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81.
Onur Erturk Orhan Arikan Feza Arikan 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
Electron density distribution is the major determining parameter of the ionosphere. Computerized Ionospheric Tomography (CIT) is a method to reconstruct ionospheric electron density image by computing Total Electron Content (TEC) values from the recorded Global Positioning Satellite System (GPS) signals. Due to the multi-scale variability of the ionosphere and inherent biases and errors in the computation of TEC, CIT constitutes an underdetermined ill-posed inverse problem. In this study, a novel Singular Value Decomposition (SVD) based CIT reconstruction technique is proposed for the imaging of electron density in both space (latitude, longitude, altitude) and time. The underlying model is obtained from International Reference Ionosphere (IRI) and the necessary measurements are obtained from earth based and satellite based GPS recordings. Based on the IRI-2007 model, a basis is formed by SVD for the required location and the time of interest. Selecting the first few basis vectors corresponding to the most significant singular values, the 3-D CIT is formulated as a weighted least squares estimation problem of the basis coefficients. By providing significant regularization to the tomographic inversion problem with limited projections, the proposed technique provides robust and reliable 3-D reconstructions of ionospheric electron density. 相似文献
82.
T. Sindelarova D. BuresovaJ. Chum F. Hruska 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
Ionospheric effects of meteorological origin observed by the continuous HF Doppler sounder over the Czech Republic are reported in this paper. We focused on detection of waves of periods 1–10 min. We discuss the influence of dynamics and intensity of active weather systems on the occurrence of short period waves and dependence of the observed ionospheric effects on the height of reflection of the sounding radio wave. We observed 3–5 min waves during a severe weather event in summer and 2.5–4 min waves during a severe weather event in winter. We excluded possible geomagnetic origin of these oscillations by the analysis of fluctuations of the local geomagnetic field. In eight cases of 10, wave activity in the analysed period range was not significantly increased comparing to quiet days. The intensity of weather systems as well as the location of potential sources of waves towards the points of HF Doppler shift observation influence significantly the occurrence of infrasonic waves in the ionosphere. The results in Central Europe differ considerably from those previously obtained in North America. As a possible reason, we discuss different intensity and dynamics of weather systems in both regions. 相似文献
83.
P.I.Y. Velinov P.T. Tonev 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,42(9):1569-1575
Conduction and displacement currents, and their sum the Maxwell current, generated over a thunderstorm (TS) with recurrent lightning discharges are investigated theoretically. The aim is to study better the influence of different factors on these currents, which form the link between thunderstorms and the ionosphere in the global atmospheric electrical circuit. The factors studied concern the thunderstorm characteristics (the charge separation current, and the lightning discharge parameters), as well as the atmospheric and cloud conductivity. Some of these factors may show long-term changes with the 11-year solar cycle, possibly realized through an inverse dependence of the cosmic ray flux on solar activity. Earlier investigations have suggested that the lightning-related charge redistribution and subsequent relaxation, rather than the high intensity current, is mainly the source of the energy coupled to the ionosphere. With respect to this, a quasi-electrostatic analytical model is proposed, based on Maxwell’s equations. The currents are generated by a TS modeled as a positive vertical dipole with charges which are first accumulated and then destroyed by lightning. Our computations show that the mean and peak values of the conduction and total Maxwell currents to the ionosphere depend significantly on the charge moment change. The mean currents are also sensitive to the reduction of the conductivity in thunderclouds. Small variations of the stratospheric conductivity (20% at geomagnetic latitude 40° and 40–50% at 55°) with the solar activity do not influence the currents to the ionosphere very much. 相似文献
84.
利用不同时刻系统测量噪声具有独立同分布的特点,对脉冲响应进行相关处理,可以提高信噪比。为解决传统的基于脉冲响应的模态辨识算法的辨识精度问题,用某点脉冲响应同参考点响应之间的相关函数代替该点的脉冲响应以形成基于脉冲响应相关函数的模态辨识算法,并从理论上证明脉冲响应相关函数算法的辨识精度优于传统的脉冲响应算法。通过对一个四自由度系统模态频率和阻尼比的仿真辨识,验证了脉冲响应相关函数辨识算法的有效性。 相似文献
85.
高晓阳 《中国空间科学技术》2006,26(3):67-71
回顾世界无线电通信大会对卫星广播业务规划修改的历程,并以国际电联的相关建议为基础,通过对实例的分析、比较,认为空间业务极化方式的选择应综合考虑载波的传输特性、空间业务的继承性、《无线电规则》的相关条款、是否有利于网络的兼容性以及易于用户接收设备的安装等因素。 相似文献
86.
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88.
Shaolin Zhu Dongjie Yue Lina He Jian Chen Zhiqiang Liu 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2021,67(12):3994-4010
Single-frequency precise point positioning (SF-PPP) has attracted increasing attention due to its high precision and cost effectiveness. With various strategies to handle the dominant error, i.e., ionosphere delay, the ionosphere-float (IF), ionosphere-free-half (IFH), ionosphere-corrected (IC), and ionosphere-weighted (IW) SF-PPP models are certain to possess different characteristics and performance levels. This study is dedicated to assessing and comparing the four models from model characteristics, positioning performance, and atmosphere delay retrieval. The model comparison shows that IC and IW models are full-rank while IF and IFH models have a rank deficiency of size one that will result in biased estimations, which means the better solvability of IC and IW models. The experiments are carried out based on the 7-day Global Positioning System (GPS) observations collected at 57 global Multi-GNSS Experiment (MGEX) stations and Global Ionosphere Map (GIM) products. The results indicate that the IW model can accelerate SF-PPP convergence and achieve higher positioning accuracy compared to the other three SF-PPP models, especially in kinematic mode. With convergence criteria of 0.25 m in horizontal and 0.5 m in vertical, the east/north/up convergence times of IW model are 0.5/15.0/25.0 min and 0.5/16.0/36.5 min for static and kinematic modes, respectively. The IW model is able to achieve an instantaneous positioning accuracy of 0.28/0.35/0.75 m. In addition, a real kinematic test also demonstrates the best positioning solutions of IW model. Regarding troposphere delay retrieval, the IF, IFH, and IW models obtain a comparable daily accuracy of 3.0 cm on average, while the IC model achieves the worst accuracy of 8.0 cm. For precise ionosphere delay estimation, IW model only needs an average initialization time of 34.3 min, but a longer initialization time of 51.6 min is required for IF model. The daily precision of ionosphere delay estimation for IW model can reach up to 10.8 cm. At the present accuracy of GIM products, it is suggested that the IW model should be adopted for SF-PPP first due to its superior performance in positioning and atmosphere delay retrieval. 相似文献
89.
本文利用西藏、广州地区1985—1988年夏季的7、8月份的实测资料,对电离层特征参数,f_oE,f_oE_s,f_bE_s,进行了统计分析,得到它们的变化规律与特点。 从图1中看出,西藏高原地区的f_oE值,其变化规律,受太阳黑子的影响较大且为正 相似文献
90.
IRI-90及其与中国电离层资料f_0F_2的比较 总被引:1,自引:0,他引:1
本文介绍了国际参考电离层IRI-90版中新增的内容,比较了它与IRI-86的主要差别。与我国电离层临频数据的统计分析表明,新版本对我国地区的适用程度有了改进。 相似文献