This paper presents our research effort to develop an assimilation model for the FORMOSAT-3/COSMIC (F3/C) GPS Occultation Experiment (GOX) observations by means of ensemble Kalman filtering using the NCAR Thermosphere Ionosphere Electrodynamics General Circulation Model (TIE-GCM) as a forecast model. More than 2000 electron density profiles are retrieved daily from the F3/C GOX, uniformly distributed around the globe. They provide an excellent opportunity to monitor the global ionospheric electron density structure, especially over oceans, deserts, and the polarregions. The NCAR TIE-GCM is a three-dimensional, time-dependent model of the Earth's neutral upper atmosphere and ionosphere which provides self-consistent solutions for the coupled nonlinear equations of hydrodynamics, thermodynamics, and continuity of the neutral gas and plasma. The Data Assimilation Research Testbed (DART), an open-source community facility for ensemble Kalman filtering (EnKF), is also used in our research.

The F3/C GOX observations are combined with the TIE-GCM simulations by EnKF algorithms implemented in DART to compute the expected value of electron density, which is 'the best' estimate of the current model state. A number of filtering experiments are conducted with real F3/C electron density profiles and the results are compared with independent ground-base observations. This paper shows the improvement of the critical ionospheric modeled parameters such as NmF2 and hmF2, and furthermore discusses the limitations of the model and the impact of ensemble member creation on the assimilation results.