The SWOT (Surface Water and Ocean Topography) satellite was launched on December 16, 2022, into an approximately 850 km altitude orbit. The platform carries, among other equipment, a GNSS receiver with a navigation function. Positions generated by this equipment are used as measurements at the CNES ground control center to perform the routine orbit determination of the satellite by means of a classical least squares filter.
All relevant perturbing forces are taken into account (geopotential, terrestrial tides, Moon and Sun attraction, solar radiation pressure, and atmospheric drag). The SWOT vehicle is modeled by equivalent shapes to faithfully represent the real complex vehicle. The satellite attitude is taken into account. Due to the uncertainties in the atmosphere modeling, a multiplicative coefficient on the atmospheric drag force is estimated in the orbit determination process. The precision of the orbit obtained is less than one meter.
The GNSS receiver has been turned on the day after the launch, turned off for the main instrument deployment and turned on for good the 7th day. Starting from this moment, negative drag coefficients were observed, dropping to -4 at the beginning of January, indicating that the satellite seemed to naturally gain altitude. Therefore, an additional unexpected force appeared to be at play. This paper will present the studies conducted to characterize this force, particularly its evolution over time, the impact of a yaw flip attitude maneuver, and that of the operations carried out on various instruments on the satellite (heating, activations). Orbit determinations estimating empirical accelerations instead of a multiplicative coefficient on the atmospheric drag have been also performed, allowing a more accurate analysis. We will show that the leading hypothesis to explain this phenomenon is outgassing of the materials.