低纬电离层延迟的精确建模与修正一直是GNSS电离层研究的热点和难点之一。近年来，北斗系统在我国南方地区的推广应用对此提出了更为迫切的需求。现有广域增强电离层延迟建模方法应用于低纬地区时通常难以达到其在中纬地区的修正效果。针对此，本项目拟结合我国低纬电离层活动特点，从"建立相对合理的电离层多层假设"以及"构造自适应描述电离层延迟相关性的协方差函数"两方面入手，构建提高我国低纬GNSS电离层延迟建模精度的新方法，旨在站间距400-500公里的条件下，实现平均优于0.4米的修正精度，较现有方法提高20%左右。新方法不仅一定程度上考虑了电离层电子密度在垂直方向上的分布结构，而且更好地顾及到了电离层固有的随机特性。 相关成果将进一步丰富现有GNSS电离层延迟建模的理论与方法，并可为北斗广域增强系统、网络RTK以及精密单点定位等用户的电离层延迟修正提供技术支撑，具有重要的理论意义与广阔的应用前景。

The precise model and mitigation of ionospheric delay over low latitudes has been a hot and difficult problem in the research area of GNSS-related ionosphere. Recently, the extensive application of BeiDou Navigation Satellite System in southern China makes higher and urgenter requirement of ionospheic delay modeling and correction over low latitudes of China.Generally, the performace of existing methods of WAAS-related ionospheric modeling directly used over low latitudes is much worse than that in middle latitudes and cann't meet the demand of preceise GNSS user. In view of this, a new approach is proposed in this project to further improve the existing method by introducing an ionospheric multi-layers assumption and adaptive covariance function of ionospheric delay. The proposed approach aims to achieve a correction of better than 0.4m （about 20% better than the performance of current methods） over low latitudes with the inter-station distation about 400-500km. The proposed approach takes the vertical distribution of ionospheric electron density and the inherent random characteristics of ionospheric delay into account, while that has been ignored in the existing methods. The achievement of this project can further enrich the theory of GNSS-based ionospheric delay modeling and provide a techinque support for the users of GNSS/Compass wide area real-time precise positioning, RTK, PPP and so on to obtain the ionospheric delay with a high precison. Therefore, this research has not only an important academic value, but also broad application prospect.