遗传算法优化的BP神经网络卫星钟差预报

针对BP (Back Propagation)神经网络模型预测卫星钟差中权值和阈值的最优化问题, 提出了基于遗传算法优化的BP神经网络卫星钟差短期预报模型, 给出了遗传算法优化BP神经网络的基本思想、具体方法和实施步骤. 为验证该优化模型的有效性和可行性, 利用北斗卫星导航系统(BeiDou navigation satellite system, BDS)卫星钟差数据进行钟差预报精度分析, 并将其与灰色模型(GM(1,1))和BP神经网络模型预报的结果比较分析. 结果表明: 该模型在短期钟差预报中具有较好的精度, 优于GM(1,1)模型和BP神经网络模型.     

泛函网络在导航卫星钟差中长期预报中的应用

为了更好地反映导航卫星钟差特性以及提高导航卫星钟差中长期预报精度,在对卫星原子钟差预报建模时,除考虑卫星原子钟频移、频漂和频漂率等物理性质外,还考虑了卫星钟差的周期性变化和随机性等特点.在传统多项式预报模型基础上,采用泛函网络对卫星钟差的周期项和随机项部分进行建模,利用GPS导航卫星钟差数据进行预报实验,并与传统的多项式模型、灰色系统模型、差分自回归滑动平均(ARIMA)模型以及Kalman滤波方法的预报结果进行比对,结果表明,基于泛函网络建立的混合预报模型能有效减小导航卫星钟差的中长期预报误差.     

基于Takagi-Sugeno模糊神经网络模型的卫星钟差预报方法

卫星钟差预报精度的不断提升是精密导航的关键问题.为了进一步提高钟差的预报精度和更好地反映钟差的变化特性,提出一种基于Takagi-Sugeno模糊神经网络(Fuzzy Neural Network,FNN)的钟差预报方法.该方法首先根据钟差数据的特点对钟差进行预处理,然后以预处理后的数据建立一种高精度预报钟差的Takagi-Sugeno模糊神经网络算法.采用IGS(International Global Navigation Satellite System Service)不同采样间隔的精密钟差数据进行了短期预报试验,并与ARIMA(Auto-Regressive Integrated Moving Average)模型、GM(1,1)模型及QP(Quadratic Polynomial)模型进行了对比试验,分析结果表明:对不同类型原子钟,该方法用于钟差短期预报是可行的、有效的,其获得的卫星钟差预报结果明显优于常规方法.     

基于Vondrák滤波1阶差分灰色模型的卫星钟差预报方法

为了提高卫星钟差预报的精度,提出基于Vondrák滤波1阶差分的灰色模型算法.首先,对原始钟差数据序列采用Vondrák滤波处理后,得到1组新的钟差数据序列.然后,对相邻历元的钟差数据序列作1阶差分处理.最后,基于Vondrák滤波后的1阶差分的钟差差值数据序列,建立了GPS钟差预报的灰色模型.此外,采用了IGS(International Global Navigation Satellite System Service)公布的精密卫星钟差数据进行了预报试验,通过与卫星钟差预报中常用的2次多项式模型和直接采用原始钟差数据建立灰色预报模型预报结果的对比分析,结果表明:该方法可以对GPS卫星钟差进行高精度的中短期预报.在12 h、16 h、20 h和24 h的平均预报精度分别为0.50 ns、0.85 ns、1.08 ns和1.27 ns,相比于2次多项式模型的平均预报精度分别提高了24.24%、15.84%、12.90%和11.81%;相比于直接采用原始钟差数据建立灰色预报模型的平均预报精度分别提高了56.14%、49.40%、48.82%和47.80%.     

附有周期项的预报模型及其在GPS卫星钟差预报中的应用研究

为了有效进行GPS卫星钟差预报和更好地反映卫星钟差特性,除了考虑卫星原子钟频移、频漂和频漂率等物理性质外,还应考虑到卫星钟差的周期性变化特点.在二次多项式模型基础上,增加了周期项因素,构造了新的预报模型.选取部分GPS卫星铯钟(Cs.clock)和铷钟(Rb.clock)钟差资料,根据钟差变化趋势分3种情况,按不同时间长度进行钟差预报分析,并与二次多项式模型的预报结果比较分析,大量数据分析表明:附有周期项的二次多项式模型预报精度优于二次多项式模型,铷钟预报精度略优于铯钟.     

灰色模型修正及其在实时GPS卫星钟差预报中的应用研究

在GPS实时精密单点定位中,卫星钟差的实时可靠预报是实现GPS实时高精度单点定位的关键之一.星载GPS原子钟频率高,非常敏感,极易受到外界及其本身因素的影响,从而很难掌握其复杂细致的变化规律,这些属性符合灰色系统理论的特点.因此,考虑将钟差的变化过程看作一个灰色系统.在探讨二次多项式和灰色模型卫星钟差预报局限性基础上,提出了利用改进的灰色模型实时预报GPS卫星钟差的研究,最后利用3个不同时段的GPS卫星钟差资料进行不同采样间隔钟差预报精度分析、灰色模型指数系数与预报精度的关系、与二次多项式预报精度比较分析,总结不同卫星钟差类型与模型指数系数的一般关系,并与IGS最终钟差星历产品比较,验证本文提出的改进预报模型的可行性和有效性.为实时GPS动态精密单点定位提供较高精度的卫星钟差产品.     

