COMPASS time synchronization and dissemination—Toward centimetre positioning accuracy

In this paper we investigate methods to achieve highly accurate time synchronization among the satellites of the COMPASS global navigation satellite system (GNSS). Owing to the special design of COMPASS which implements several geo-stationary satellites (GEO), time synchronization can be highly accurate via microwave links between ground stations to the GEO satellites. Serving as space-borne relay stations, the GEO satellites can further disseminate time and frequency signals to other satellites such as the inclined geo-synchronous (IGSO) and mid-earth orbit (MEO) satellites within the system. It is shown that, because of the accuracy in clock synchronization, the theoretical accuracy of COMPASS positioning and navigation will surpass that of the GPS. In addition, the COMPASS system can function with its entire positioning, navigation, and time-dissemination services even without the ground link, thus making it much more robust and secure. We further show that time dissemination using the COMPASS-GEO satellites to earth-fixed stations can achieve very high accuracy, to reach 100 ps in time dissemination and 3 cm in positioning accuracy, respectively. In this paper, we also analyze two feasible synchronization plans. All special and general relativistic effects related to COMPASS clocks frequency and time shifts are given. We conclude that COMPASS can reach centimeter-level positioning accuracy and discuss potential applications.     

Compaction of icy/rocky porous mixtures at low temperatures (from 262K TO 77K) - preliminary results

Abstract

Rheology of icy/rocky porous mixtures relevant to material of icy satellites of giant planets was studied experimentally. The lower limit of temperature was that of liquid nitrogen and the pressure was up to 17.7 MPa. The compaction rate was studied and empirical formula was fitted. The dependence of formula parameters upon pressure, temperature and rock concentration was established.     

ROSETTA and FIRST: Two cornerstone missions of ESA long-term space science programme

Summary The paper reviews the objectives and status of definition of ROSETTA (Comet Nucleus Sample Return) and FIRST (Far Infrared Space Telescope), which are two cornerstone missions of the ESA long-term scientific programme. ROSETTA is a joint ESA/NASA programme and it will address problems centered on the origin of planetary material and the pre-biotic evolution of organic matter. The ROSETTA spacecraft will be launched in 2001; it will land into the nucleus of comet Churyumov-Gerasimenko, take about 10 kg of cometary samples (ice, dust, organics) and will return the Earth in 2008. FIRST will explore the 10 μm to 1 mm region of the electromagnetic spectrum, where a large number of very important atomic and molecular spectral lines are hosted. The major scientific objectives are the physics of the interstellar medium of the star formation and cosmological studies. The mission is based on a 4.5 to 8 m telescope equipped with imaging spectrometers and operating as a near-real-time observatory from a high elliptical orbit of 24 h period. Low operating temperatures for the payload will be achieved by using a combination of mechanical coolers and a liquid helium cryostat. To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee.     

13C and19F NMR study of α,β-difluorostyryl derivatives of transition metal carbonylates. A method of signal assignment based on the carbon—fluorine spin-spin coupling constants

The¹³C and¹⁹F NMR spectra ofZ- andE-isomers of β-X-substituted α,β-difluorostyrenes (X=F, Cl, CpFe(CO)₂, Re(CO)₅, Re₂(CO)₉Na) were studied. Direct and long-range (across 1–5 bonds) spin-spin coupling constants and the (¹³C−¹²C) isotope shifts in the¹⁹F NMR spectra were determined. The study of the¹³C satellites in the¹⁹F NMR spectra of substituted difluorostyrenes permitted assignment of the¹³C NMR signals of the vinylic carbon atoms. Similarly, the signals in¹⁹F NMR spectra were assigned based on coupling constants of fluorine withipso-carbon. These assignments were found to be in good agreement with the data available from the literature (X=F, Cl). The developed approach was applied to the elucidation of the structure ofZ−PhCF=CClFe(CO)₂Cp. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya. No. 8, pp. 1575–1579, August, 1998.     

Kα X-ray satellite spectra of Ti, V, Cr and Mn induced by photons

K X-ray emission spectra of Ti, V, Cr and Mn generated by photon excitation have been studied with a crystal spectrometer. The measured energy shifts of K_α satellite relative to the diagram line are compared with values obtained by electron excitation and with different theoretical estimates. The present experimental values of K_αL¹/K_αL⁰ relative intensities are compared with values obtained by electron excitation.      

