对地方电视台转播CCTV时延的测定

讲述了同步广播卫星电视时间信号的测量方法和测量结果，获得了ＣＣＴＶ、ＣＣＴＶ２通过同一颗卫星（亚太－１Ａ）转播的时延差值为１６３．３３μｓ，测量精度在１０ｎｓ以内；ＣＣＴＶ２、ＣＣＴＶ４分别经两颗卫星（亚太－１Ａ，亚洲－Ⅱ）转播，在陕西天文台卫星地面接收站时延差值为１６４４．２０μｓ，严谨为５０μｓ，并分析了影响时延差值和精度的原因，同时测量了地方电视台转播亚太－１Ａ的ＣＣＴＶ信号与直接接收亚太－     

同步广播卫星电视中秒信号的测量结果及其分析

简单介绍了国家授时中心的卫星电视比对系统,给出了利用时间比对系统测量CCTV信号中插入的时间信号(它们或仅由卫星转发或由卫星转发后再经地方电视台转播)时延的方法和测量结果,并对其进行了分析,对定时用户具有参考价值。     

三站时差法荧光屏卫星(714MHz)定位及时刻同步

利用同步的三个地面站,同时接收荧光屏卫星(714MHz)的信号,由卫星信号至各地面站的传播时延差,计算出卫星的位置,定位精度在0.1以内。国内任何接收点,接收卫星信号并作卫星位置的时延修正,则与陕西天文台时刻同步误差小于5μs(1σ)。     

基于伪随机码的低压电力线信道时延测量技术研究

提出了一种基于伪随机码的低压电力线信道时延测量方法,是低压电力线授时的关键技术,利用现有的低压电力线网络,无需搭建额外的通信线路,不受外界物体遮蔽或天气影响。根据双向时间同步原理:服务器首先与标准时间同步,使用BPSK调制方式将1号C/A码经低压电力线传输至客户端,客户端对1号C/A码信号解调捕获后回传服务器2号C/A码,服务器根据信号发射时刻和接收时刻时间差计算出信道传输时延。实验表明:该方法可以用于低压电力线信道的时延测量,在80 m电力线距离内,时间同步误差优于100μs。     

低频时间信号周期修正项的扣除和时延、场强的色散修正

本文从理论上说明用定时接收机接收低频时间信号进行台站时间同步时周期修正项是30.18(≈30.2)μs而不是30μs;说明在—定的条件下脉冲色散对信号时延和场强测量的影响。导出定时信号色散影响不能忽略时场强和时延的修正公式。     

直播卫星(714MHz)信号至两地面站传播时延差的测定

位于东经99°赤道上空的直播电视卫星(714MHz)是静止轨道的同步卫星,陕西天文台和北京天文台合作,同时接收卫星的电视信号,提取规定的某行同步脉冲,记录本地原子钟的秒信号与该脉冲的时刻差,并且利用Loran-C长波定时信号使两天文台的原子钟同步,对卫星比对结果作钟差修正,从而获得卫星信号至两地面站的传播时延差。初步结果表明:卫星信号至陕台、北台的时延差变化范围是60μs,用二次曲线分段拟合,标准偏差(10)在1μs左右。这结果对于卫星定位、研究静止卫星轨道的运动及初步的时刻同步是有意义的。     

双向不对等几何路径对卫星双向时间频率传递的影响

分析了在卫星双向时间频率传递中,由地面站间钟差和卫星运动引起的双向几何路径不对等导致的双向几何路径时延差对双向时间比对计算结果的影响。选取了3颗卫星（中卫1号、北斗3G、IGSO70）和3组地面站（北京-成都、北京-喀什、北京-三亚）组成的9条卫星双向时间频率传递链路作仿真计算。对于这9条链路,仿真结果显示：1）当两地面站间钟差在1μs～10 ms范围内时,通过GEO卫星比通过IGSO卫星的双向不对等几何路径时延之差对双向时间比对计算结果的影响（τ值）较小;2）假设地面站间钟差在1 ms内时,通过 GEO卫星的卫星双向时间比对链路所对应的τ值均在皮秒量级,一般可忽略;通过 IGSO 卫星的卫星双向时间比对链路所对应的τ值均在纳秒量级,一般不可忽略。     

荧光屏电视广播卫星的初步定位及卫星时刻同步

利用三个地面站同时接收卫星电视信号,分离出Line—319同步脉冲,并记录本地钟与分离脉冲的时刻差。在三个地面站时钟同步的情况下,可得卫星信号至各地面站的传播时延差;在假定地心距为常数的情况下,解算出卫星的位置。确定卫星经纬度,国内任何可接收卫星信号的地方,通过卫星与陕西天文台标准时刻同步的均方误差小于±3.5μs     

非坐标方法确定卫星时间信号的传递时延

在卫星电视授时系统方案中，为确定标准秒信号从发射站经卫星到用户接收站的传播时延值，需要知道收、发地面站及卫星的坐标，所以，在高精度的卫星电视授时系统中，需要建立高精度的同步卫星定位同。本文提出在没有卫星定位同的条件下，用非坐标方法来确定卫星时间信号到用户的传递时延，其授时精度为几μｓ。     

非坐标方法确定卫星时间信号的传递时延

在卫星电视授时系统方案中，为确定标准秒信号从发射站经卫星到用户接收站的传播时延值，需要知道收，发地面站及卫星的坐标，所以，在高精度的卫星电视授时系统中，需要建立高精度的同步卫星定位网。本文提出在没有卫星定位网的条件下，用非坐标方法来确定卫星时间信号到用户的传递时延，其授时精度为几μｓ。     

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.     

