共查询到20条相似文献，搜索用时 540 毫秒
1.
针对中国区域定位系统(China Area Positioning System,CAPS)接收机采用圆极化天线接收线极化信号引入极化损失的问题,提出了一种基于双线性极化天线的信号合成捕获算法.算法首先对双线性极化天线接收的两路CAPS导航信号实现捕获,并根据捕获结果进行信号同步;进而利用最大比合并合成两路信号以提高接收信号的信噪比.应用于实测数据的分析表明,合成信号的信噪比相比单路信号信噪比有1.5 dB左右的改善. 相似文献
2.
3.
本中，采用多元线性逐步回归分析方法进行了接收参数－－信噪比（SN）、仰角（EL）、方位角（AZM）、电离层（ION）对GPS时间比对精度（在数据跟踪持续时间15分钟内取样平均时间为10秒的时间起伏均方差）影响的一些比较分析，结果表明：信噪比这个参数在多数情况中是影响GPS时间比对精度的主要因素。 相似文献
4.
为有效统计射电天文台址常驻电磁干扰的变化,提高实时电磁环境监测效率,提出一种基于先验信息的常驻电磁干扰信号检测与识别方法.首先通过信噪分离,信号能量估值从电磁环境历史监测数据中提取移动通信,地面数字电视等常驻大信号的中心频率、极化方式、来波方向等特征,经对无线信号传播信道的衰落方式进行分析,提出信号能量模型服从的分布函数假设并采用K-S检验验证该假设,合理设置能量阈值,建立信号模板库.其次,根据信号模板库中的信号特征对实时频谱数据进行双门限能量检测、信号相关性识别,从而提高信号检验的准确率,实现了常驻信号的快速统计.针对射电天文台址实测频谱,运用该方法能够有效识别并统计频谱中的常驻信号,从而为台址干扰缓解策略制定提供重要依据. 相似文献
5.
6.
利用GPS系统传递时间信息,受到各国时频工作者的关注,中国北京无线电计量测试研究昕(BIRM)和中国科学院武汉测量与地球物理研究所(WTO)等单位于1987年7月12至18目在北京、西安、临潼三地同时进行中国首次GPS共视法时间传递与同步试验,得到接收点均方根时间起伏值优于3.5ns,单站接收一个GPS卫星时间传递与同步误差可达5～16ns,两站共视接收GPS信号的时间传递与同步误差可以减小5ns左右,利用搬运钟验证,站间时间同步准确度可优于50ns。 相似文献
7.
8.
对全球定位系统(Global Positioning System,GPS)信号捕获中的延时相乘捕获算法的检测率进行了仿真分析.延时相乘捕获算法消除了多普勒频偏的影响,适用于高动态环境下信号的捕获.但算法的缺点是延时相乘操作增加了噪声量,从而降低了信噪比,影响检测性能.针对算法的缺点,通过理论计算与蒙特卡罗方法仿真,分析了算法中检测变量的噪声分布.从仿真结果看,检测变量的噪声符合高斯分布.根据噪声分布,计算了在不同信号载噪比(C/N0)条件下延时相乘捕获算法的检测率与虚检率的关系.通过减小噪声带宽和延长积分时间,可以提高算法的检测率. 相似文献
9.
基于泊松模型的X射线脉冲星信号的最大似然TOA估计 总被引：1，自引：0，他引：1
讨论了X射线脉冲星光子到达的周期平稳Poisson模型和到达时刻(Time of arrival,TOA)估计问题.在此基础上,得到了脉冲到达时刻的最大似然估计和克拉美-罗限(Cramer-Rao boundary,CRB),并推导了低信噪比情况下的似然函数近似表达和克拉美-罗限.利用解析的脉冲轮廓,对PSR B1821-24的TOA估计进行了Monte Carlo仿真,讨论了不同观测时间和信噪比下的估计误差,给出了相应的信噪比门限.研究表明:该分析方法能够有效估计X射线脉冲星的TOA定时精度,有利于评估其在各种应用中的性能. 相似文献
10.
11.
Laura Schaefer 《Icarus》2004,169(1):216-241
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 ONaKFeSiMgCaAlTi 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. 相似文献
12.
13.
14.
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. 相似文献
15.
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. 相似文献
16.
Marcelo Samuel Berman 《Astrophysics and Space Science》2008,314(1-3):79-82
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).
相似文献
17.
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.
相似文献
18.
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). 相似文献
19.
白矮星演化与宇宙纪年学 总被引：2，自引：0，他引：2
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。 相似文献
20.
CHENG Zheng ZHANG Cheng-min ZHAO Yong-heng WANG De-hua PAN Yuan-yue LEI Ya-juan 《Chinese Astronomy and Astrophysics》2014
By reviewing the methods of mass measurements of neutron stars in four different kinds of systems, i.e., the high-mass X-ray binaries (HMXBs), low-mass X-ray binaries (LMXBs), double neutron star systems (DNSs) and neutron star-white dwarf (NS-WD) binary systems, we have collected the orbital parameters of 40 systems. By using the boot-strap method and the Monte-Carlo method, we have rebuilt the likelihood probability curves of the measured masses of 46 neutron stars. The statistical analysis of the simulation results shows that the masses of neutron stars in the X-ray neutron star systems and those in the radio pulsar systems exhibit different distributions. Besides, the Bayes statistics of these four different kind systems yields the most-probable probability density distributions of these four kind systems to be (1.340 ± 0.230)M8, (1, 505 ± 0.125)M8,(1.335 ± 0.055)M8 and (1.495 ± 0.225)M8, respectively. It is noteworthy that the masses of neutron stars in the HMXB and DNS systems are smaller than those in the other two kind systems by approximately 0.16M8. This result is consistent with the theoretical model of the pulsar to be accelerated to the millisecond order of magnitude via accretion of approximately 0.2M8. If the HMXBs and LMXBs are respectively taken to be the precursors of the BNS and NS-WD systems, then the influence of the accretion effect on the masses of neutron stars in the HMXB systems should be exceedingly small. Their mass distributions should be very close to the initial one during the formation of neutron stars. As for the LMXB and NS-WD systems, they should have already under- gone the process of suffcient accretion, hence there arises rather large deviation from the initial mass distribution. 相似文献