【作者】 吴小成；

【导师】 胡雄；

【作者基本信息】 中国科学院研究生院（空间科学与应用研究中心） ， 空间物理， 2008， 博士

【摘要】 无线电掩星技术作为一种既经济又强大的探测手段,从20世纪60年代就开始应用于行星电离层和大气探测,至今仍是行星探测的一种重要手段。无线电掩星技术作为全球卫星导航定位系统应用新的增长点,也将成为21世纪最先进的空间探测技术之一。该技术可测量电离层电子密度剖面和中性大气气象场,具有覆盖面广、全天候观测、长期稳定、高精度和高垂直分辨率等优点;该技术提供的观测资料,将对空间环境监测与研究、数值天气预报和气候研究具有极大的推动作用,在空间、天文、气象以及国防领域具有广泛的应用前景。本文介绍了无线电掩星技术的发展现状和基本原理,围绕电离层掩星理论和反演算法、验证及其应用等开展了诸多研究和探讨,主要内容如下:1、介绍无线电掩星观测模拟方法,包括轨道模拟和观测数据模拟。2、系统地研究了基于电子密度分布局部球对称近似的电离层掩星反演方法,着重研究如何处理LEO轨道高度以上电离层对无线电波的影响,发展了改正TEC反演方法、利用Chapman模式和IRI模式辅助反演共三种方法来处理此问题。统计比较结果表明,改正TEC反演方法是最佳的反演方法。当非掩星侧数据无法获得时,利用Chapman模式和IRI模式辅助反演的精度优于传统的弯曲角指数外推方法和忽略顶部电离层影响的方法。3、分析电离层掩星误差源,指出电子密度分布的局部非球对称性是电离层掩星的主要误差源。通过大量模拟掩星数据反演误差的统计分析,总结出以下特点:(1)反演误差随太阳活动水平的增强而增大;(2)反演误差在冬季最大,春秋季次之,夏季最小;(3)总的来说,中纬的反演误差比低纬和高纬小;(4)不考虑季节和纬度影响,白天的反演误差比夜间和晨昏小。4、利用非相干散射雷达和垂测仪数据,对COSMIC电离层掩星数据进行比较验证。结果表明,电离层掩星电子密度剖面与非相干散射雷达探测结果符合得较好;由掩星数据得到F层峰值电子密度NmF2与垂测仪探测结果具有很好的相关性,相关系数为0.937,相对偏差的标准差为20.7%。通过分类比较发现,COSMIC掩星探测得到的NmF2与垂测仪的相对偏差随季节、地磁纬度、地方时变化的特点,与模拟掩星数据反演误差统计结果符合得很一致。5、深入分析电子密度分布非球对称性影响电离层掩星反演的机制,为电离层掩星定义了非球对称因子,研究表明非球对称因子与NmF2的相对误差具有很好的线性关系,非球对称因子被用于反演结果的修正,大大降低反演误差。6、研究三维约束的电离层掩星反演方法。采用三维经验电离层模式作为约束,应用于模拟掩星数据反演,大大降低了反演误差,对实测数据反演结果与垂测仪的比较表明,反演结果合理可靠。首次尝试用球谐函数拟合,对全球分布的掩星电子密度进行建模,再将建模结果作为约束条件用于掩星反演,结果表明,这种方法可以大幅降低反演误差,可以应用于将来的掩星星座观测数据的反演。7、利用COSMIC掩星数据分析太阳活动低年的电离层气候特征。8、对火星电离层掩星探测进行仿真研究。研究表明,中俄联合火星探测中,星-星掩星双频探测得到的火星电离层电子密度,将会达到前所未有的测量精度;星-地掩星单频探测,可以有效获得白天火星电离层电子密度剖面。更多还原

【Abstract】 The radio occultation technique had been applied successfully to explore the planetary atmosphere and ionosphere since 1960s. It is a new application of Global Navigation Satellite Systems, and one of the most advanced space exploring technique in the 21st century. This technique can observe the profiles of electron density in the ionosphere and the meteorological parameters of neutral atmosphere. It can operate all-weather with long-term stability and global coverage. The global distributed occultation data will greatly promote the space weather researches, numerical weather prediction and climate researches.This thesis focuses on the Ionospheric Radio Occultation (IRO) technique and its application. The main works can be summarized as following:1. The method for the simulation of the radio occultation is introduced, including the orbit simulation and the observation simulation.2. The inversion methods for IRO are studied, which are based on the approximation that the electron distributes symmetrically in local ionosphere. It is important to deal with the upper boundary condition in the inversion of IRO. Here three methods are proposed to do it: calibrated TEC (Total Electron Content) method, inversion method assisted with Chapman model or IRI (International Reference Ionosphere) model. The research shows that the calibrated TEC method is best. If there is no data at the non-occultation side, the inversion method assisted with Chapman model or IRI model is better than the traditional methods– bending angle extrapolation method or ignoring the contribution of the ionosphere above the LEO.3. The error sources of IRO are analyzed. The spherical symmetry approximation of electron density is the main source of the inversion error. The statistical results reveal some characters of the inversion errors. (1) The relative error increases with enhanced solar activity; (2) It is lager in winter than equinox season, and smallest in summer; (3) Generally, it is smaller at middle latitude than low and high latitude; (4) Regardless of season and geomagnetic latitude, it is smaller at daytime than other time.4. The IRO data from COSMIC is compared with measurements of ISR (Incoherent Scanter Radar) and ionosondes. The comparison shows that the electron density profiles of IRO are consistent with the ISR. The peak electron density (NmF2) of IRO is consistent with the measurement of ionosonde, and the correlation coefficient is 0.937, and the standard deviation of their relative differences is 20.7%. The differences between the NmF2 of IRO and ionosondes are studied, and their dependences on season, geomagnetic latitude and local time are consistent with the characters of the inversion errors of the simulation data.5. The effect of the asymmetry of electron density on the inversion of IRO is studied. And an asymmetry factor is defined for IRO. The study shows that the asymmetry factor is consistent with the relative error of inversed NmF2. Then the asymmetry factor is applied to correct the inversion result of IRO, and the inversion error is reduced greatly.6. The three-dimensional (3D) constrained inversion method is studied. With the help of 3D empirical ionosphere model, the method is applied in the inversion of IRO simulation data, and the inversion error is greatly reduced. Then the method is applied in the inversion of real IRO data with IRI2001 as constraint, and the comparison between the inversion result and measurement of ionosonde shows that the inversion result is reasonable and credible. An ionosphere model based on global electron density of IRO is constructed with spherical harmonic function, then the model is used as constraint in the inversion of IRO simulation data, and the inversion error is greatly reduced.7. The IRO data from COSMIC is applied to analyze the climate character of the ionosphere in the low solar activity.8. The exploration of Mars ionosphere with IRO technique is studied with simulation method. The research shows that in the China-Russia joint exploration of Mars, the electron density acquired from satellite-satellite IRO by dual frequencies will reach the unprecedented precision. The satellite-Earth IRO by single frequency can detect the electron density profile of Mars ionosphere in daytime.更多还原