Prediction and ephemeris fitting of LEO navigation satellites orbits computed at the antenna phase center

Nowadays, Low Earth Orbit (LEO) satellites are proposed to augment the Positioning, Navigation and Timing (PNT) service of the GNSS satellites by directly transmitting navigation signals. In such cases, the users eventually need the orbits at the Antenna Phase Center (APC) of the antenna broadcasting navigation signals toward the Earth instead of those at the satellite Center of Mass (CoM). Using real attitudes of Sentinel satellites and simulated attitudes of different source types with enlarged instabilities, the influences of the attitude instability on the prediction and ephemeris fitting of the APC orbits are studied. It was found that different scenarios of attitude stabilities could lead to prediction degradations with a 3D RMS from a few millimeters to more than 4 cm. The study also showed that the ephemeris fitting errors of the APCs are not significantly impacted, considering both the real attitudes of Sentinel-6A and the simulated attitude instabilities.