The transition region from the ionosphere to the plasmasphere remains not fully explored since the discovery of the ionosphere a century ago. The EULI project will advance in understanding how to represent this region by including dense measurements collected by satellites placed in low Earth orbits (LEO). Our primary goal is to enhance Global Navigation Satellite Systems (GNSS) by developing a unified mathematical expression of the ionosphere and plasmasphere. A detailed analysis will be conducted to improve positioning of mass-market GNSS receivers and satellite orbit determination, which are vital aspects for modern technology-driven societies. The new formulation is foreseen to also provide benefits in space weather forecasting and space physics, which holds particular importance in optimize resource and space exploitation.