顾及电离层变化规律的自适应层析迭代反演算法研究

GNSS-based ionospheric tomography technique can be used to reconstruct the large-scale ionospheric structure of three-dimensional electron densities, and has become an important ionospheric sounding tool. Current researches on GNSS-based ionospheric tomography iterative methods pay more attention to the iterative models, and we will improve the iterative algorithm of tomography from two aspects. On the one hand, for the conventional GPS iterative algorithms of ionospheric tomography, the modification to a density in the assigned voxel is only proportional to the intersection length of GNSS ray with each ray-voxel. But a voxel’s contribution to TEC includes the geometry contribution of the intersection lengths of the ray-voxel, and the contribution of the electron density of that voxel. Furthermore, the inversion error of electron densities retrieved from GPS tomographic technique is the dominant source of the discrepancy between the real measured and calculated TEC, and the intersection length only has an amplification effects on the error of electron density. Thus, it is not reasonable for the current algorithms to determine the assigned value of TEC difference in a particular voxel only by the intersection length with that voxel, while the contribution of the electron density is ignored. Therefore, a new iterative model used for the GNSS ionospheric tomography will be developed in the geomagnetic frame to redistribute the discrepancy between GNSS TEC and the calculated TEC among the ray-voxels according to the voxel's contribution to TEC instead of the ratio of the length of the ray-voxel intersection to the sum of the lengths of the all ray-voxel intersections. On the other hand, in conventional tomographic techniques, the relaxation factor is always set to a fixed constant, but ignoring the effects of the relaxation factor which controls the tradeoff between GNSS ionospheric tomography inversion accuracy and the smoothness of the electron density. This work will propose a new method to appropriately choose the relaxation factor accounting the variations of ionosphere for GNSS tomographic iterative algorithms of imaging the ionosphere, which is controlled by the ionospheric noise level and gradient intensity. At the same time, two related algorithms will be carried out to estimate the ionospheric noise level and gradient intensity in parallel on the basis of the voxel-based electron densities considering the variation of the ionosphere. Finally, we will establish an innovational adaptive ionospheric tomography based on the GNSS observations to monitor the ionospheric disturbance. It is expected that this study will promote the inter-disciplinary research between the satellite geodesy and space weather, and is of important academic significance. Meanwhile, this research has the important application values in monitoring the disturbances of the ionosphere state and the ionospheric delay corrections in the related engineering fields.

GNSS电离层层技术以其独特的空间大尺度电子密度三维探测能力引起广泛重视。以往迭代型GNSS电离层层析研究仅关注迭代模型，本项目将从两方面入手提高电子密度层析反演精度：①现有的迭代模型仅与对电子密度误差起放大作用的GNSS射线截距权重相关，本项目以射线截距和电子密度乘积的组合为变量，研究地磁坐标系下的新迭代模型，实现按像素格网内的电子密度对TEC的贡献来分配TEC反演值与实测值之间的差距；②传统迭代算法中松弛因子常取经验固定常数，忽略松弛因子对电子密度精度和平滑程度的调节作用，本项目提出顾及电离层实际变化构造松弛因子的新思路，通过设计电子密度噪声水平和梯度强度估计方法，进而联合构造自适应松弛因子模型。最终建立一套适用于电离层扰动监测应用的GNSS自适应电离层层析迭代反演新方法，促进卫星大地测量学与空间天气学的交叉研究，具有重要学术意义，同时在电离层扰动监测及延迟修正等领域具有良好应用前景。

利用GNSS 数据反演电离层电子密度三维结构，是监测与研究电离层时空变化规律的重要探测技术，同时由于其能够对受电离层影响的无线电信号进行修正，是导航与定位领域关注的重点和热点技术。区别于以往迭代类GNSS电离层层析研究主要关注迭代重构模型的研究思路，本项目考虑电离层电子密度的实际物理变化状态，改进GNSS电离层层析算法，取得的主要研究成果如下：.（1）提出一种顾及电离层变化的GNSS层析反演算法，引入在层析格网像素内的射线截距与电子密度的乘积为组合自变量（该组合变量表示GNSS TEC在观测射线方向上对应层析像素格网内的电子总量分量），合理分配不同电子密度像素格网内TEC实测值与反演值之间的误差，同时，构造了与电子密度变化相关的松弛因子表达式，抑制传播噪声对电子密度估值的影响，实现平衡与调节层析反演的电子密度精度与结果的平滑程度。.（2）提出在地磁坐标系下沿着地磁经度、地磁纬度、高度划分与建立GNSS电离层层析电子密度像素格网，确保能够更加合理地约束不同层析系统格网内的电子密度，提高反演结果可靠性。.（3）提出在电离层高度方向上将电子密度变化较大的区域划分较小的电子密度像素格网，在电子密度变化较小的区域划分较大的像素格网，尽可能在电离层变化较大的高度方向上提高像素格网的空间分辨率。.此外，参与提出被我国北斗全球导航系统采纳使用的广播电离层时延修正模型，目前中国卫星导航系统管理办公室在北斗卫星导航系统空间信号接口控制文件中以中英文两种形式在向国内外发布该模型（BDGIM）。.本项目成果以论文发表于《Journal of Geodesy》，《GPS Solutions》，《Advances in Space Research》，《Remote Sensing》，《地球物理学报》，《测绘学报》等国内外知名学术期刊上，总共发表论文22篇（其中，SCI收录17篇，EI收录5篇）。项目负责人受邀担任IGS实时电离层工作组成员；培养博士生/硕士生6名/3名（其中，在读博士生2名/在读硕士生2名）；10人次参加国际学术会议，3人次参加国内学术研讨会。