【作者】 赵莹；

【导师】 刘经南； 张小红；

【作者基本信息】 武汉大学 ， 大地测量学与测量工程， 2011， 博士

【摘要】 无线电掩星技术起源于19世纪60年代中期,最早被美国斯坦福大学用于研究太阳系行星的大气层和电离层。随着1995年GPS/MET掩星探测计划的成功,该技术被广泛地应用于地球中性大气和电离层探测。GNSS电离层掩星反演技术具有全天候、全球覆盖、高垂直分辨率、准实时、自校准等其他电离层探测手段所不具备的特点,尤其能够弥补特殊区域(如：海洋、极地)电离层观测资料的不足,为电离层研究提供了更多的资料及更多的选择。利用该技术获得的全球性的电离层电子密度分布资料,将极大地推动空间环境监测、数据同化及空间天气效应等研究的发展,具有十分重要的科学意义。论文详细地介绍了GNSS无线电电离层掩星反演技术的研究现状及基本原理,围绕掩星反演方法、反演结果验证及应用等开展了一系列研究和讨论,主要内容和成果如下：(1)系统地介绍了GNSS电离层掩星反演方法,包括：Abel积分反演方法、“洋葱分层”反演方法以及最小二乘反演方法,着重对它们的反演结果进行了比较和分析。实验结果表明,对于轨道高度约800km的COSMIC电离层掩星数据,Abel积分反演方法与“洋葱分层”反演方法反演得到的电子密度廓线在F2层以下基本一致,但在500km以上的电离层上部,采用不同的反演方法会引起结果的差异；而最小二乘反演方法与“洋葱分层”反演方法得到的电子密度廓线基本一致。(2)对COSMIC掩星数据的时空分辨率进行了分析。结果表明,掩星电离层反演的垂直分辨率DF和水平分辨率ΔH分别为1.5km和300km,具有较高的垂直分辨率,但水平分辨率和时间分辨率较低,对于一些对时间分辨率要求较高的应用(如：地震-电离层异常探测),现阶段掩星数据还只能作为其他电离层观测手段的补充,不足以独立利用自身数据进行研究。(3)利用IRI模型和电离层垂测仪观测数据,对掩星结果进行了比较验证及统计分析。结果表明,利用COSMIC掩星数据反演得到的电离层垂直电子密度剖面与垂测仪结果在整体趋势和结构上符合的比较好；推求出的foF2与垂测仪和IRI模型的结果均具有较高的一致性,相关系数分别为0.9和0.88,但hmF2相较于垂测仪和IRI模型的结果存在较大偏差,相关系数分别只有0.339和0.42。(4)对于采用Abel积分反演方法反演电子密度的计算式中存在积分下奇点的问题,论文全面讨论了处理积分奇点的几种不同方法,并首次将“奇点分离法”用于对Abel积分反演中积分下奇点的处理,该方法与常用的Hφeg的“变换法”相比,更为简单直接、更易于编程实现,且反演得到的电子密度剖面与UCAR公布的电子密度产品符合的很好。(5)针对低轨卫星轨道高度以上的电子含量对反演结果造成的影响,论文采用"Calibrated TEC",即改正TEC方法来进行反演,利用非掩星时段观测数据修正掩星时段观测数据,以消除低轨卫星轨道高度以上的电离层对掩星观测的影响。结果显示,改正TEC算法消除了低轨卫星轨道高度以上电子含量的影响,对于轨道较低的低轨卫星而言是一种有效的电离层掩星反演方法。(6)对于CHAMP这类轨道高度较低的低轨卫星,论文提出利用变标高的Chapman函数对CHAMP卫星轨道高度以上部分的电离层电子密度垂直分布进行重建。结果显示,通过Chapman函数恢复出的电子密度剖面补充了CHAMP卫星轨道高度以上的电离层电子密度信息,包括了电离层的主要部分,且在F2层峰值高度至CHAMP卫星轨道高度这一区间与Abel积分反演结果吻合的很好。(7)分析了非球对称性对电离层掩星反演的影响,将已在模拟数据中取得良好修正效果的非球对称因子AF用于实测COSMIC掩星数据掩星反演结果的修正,结果表明,引入非球对称因子AF改善局部球对称假设后,电离层掩星反演结果有显著改善,但修正效果还需通过更多的实测数据来验证。(8)对E层负值的出现与掩星切点水平漂移的关系进行了统计分析,深入分析了E层产生负值的原因；对反演结果中E层出现的异常情况进行了归纳分类,并统计了四种异常情况所占比率随纬度的变化。在此基础上,利用非线性最小二乘方法以及非负最小二乘方法对E层负值进行修正,并首次提出利用Chapman函数对E层反演异常情况进行重建。结果显示,利用非线性最小二乘方法以及非负最小二乘方法对E层负值进行修正只是一种数学上的处理,不能给出物理解释；利用Chapman函数进行E层重建,能够得到具有物理意义的E层电子密度廓线,大大提高了掩星数据的可利用性。(9)利用COSMIC掩星数据对全球电离层变化、日全食-电离层效应、地震-电离层效应以及由太阳风暴引发的电离层暴等进行了详细的分析和讨论。结果表明,COSMIC掩星电离层反演能够观测到全球电离层的各种变化,并能够探测到各种现象引起的电离层效应,为全球电离层运行机制和电离层扰动的监测与解释提供了一种新的观测手段。更多还原

