# Improvement of Global Ionospheric TEC Derivation with Multi-Source Data in Modip Latitude
###### Abstract  
Global ionospheric total electron content (TEC) is generally derived with ground-based Global Navigation Satellite System (GNSS) observations based on mathematical models in a solar-geomagnetic reference frame. However, ground-based observations are not well-distributed. There is a lack of observations over sparsely populated areas and vast oceans, where the accuracy of TEC derivation is reduced. Additionally, the modified dip (modip) latitude is more suitable than geomagnetic latitude for the ionosphere. This paper investigates the improvement of global TEC with multi-source data and modip latitude, and a simulation with International Reference Ionosphere (IRI) model is developed. Compared with using ground-based observations in geomagnetic latitude, the mean improvement was about 10.88% after the addition of space-based observations and the adoption of modip latitude. Nevertheless, the data from JSON-2 satellite altimetry and COSMIC occultation are sparsely-sampled, which makes the IRI TEC a reasonable estimation for the areas without observation. By using multi-source data from ground-based, satellite-based and IRI-produced observations, global TEC was derived in both geomagnetic and modip latitudes for 12 days of four seasons in 2014 under geomagnetic quiet conditions. The average root-mean-square error (RMSE) of the fitting was reduced by 7.02% in modip latitude. The improvement was largest in March and smallest in June. global ionospheric derivation; total electron content (TEC); multi-source data; modip latitude 202124343103390/atmos1204034 developed in the ionospheric derivation by a single station [1], and then applied to multiple stations in network [2]. Daily global TEC and DCBs are routinely generated with more receivers being set up [3]. Currently, the International GNSS Service (IGS) provides global ionospheric maps (GIMs) and DCB estimations based on the results from seven members called Ionospheric Associate Analysis Centers (IAACs), each of which uses an individually developed algorithm based on ground-based GNSS observations to provide ionosphere products, and IGS combines these products with corresponding weights [4]. Global TEC data from the Center for Orbit Determination in Europe (CODE), European Space Agency (ESA) and Natural Resources Canada (NRCan/EMR) are obtained from series of spherical harmonic (SH) functions up to 15 degrees and orders but with several differences in details [5; 6; 7]. The Jet Propulsion Laboratory (JPL) uses a linear composition of bi-cubic splines with 1280 spherical triangles and the Polytechnic University of Catalonia (UPC) uses a rectangular grid with two layers model and an interpolation scheme [8; 9]. Wuhan University (WHU) obtains the global TEC by using a SH function and inequality-constrained least-squares method, and the approach used by Chinese Academy of Sciences (CAS) is integrating the spherical harmonic and the generalized trigonometric series functions on global and local scales, respectively [10; 11]. However, comparative research has shown that the relative errors between GIMs from different centers can reach 20-30% in equatorial and oceanic areas of the Southern Hemisphere where the ground observation is not sufficiently set or there is no observation [12]. Recently, satellite-based observations have been used together with ground-based GNSS observations to improve GIM results. Satellite altimetry missions such as JASON-2/3 provide precise ionospheric information over the oceanic areas, and low-Earth orbiting (LEO) satellites such as Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) occultation provide well-distributed information about the ionosphere on a global scale, which can be powerful supplements to the ground-based observations. [PERSON] et al.