卫星钟差预报模型中周期项的选取方法及性能分析

针对卫星钟差预报(SCB)中周期项选取方法存在的问题,在分析卫星钟周期波动特性的基础上,给出了正确的周期项选取方法,并与现有方法进行了比较.利用IGS(International GNSS Service)的卫星钟差数据,比较分析了二次多项式加周期项模型与传统模型的预报精度.从理论上分析了周期项对传统模型的改善程度及适用条件.结果表明:按照提出方法得到的周期项更符合实际,将其应用于钟差预报时能获得更高的预报精度,大量仿真实验还表明卫星钟周期性波动相对较大时周期项对传统模型有明显改善.     

BDS/GPS星载原子钟的短期钟差预报模型研究

针对BDS(BeiDou Navigation Satellite System)/GPS(Global Positioning System)星载原子钟特性和卫星钟差预报模型研究中存在的若干问题,在介绍4种单一模型(多项式模型(QR)、灰色模型(GM)、时间序列模型(ARMA)和广义回归神经网络模型(GRNN))的基础上,引入了经典权组合模型(CM)和修正经典权组合模型(Modified CM).利用武汉大学卫星导航定位技术研究中心的卫星精密钟差产品对BDS/GPS星载原子钟的短期钟差预报模型进行研究,并对比了不同卫星钟和不同模型的预报效果.试验结果表明:单一模型对于BDS卫星钟(C04(GEO Rb)、C07(IGSO Rb)、C14(MEO Rb))的钟差预报精度跳跃性很大;而对于GPS卫星钟(G04(Block IIA Rb)、G09(Block IIA Cs)、G16(Block IIR Rb)、G31(Block II-M Rb)、G27(Block IIF Rb)、G24(Block IIF Cs))的预报精度变化比较平稳;同一种预报模型应用在不同类型的卫星钟序列中,预报精度差异较大.然而,修正经典权组合模型在保证预报可靠性的前提下提高了预报精度,在一定程度上优于其他模型.     

基于修正线性组合模型的原子钟钟差预报

原子钟钟差预报在时频工作中起着重要作用.目前常用的预报模型各有优缺点,为了综合各种预报模型特点,可以考虑组合这些模型预报的结果.针对线性组合模型预报钟差时存在的问题,提出了学习权的概念,以有效地利用各种精度信息,建立了修正组合预报模型.为了验证该方法的有效性,选取了4颗GPS卫星IGS(International GN...     

基于ARMA模型的导航卫星钟差长期预报

卫星钟差长期可靠预报是实现卫星自主导航定轨所要解决的重要前提之一.针对多项式模型(PM)、灰色模型(GM)等常用的钟差预报方法存在的预报误差较大的情况,为了有效地进行卫星钟差预报和更好地反映卫星钟差变化特性,将ARMA(Auto-Regressive Moving Average)模型引入到卫星钟差预报中,利用IGS(International GNSS Service)提供的卫星钟差观测数据进行90 d的长期预报,根据各个卫星钟差的变化特性,对其进行模式识别、建模和预报,并与其它3种模型进行了较为细致的比较.计算结果表明,采用ARMA模型可以有效地提高卫星钟差的长期预报精度.     

Analysis of pulsations

There is no universal method of data analysis. The scientist must first know what he intends to measure; then, it is possible to select the most suitable methods and give practical applications. We limit our investigation here to three domains.

In the first one, estimation of the parameters, the number of degrees of freedom is estimated, taking into account the a priori knowledge in magnetic pulsation models. The random and deterministic parts of the signal are evaluated and the meaning of the confidence interval of the measurements is discussed. The use and limitations of maximum entropy and autoregressive filters for analysing multicomponent signals are surveyed.

In the second one, we show that the filtering of a multicomponent signal consists of defining the sub-space containing the expected part of the signal and projecting the recorded data on this sub space. With such a representation just a few coefficients are sufficient to ensure a good determination of the physical parameters of pulsation signals.

In the last one, an example of detection in multichannel data is presented. The art of geophysics is to know what assumptions may be used to define the set of signals and to test this choice.     

