中文摘要：

区别于以往GPS电离层层析研究主要关注迭代模型的思路,本文从两方面入手提高GPS电离层层析迭代算法的反演精度：一方面,顾及传统电离层层析迭代模型仅与对电子密度误差起放大作用的GPS射线截距权重相关的不足,提出考虑层析像素格网中的电子密度对GPS TEC的贡献建立新的迭代模型,在不同电子密度像素格网内重新分配GPS TEC实测值与其反演值之间的差距;另一方面,顾及电离层层析迭代算法中松弛因子对反演结果的影响,提出考虑电子密度变化构造新的松弛因子,抑制传播噪声对电子密度反演精度的影响.实验结果显示,相对于传统代数重构算法（ART）,新方法反演的电离层电子密度剖面更接近于电离层测高仪观测的电子密度剖面,提高了电子密度反演精度.

英文摘要：

GPS-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 GPS-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 GPS 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 effect 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 GPS ionospheric tomography will be developed to redistribute the discrepancy between GPS TEC and the calculated TEC among the ray-voxels according to the voxelrs contribution to TEC instead of the length of the ray-voxel intersection. 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 GPS 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 GPS ionospheric tomography. Finally, w