中纬度地区电离层偶发E层对量子卫星通信性能的影响

电离层偶发E层是指在距离地面高度80—150 km之间,在风剪切作用下,电子密度急剧增加的不规则电离薄层,它会对量子卫星光信号的传输造成极大的影响.然而,有关电离层偶发E层与星地间量子通信信道参数关系的研究,迄今尚未展开.为了研究偶发E层对量子卫星通信性能的影响,首先分析了它的形成过程,得出自由电子密度随高度变化的关系;然后建立了自由电子密度、偶发E层的厚度对量子卫星链路衰减的模型;针对振幅阻尼信道,给出自由电子密度对信道容量、纠缠保真度、误码率和安全密钥产生率的定量关系.理论分析和仿真结果表明,当偶发E层的厚度为1 km、电子密度由3×10~5cm~(-1)增加到27×10~5cm~(-1)时,信道容量由0.8304衰减到0.1319,纠缠保真度由0.9386下降到0.3606,量子误码率由0.0093增加到0.0769,安全密钥产生率由9.968×10~(-5)减小到1.91×10~(-6).由此可见,电子密度的大小和偶发E层的厚度对量子卫星通信性能有显著的影响.因此,在进行量子卫星通信时,应根据对电离层参数的探测情况,自适应调整卫星系统的各项指标,以确保量子通信的可靠性.     

肖特基二极管整流的计算与测量

 提出了改进型的肖特基二极管整流数理模型,用加速迭代法和四阶精度龙格-库塔法编制了计算程序,并结合实验测量得到了整流效率与输入功率、频率和负载等关系曲线:负载一定时,输入功率从零开始增大,整流效率先快速上升,然后上升趋势变缓;输入功率一定时,负载从零开始增大,整流效率先增大后减小,对于某一固定的输入功率,存在着一个最佳负载值;当输入功率和负载都相同时,降低工作频率,整流效率上升。型号为2DV10B的X波段管子在负载为525 W、输入功率为10 mW、频率为3.2 GHz时获得的整流效率为75.2%;频率为10 GHz时获得50.2%的效率,实验测量结果与理论分析一致。     

Ion-acoustic waves at the night side of Titan's ionosphere: higher-order approximation

Nonlinear solitary waves are investigated for a plasma system at the night side of Titan's ionosphere. The plasma model consists of three positive ions, namely C_2H_5~+, HCNH~+, and C_3H_5~+, as well as Maxwellian electrons. The basic set of fluid equations is reduced to a Korteweg de-Vries(KdV) equation and linear inhomogeneous higher order KdV(LIHO-KdV) equation.The solitary wave solutions of both equations are obtained using a renormalization method. The solitary waves' existence region and the wave profile are investigated, and their dependences on the plasma parameters at the night side of Titan's ionosphere are examined. The solitary waves' phase velocities are subsonic or supersonic, and the propagating pulses are usually positive. The effect of higher-order corrections on the perturbation theory is investigated. It is found that the higher-order contribution makes the amplitude slightly taller, which is suitable for describing the solitary waves when the amplitude augments.     

准分子宽带泵浦碱金属激光器

基于国内外相关研究成果,对准分子宽带泵浦碱金属激光器(XPAL)的原理进行了阐述,对其优缺点进行了分析,针对半导体泵浦碱金属蒸气激光器(DPAL)发展中所面临的困难,分析了XPAL解决这些困难的潜力。最后综述了XPAL近年的发展,提出了下一步的发展方向。     

Evaluation of DNA bending models in their capacity to predict electrophoretic migration anomalies of satellite DNA sequences

DNA containing a sequence that generates a local curvature exhibits a pronounced retardation in electrophoretic mobility. Various theoretical models have been proposed to explain relationship between DNA structural features and migration anomaly. Here, we studied the capacity of 15 static wedge‐bending models to predict electrophoretic behavior of 69 satellite monomers derived from four divergent families. All monomers exhibited retarded mobility in PAGE corresponding to retardation factors ranging 1.02–1.54. The curvature varied both within and across the groups and correlated with the number, position, and lengths of A‐tracts. Two dinucleotide models provided strong correlation between gel mobility and curvature prediction; two trinucleotide models were satisfactory while remaining dinucleotide models provided intermediate results with reliable prediction for subsets of sequences only. In some cases, similarly shaped molecules exhibited relatively large differences in mobility and vice versa. Generally less accurate predictions were obtained in groups containing less homogeneous sequences possessing distinct structural features. In conclusion, relatively universal theoretical models were identified suitable for the analysis of natural sequences known to harbor relatively moderate curvature. These models could be potentially applied to genome wide studies. However, in silico predictions should be viewed in context of experimental measurement of intrinsic DNA curvature.