Random-Lag Singular Cross-Spectrum Analysis

We compare the distribution of stars of different spectral types, and hence mean age, within the central SMC and find that the asymmetric structures are almost exclusively composed of young main-sequence stars. Because of the relative lack of older stars in these features and the extremely regular distribution of red giant and clump stars in the SMC central body, we conclude that tides alone are not responsible for the irregular appearance of the central SMC. The dominant physical mechanism in determining the current-day appearance of the SMC must be star formation triggered by a hydrodynamic interaction between gaseous components. These results extend the results of population studies (see Gardiner & Hatzidimitriou) inward in radius and also confirm the suggestion of the spheroidal nature of the central SMC based on kinematic arguments (Dopita et al.; Hardy, Suntzeff, & Azzopardi). Finally, we find no evidence in the underlying older stellar population for a "bar" or "outer arm," again supporting our classification of the central SMC as a spheroidal body with highly irregular recent star formation.     

GPS定位中的误差分析

介绍了GPS的空间部分、地面控制部分和用户部分的发展状况及其作用。并对GPS定位的基本原理做了阐述。GPS定位误差由几何精度因子和站星距离的测量误差决定,站星距离的测量误差由3大类误差因素决定,它们是与GPS卫星有关的误差、与GPS卫星信号传播有关的误差、与GPS信号接收设备有关的误差。对上述重要误差进行了系统全面的分析,并提出了具体的误差修正模型及各种减小或消除误差的方法。     

Experimental Analysis of Hydrodynamic Solitons

Nonlinear interactions between solitary waves currently observed in Plasma Physics are simulated in one and two dimensional water channels.     

Analysis of dust through extinction

Extinction is treated through the optical thickness . Some important characters of dust, such as the composition and the size of grains, are reviewed in the light of observational evidence. Extinction is investigated through three kind of parameters: (1) those affecting it directly such as the geometrical cross sections of the grains (through the radius) and the wavelength of radiation interacting with them; (2) those connected witht the solid state constitution of grains, such as the dielectric constant and the conductivity; and (3) those connected with the distribution of dust in the intergalactic medium (IGM), such as the dust density and the geometrical thickness of dust slabs. The results are discussed and compared with observations.     

太阳黑子数的子波分析

本文讨论了子波变换用于信号突变检测的原理，用它分析了１７００－１９９３年间的太阳黑子数的年均值．精确地检测到了太阳活动的突变点，用相邻两个突变点的时间长度求得了不同尺度下太阳黑子变化的周期．结果表明：利用子波变换检测太阳黑子周期与传统方法相比具有独到之处．     

Analysis of Sunspot Activity Cycles

A nonlinear analysis of the daily sunspot number for each of cycles 10 to 23 is used to indicate whether the convective turbulence is stochastic or chaotic. There is a short review of recent papers considering sunspot statistics and solar activity cycles. The differences in the three possible regimes – deterministic laminar flow, chaotic flow, and stochastic flow – are discussed. The length of data sets necessary to analyze the regimes is investigated. Chaos is described and a chronology of recent results that utilize chaos and fractals to analyze sunspot numbers follows. The parameters necessary to describe chaos – time lag, phase space, embedding dimension, local dimension, correlation dimension, and the Lyapunov exponents – are determined for the attractor for each cycle. Assuming the laminar regime is unlikely if chaos is not indicated in a cycle by the calculations, the regime must be stochastic. The sunspot numbers in each of cycles 10 to 19 indicate stochastic behavior. There is a transition from stochastic to chaotic behavior of the sunspot numbers in cycles 20, 21, 22, and 23. These changes in cycles 20 – 23 may indicate a change in the scale of turbulence in the convection zone that could result in a change in the convective heat transfer and a change in the size of the convection region for these four cycles.     

Timescale Analysis of Spectral Lags

A technique for timescale analysis of spectral lags performed directly in the time domain is developed. Simulation studies are made to compare the time domain technique with the Fourier frequency analysis for spectral time lags. The time domain technique is applied to studying rapid variabilities of X-ray binaries and γ-ray bursts. The results indicate that in comparison with the Fourier analysis the timescale analysis technique is more powerful for the study of spectral lags in rapid variabilities on short time scales and short duration flaring phenomena.     

Analysis of gamma-burst space distribution

The distribution of BATSE-events over the sky is studied by the method based on the probability theory. The short presentation of the method is given. The results might indicate the presence of some anomaly in a broad region around the Andromeda nebula.     

Multifractal Analysis Of Solar Magnetograms

As solar observational techniques improve, fine small-scale structures observed on the solar surface become more pronounced. Complex filigree structures of solar granulation, sunspots, photospheric magnetic and velocity fields cannot be described adequately by a single parameter (e.g., filling factor, fractal dimension, or power-law index). Methods which incorporate parameters that are a function of scale (multiscale methods) to describe the complexity of a field under study, should be involved. The multifractal approach offers such a possibility. In this paper the scaling of structure functions is proposed in order to analyze multifractality. Application of the approach to SOHO/MDI high-resolution magnetograms of active regions show that the structure functions differ for all active regions studied. For a given active region, the functions may maintain their shape during several hours; however, they can significantly change during a day. Flare-quiet active regions tend to possess a lower degree of multifractality than flaring active regions do. The increase in multifractality is a signal that a magnetic structure is driven to a critical state, thus gaining tangential discontinuities of various length scales.