# Improvement of Global Ionospheric TEC Derivation with Multi-Source Data in Modip Latitude
###### Abstract
[13] obtained GIMs in geomagnetic latitude through combining observations from GNSS, satellite altimetry and occultation measurements, and the results show that the root-mean-square error (RMSE) of the combined GIMs is decreased when compared with the results of fitting with ground-based GNSS data. [PERSON] et al. [14] derived global TEC with ground-based GNSS, satellite altimetry and COSMIC observations in a solar-geomagnetic reference frame using an SH function. The performance of using both ground- and satellite-based data has been improved and was validated by satellite altimetry observations over long periods. Since satellite observations have been sparsely sampled, their supplementary effect on ground-based observation is limited. For obtaining TEC on a global scale, estimation from empirical ionosphere models plays an important role for those areas with limited observations. [PERSON] et al. [15] used observations of multi-GNSS and satellite altimetry, and a priori values from an International Reference Ionosphere (IRI) model to map the ionosphere in a solar-geomagnetic reference frame, and the validation results suggest the GIMs from this model are further improved through employing multi-source data, especially for the ionosphere over the oceans. Later, the same group only used ground-based observations and the IRI model to obtain the GIMs, and concluded their method has a higher precision than IGS GIMs, even for ionosphere disturbances during geomagnetic storms [16]. The coordinate system is another important factor since an unsuitable one has a risk of over fitting and spurious peaks. The dip latitude calculated from magnetic elements is unsuitable for global mapping of the ionosphere due to the anomaly of meridional variation near South Africa, and the usual solution of geomagnetic latitude defined by the dipole approximation does not coincide with the true magnetic equator, and is distorted from the real magnetic field of the Earth [17]. In 1963, [PERSON] [18] introduced the modified dip (modip) latitude, and investigated the benefits of describing the variability of TEC at low and middle latitudes in 1984 [19]. [PERSON] et al. [20] used ground-based GNSS data in a modip latitude to derive the global ionospheric TEC, and results show that the accuracy of global vertical TEC (VTEC) representation was improved by more than 10% compared with fitting in geomagnetic latitude, particularly in the region of the equatorial anomaly. This suggests that the modip latitude is more suitable for TEC derivation through fitting with observation in the global scale. However, this performance was only studied over three months (March, April and May) of 1999 and validated in areas with existing Topex observations, so the improvement is not clear from a global perspective. We conclude therefore that the impact of multi-source data and the coordinate system on global TEC derivation is still an open question. Simulation and data analysis can enhance our understanding. In this paper, Section 2 introduces the multi-source data and analysis method in the global ionospheric TEC derivation. In Section 3, due to the non-homogeneous distribution of observational data on a global scale, we study the impacts of data distribution and coordinate system in global TEC derivation by using the reference data generated from the IRI-2016 model. In Section 4, global TEC are derived in a solar-modip coordinate system using an SH function with multi-source data, including observations from GNSS, satellite altimetry (JASON) and occultation (COSMIC), and virtual observations from the IRI-2016 model. Data for 12 days in quiet geomagnetic conditions for four seasons in 2014 were collected and processed, and the final results were examined.