Evaluation of total electron content derived from the spaceborne receivers of GRACE/GRACE-FO missions

The Gravity Recovery and Climate Experiment (GRACE) and its follow-on mission GRACE-FO are gravity satellites jointly developed by the National Aeronautics and Space Administration (NASA) and German Aerospace Center (DLR), which are composed of two satellites. Such tandem satellite missions provide us with a good opportunity to evaluate the ionospheric total electron content (TEC) derived from their onboard global positioning system (GPS) receivers. In addition, the K-band ranging system (KBR) between two satellites provides also the in-situ electron density (Ne) at the satellite orbits, which can help further to evaluate the reliability of TEC. By combing the observations from GRACE and GRACE-FO, 20 years of data (from 2002 to 2022) have been accumulated to analyze the solar cycle dependence of TEC and Ne at the topside ionosphere. Our results show that the TEC from the tandem satellites is generally the same, but slight differences can still be found, showing solar cycle and local time dependences. In addition, we found that the TEC differences between the tandem satellites of GRACE are somehow smaller than that of GRACE-FO, and the consistency between the TEC and inter-satellite electron density results of GRACE is also better.