Status of CAS global ionospheric maps after the maximum of solar cycle 24

As a new Ionosphere Associate Analysis Center(IAAC)of the International GNSS Service(IGS),Chinese Academy of Sciences(CAS)started the routine computation of the real-time,rapid,and final Global Ionospheric Maps(GIMs)in 2015.The method for the generation of CAS rapid and final GIMs and recent updates are presented in the paper.The quality of CAS post-processed GIMs is assessed during 2015-2018 after the maximum of solar cycle 24.To perform an independent and fair assessment,Jason-2/3 Vertical Total Electron Contents(VTEC)are first used as the references over the ocean.GPS differential Slant TECs(dSTEC)generated from 55 Multi-GNSS Experimental(MGEX)stations of the IGS are also employed,which provides a complementing way to evaluate the ability of electron content models to reproduce the spatial and temporal gradients in the ionosphere.During the test period,Jet Propulsion Laboratory(JPL)GIMs present significantly positive deviations compared to the Jason VTEC and GPS dSTEC.Technical University of Catalonia(UPC)rapid GIM UQRG exhibits the best performance in both Jason VTEC and GPS dSTEC analysis.The CAS GIMs show comparable performance with the results of the first four IAACs of the IGS.As expected,the poor performance of all GIMs is in equatorial regions and the high latitudes of the southern hemisphere.The consideration of generating multi-layer or three-dimensional ionospheric maps is emphasized to mitigate the inadequacy of ionospheric single-layer assumption in the presence of pronounced latitudinal gradients.The use of ionospheric observations from the new GNSS constellations and other space-or ground-based observation techniques is also suggested in the generation of future GIMs,given the sparse GPS/GLONASS stations in the southern hemisphere.