ST3LLARsat-1 "BOIRA", the first student CubeSat at Universidad Carlos III de Madrid (UC3M), requires a robust onboard orbit determination (OD) capability for its Attitude Determination and Control System (ADCS). This article presents and validates a unified OD architecture centered on a single Simplified General Perturbations #4 (SGP4) propagator, comparing it against an initial dual-propagator (Numerical + SGP4) concept. The system centers on a Pseudo-TLE Generation Algorithm that converts real-time GNSS data into standardized Two-Line Element (TLE) formats. By using a weighted Levenberg-Marquardt fit, the algorithm ensures that these "pseudo-TLEs" are mathematically precise. This dual-source approach allows the SGP4 propagator to remain highly accurate at all times: using live GNSS data when available, and automatically falling back to traditional ground-based TLEs if the signal is lost. Using the full Space-Track catalog (31,753 objects), the algorithm reconstructs TLEs with epoch position errors in the [10-7, 10-6] m bin for 99.94% of cases. In Initial Orbit Determination (IOD) test scenarios using simulated telemetry, a 24-hour data arc allowed for precise drag (