The investigators will develop a data assimilation ionospheric model to be driven mainly by data obtained from the Global Positioning System (GPS). This model will overcome limitations experienced in obtaining measurements through total electron content tomography, relying on the first principle dynamical equations governing the state of the ionosphere. The main goals of the project are: (1) developing a mid- and low-latitude data assimilation model based on an existing physics-based model, (2) using the total electron content data obtained from the continuously tracking ground network of GPS receivers as input to the model, (3) using the total electron content data obtained from a low-Earth orbiter tracking GPS as input to the model, (4) deriving a 3-D field of electron density, (5) evaluating the accuracy of the model by comparing to independent measurements, and (6) evaluating the predictive power of the model by comparing predicted measurements to actual ones.