The proposed project develops the concept of orbit and clock determination for a LEO PNT system that uses high-precision on-board GNSS receivers providing POD in real time, and a reduced infrastructure in the ground segment. It also analyses the user segment, characterising the performance of the LEO PNT in the precise PPP positioning, evaluating the improvements in availability and convergence time. We rely on the experience of our G3STAR high-performance GNSS receiver, which provides the required capabilities of On-Board ODTS, using Galileo High Accuracy Service HAS. An orbit determination platform will be prototyped: the Orbit and Clock Determination OCD Software. It will collect the measurements of the LEOPNT constellation and use our experience in precise ODTS for Galileo, EGNOS and G2G. Regarding the user segment, we rely on the SW NEXA developed by Geonumerics. NEXA is currently operational providing accurate PPP positioning. It will be adapted for the study and prototyping of the use of LEO PNT signals. The main deliveries of are the system concept, the OCD SW prototype, the PPP SW and related documentation, and performance assessment.