【Abstract】 Originated in 1960’s, radio occultation technique was first to be used to study atmosphere and ionosphere of the planetary in the solar system by Stanford University. Since success of the GPS/MET program in 1995, this technology has been widely used in the Earth’s neutral atmosphere and ionospheric sounding. With advantages of all-weather, global coverage, high vertical resolution, near real-time, self-calibration, etc, GNSS ionospheric occultation inversion technique could make up for lack of ionospheric data in special areas (especially in Oceans and polar region), so as to provide more information and more choices in ionospheric studies. And the global distribution of ionospheric electron density data obtained by this technique if of extremely important scientific significance, which would greatly promote the development of space environment monitoring, data assimilation as well as space weather effects etc.This dissertation paper focuses on the GNSS ionospheric occultation inversion technique and its application. The main works and results can be summarized as follows:(1) Ionospheric occultation inversion methods are studied systematically. Comparison and analysis are made between different inversion results. Results show an agreement of the electron density profiles obtained by Abel inversion and the "onion peeling" inversion under the altitude of the F2 layer. However, at the upper ionosphere above 500km, deviation would be caused with different inversion methods. Because of the equivalence of Least squares inversion and "onion peeling" inversion method, electron density profiles obtained from these two methods are consistent.(2) Spatial and temporal resolution of the COSMIC occultation data is analyzed. Results show that there is a high vertical resolution of the occultation data, while the horizontal resolution and the temporal resolution are low. Therefore occultation data at the present stage can not do research independently but could only be used as a complement of other ionospheric observation techniques for applications with high temporal resolution demands (such as:Earthquake-ionosphere anomaly detection).(3) Inversion result from COSMIC ionospheric occultation data is compared with IRI model and ionosondes data. It shows that the trend and structure of electron density profiles of Ionospheric occultation are more consistent with ionosondes results, and the foF2 of Ionospheric occultation is consistent with both results of ionosonde and IRI model, with correlation coefficients as 0.9 and 0.88 respectively. However, there is deviation in the comparison of hmF2, with correlation coefficients as 0.339 and 0.42 respectively.(4) Several methods of dealing with integral singularity existed in Abel inversion are discussed comprehensively. "Singularity separation method" is used in handling of the integral singularity for the first time, which is more straightforward and easier programming while comparing with Hcpeg’s method and the profiles are consistent with results published by UCAR.(5) Calibrated TEC method is used to eliminate the effect of upper TEC above the LEO satellite orbit altitude on inversion Result by using observation data from non-occultation period to correct for data from occultation period. Result shows that it is an effective method for low altitude LEOs, which could eliminate the effect of upper TEC above the LEO satellite orbit altitude.(6) Modified Chapman-layer function with varying scale height is used to reconstruct the vertical distribution of electron density above the CHAMP orbit altitude. Result shows that the reconstructed electron density profile by this method includes the main part of the ionosphere, and there is an agreement between the reconstructed result and the Abel inversion result from hmF2 to the CHAMP orbit altitude.(7) Effect of the asymmetry of electron density in the inversion of IRO is studied. Asymmetry factor is used for the correction of inversion result of the measured COSMIC occultation data. Study shows that there is significant improvement in inversion result when the asymmetry factor is applied. However, its effects need more actual measuring data to verify. (8) Statistical analysis is made of the relation between the appearance of negative results in E layer and the horizontal drift of the occultation tangent point. Reason of the appearance of negative results in E layer is studied, and variations of four exceptions in E layer with respect to latitude are generalized. On this basis, methods of nonlinear least squares and non-negative least squares are used for correction of the negative results, and the Chapman function is proposed for the first time to deal with the reconstruction of abnormal E layer inversion results. Result shows that the nonlinear least squares method and the non-negative least squares method cannot give physical interpretation, while reconstruction through Chapman function could obtain the electron density profiles in E layer, which greatly improved the availability of occultation data.(9) COSMIC ionospheric occultation data is applied to analyze ionospheric effects associated with total solar eclipse、earthquake and solar storm, as well as changes of the global ionosphere. Study shows that the changes of global ionosphere and ionospheric effects associated with various kinds of phenomena could be observed through COSMIC ionospheric occultation inversion, which provides a new observational approach for monitoring of ionospheric disturbances.更多还原