Locating Algorithms in Astronomical Instruments

In astronomical observations at optical wavelengths, a fast image tracking system can be adopted to reduce the effects of the atmospheric seeing and telescopic tracking error, and therefore improve the observing efficiency. Aiming at the need of astronomical observations, totally 5 kinds of algorithms in two categories were selected to make a comparative study on their accuracies and stabilities under different noise conditions by both numerical experiment and laboratory test. The results indicate that the normalized cross-correlation method and barycenter method have not only a higher accuracy but also a better reliability against interferences, they will be applied to the high-resolution spectrograph of the Xinglong 2.16 m telescope and the scienti?c instruments of the SONG (Stellar Observations Network Group) project, respectively.     

Shear and vorticity in a combined Einstein-Cartan-Brans-Dicke inflationary lambda-Universe

A combined BCDE (Brans-Dicke and Einstein-Cartan) theory with lambda-term is developed through Raychaudhuri’s equation, for inflationary scenario. It involves a variable cosmological constant, which decreases with time, jointly with energy density, cosmic pressure, shear, vorticity, and Hubble’s parameter, while the scale factor, total spin and scalar field increase exponentially. The post-inflationary fluid resembles a perfect one, though total spin grows, but the angular speed does not (Astrophys. Space Sci. 312: 275, 2007d).