中文摘要：

针对目前地基GNSS台站在全球分布不均匀，导致南半球海洋和中高纬地区出现与实际不符的VTEC为负值的问题，利用IGS前1d的最终GIM作为虚拟观测值对TEC可能为负值的地区进行约束，结合GNSS数据建立全球电离层模型（称为SGG模型），并用2014年200多个IGS台站数据对模型进行验证。结果表明，各台站VTEC的RMS优于3TECu（赤道异常区域RMS在5～7TECu）。同时SGG能有效消除南半球海洋（40°～90°S）VTEC为负的情况，且对原有非负VTEC几乎没有影响（其变化小于2TECu）。SGG的卫星DCB与CODE相比，RMS和MEAN分别优于0．2ns和0．04ns，不同纬度带SGG与CODE的接收机DCB估值变化基本一致，两者之差基本在1ns以内。

英文摘要：

Nowadays ground-based global navigation satellite system （GNSS） stations of the interna-tional GNSS service （IGS） are distributed unevenly around the world,especially in the southern ocean area. As a result, many unreasonable negative vertical total electron content （VTEC） values appeared in corresponding areas referred above. To completely eliminate non-physical negative values, a new method, temporarily named SGG, with IGS＇s final VTEC products is proposed for constraining the area where no real data is available, and then we process the GNSS dual data of more than 200 IGS reference station in 2014（high solar activity year） with GIM constraint to validate the new method. The results have shown that the RMS of ionospheric TEC at contributed stations is within 3 TECu in the spatial domain（the RMS over the equatorial anomaly area is approximately 5-7 TECu）. Meanwhile SGG efficiently eliminates the unwanted behavior of negative VTEC values in ocean and middle-high lati- tude areas of the southern hemisphere（40°- 90°S）, and the original positive VTEC almost won＇t be affected （the variation of VTEC can be controlled within 2 TECu）. The satellite and receiver DCB of SGG is also highly consistent with IAACs, the RMS and MEAN of satellite DCB between SGG, UPC, ESA, JPL and CODE is within 0. 2 ns and 0. 04 ns respectively. The receiver DCB valuation changes between SC, G and CODE at different latitudes are basically identical（the difference is about 0-1 ns）.