Geodetic very long baseline interferometry (VLBI) observations are carried out, with the legacy system, using two well-separated frequency bands in order to determine first-order ionospheric delay corrections corresponding to the combined effect of total electron content (TEC) at two stations forming a baseline. On the other hand, it is possible to obtain the vertical TEC (VTEC) above the VLBI antennas from VLBI data. This research intends to investigate the role of the ionospheric spatial gradient on VTEC which is derived from VLBI observations based on the latest four Continuous VLBI Campaigns (CONT08 to CONT17-L1). For this purpose, station-based VTEC values were computed in two modes, with and without spatial gradients. Then, these two approaches were evaluated by comparison to global ionospheric maps (GIMs). The differences between the GIM-based VTEC values and the VLBI VTEC values derived using two parameterization approaches were used to compute the Root Mean Square Error (RMSE) for each VLBI station during the CONTs. In addition, the improvement percentages of the VLBI VTEC's error were calculated to understand the spatial gradient role. Based on the obtained results, the role of spatial gradient is more significant on the equatorial region as compared to the polar area. Using the ionospheric spatial gradient decreases the RMSE values of VLBI-derived VTEC values from CONT08, CONT11, CONT14, and CONT17-L1 by 26.4%, 37.6%, 32.4%, and 27.4%, respectively. In addition, it is also shown that this parameter reduces the RMSE values during the high solar activity, as in 2011 and 2014.

中文翻译：

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空间梯度对从大地极长基线干涉测量法观测中提取垂直总电子含量的作用：案例研究 CONT08 到 CONT17-L1

使用传统系统进行大地超长基线干涉测量 (VLBI) 观测，使用两个分离良好的频段，以确定与两个站的总电子含量 (TEC) 的组合效应相对应的一阶电离层延迟校正形成基线。另一方面，可以从 VLBI 数据获得 VLBI 天线上方的垂直 TEC (VTEC)。本研究旨在调查电离层空间梯度对 VTEC 的作用，该梯度源自基于最近四次连续 VLBI 运动（CONT08 至 CONT17-L1）的 VLBI 观测。为此，以两种模式计算基于站的 VTEC 值，有和没有空间梯度。然后，通过与全球电离层图 (GIM) 进行比较来评估这两种方法。基于 GIM 的 VTEC 值与使用两种参数化方法得出的 VLBI VTEC 值之间的差异用于计算 CONT 期间每个 VLBI 站的均方根误差 (RMSE)。此外，还计算了 VLBI VTEC 误差的改善百分比，以了解空间梯度的作用。根据所得结果，与极地地区相比，空间梯度在赤道地区的作用更为显着。使用电离层空间梯度将来自 CONT08、CONT11、CONT14 和 CONT17-L1 的 VLBI 导出的 VTEC 值的 RMSE 值分别降低 26.4%、37.6%、32.4% 和 27.4%。此外，还表明该参数降低了高太阳活动期间的 RMSE 值，如 2011 年和 2014 年。