A Robotics Perspective On Human Spaceflight

There has traditionally been a dichotomy in the space community regarding the efficacy of human versus robotic exploration of space. I argue that no such dichotomy is necessary, and that there is a natural and synergistic division of labour between man and machine, and that this division of labour will evolve in symbiotic fashion. The present state-of-the-art robotics technology is insufficient to replace the human in space, but is sufficient to act as a useful, even necessary, tool in aiding the astronaut in the conduct of useful work. I further argue that as robotics technology advances, the human will be further relieved to perform tasks best suited to human decision-making and flexibility that is unlikely in the near-term to be matched by autonomous or teleoperated machines. This revised version was published online in July 2006 with corrections to the Cover Date.     

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).      

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.      

CO J=2-1 and J=3-2 Line Observations of Molecular Clouds toward the Directions of 59 EGOs in the Northern Sky

In order to investigate the differences between the molecular clouds which are associated with the massive star forming regions and those which are not, we have performed the single-dish simultaneous observations of ¹²CO J=2-1 and J=3-2 lines toward a sample of 59 Spitzer Extended Green Objects (EGOs) as the massive star forming regions in the northern sky. Combining our results with the data of the ¹²CO J=1-0 observations toward the same sample EGOs in the literature, we have made the statistical comparisons on the intensities and linewidths of multiple ¹²CO lines between the molecular clouds associated with EGOs (EGO molecular clouds, in brief) and other non-EGO molecular clouds. On this basis, we have discussed the effects of the gas temperature, density, and velocity field distributions on the statistical characteristics of the two kinds of molecular clouds. It is found that both the EGO molecular clouds and non-EGO molecular clouds have similar mass ranges, hence we conclude that for the formation of massive stars, the key-important factor is probably not the total mass of a giant molecular cloud (GMC), but the volume filling factor of the molecular clumps in the GMC (or the compression extent of the molecular gas in the cloud).     

Orbital change experiments with a Mars general circulation model

We use a Mars general circulation model to examine the effect of orbital changes on the planet’s general circulation and climate system. Experiments are performed for obliquities ranging from 0° to 60° for two different longitudes of perihelion. Each experiment simulates a full Mars year assuming a fixed atmospheric dust distribution and fixed amount of CO₂ in the atmosphere/cap system. We find that global mean surface temperatures and pressures decline with increasing obliquity due to the increasing extent of the winter polar caps. The seasonal CO₂ cycle and intensity of the solstice circulation amplify considerably with increasing obliquity such that global dust storms are likely at both solstices. The most significant feature of the high obliquity solstice circulations is the development of an intense low-level jet associated with the return branch of the Hadley circulation.Model surface stresses are used to map regions of preferred dust lifting, which are defined in terms of an annual deflation potential. For the present obliquity, the model-predicted regions of high deflation potential are in good agreement with Cantor et al.’s (2001, J. Geophys. Res.106, 23653-23688) observations, which gives us some confidence in the model’s ability to predict where lifting might occur when Mars’ orbit parameters are different than they are today. In general we find that the dust lifting potential increases sharply with obliquity and is greatest at times of high obliquity when perihelion coincides with northern summer solstice. Over an obliquity cycle, the model global annual deflation potential ranges from several tenths of a millimeter at 0° obliquity to almost 15 mm at 60° obliquity. Much higher values are possible when the atmosphere is very dusty.We find a strong correlation between the deflation potential and surface thermal inertia: regions of high deflation potential correspond to regions of high thermal inertia (high rock abundance), and regions of low deflation potential correspond to regions of low thermal inertia (high dust/sand abundance). Furthermore, while the regions of preferred lifting (high deflation potential) expand somewhat with increasing obliquity and dust loading, the central parts of Tharsis, Arabia, and Elysium show no tendency for significant lifting at any obliquity or longitude of perihelion. These regions may therefore be very old and represent net long-term sinks for atmospheric dust. It is the topography of the planet, through its influence on surface pressure and wind systems, which ultimately determines where dust accumulates.Finally, as was found by Fenton and Richardson (2001, J. Geophys. Res.106, 32885-32909), we find no tendency for the development of east-southeasterly winds at the Pathfinder site for any of our orbital change experiments. This suggests that the ancient wind regime discussed by Greeley et al. (2000, J. Geophys. Res.105, 1829-1840) was produced by other factors, such as polar wander.     

白矮星演化与宇宙纪年学

近几年白矮星研究在理论与观测方面都取得了很大进展，这推动了白矮星宇宙纪年学的应用和发展。白矮星宇宙纪年学，即利用白矮星的演化结果计算白矮星理论光度函数，并通过与现测光度函数的对比来确定天体年龄。简要描述了白矮星的基本性质，回顾了白矮星演化研究的历史和基本方法；介绍了白矮星宇宙纪年学研究的基本原理及其在银盘、球状星团、疏散星团以及银晕年龄确定方面的应用现状。白矮星宇宙纪年学是一个很新的研究领域，但它已经在确定天体年龄方面显示出很大的潜力。最后讨论了目前研究中存在的问题，并提出今后需要进一步研究的几项工作。