Sentinel-1 is a flagship mission of the Copernicus Programme of the European Union, dedicated to Earth observation and managed by the European Space Agency. The mission, currently composed of three satellites, provides critical radar imaging data for environmental monitoring, climate studies, land and maritime surveillance, and disaster response both across Europe and globally. The satellites fly in a Sun-synchronous orbit at 6 PM and an altitude of about 700?km, following a repetition cycle of 12 days (14 + 7/12). Operational activities are conducted at ESOC and ESRIN, supported by ESTEC and industrial contractors; Thales Alenia Space Italia, as prime contractor, provides in-orbit support under ESA’s contract, managed by ESTEC’s Post-Launch Support Office.

The first satellite of the constellation, Sentinel-1A, launched in April 2014, has accumulated more than eleven years of operational activity, during which the flight dynamics team at ESOC has continuously controlled the reference ground track with a tolerance of ±120?meters. This extended time span, coupled with the strict orbit control, provides an unprecedented opportunity to investigate and understand the long-term behavior of this kind of orbits. We present an extensive analysis of Sentinel-1A’s orbital evolution based of in-flight data, as collected by the onboard Precise Orbit Determination system. By reducing these data to mean elements using the theories of Kaula and Ekstein-Ustinov, it has been possible to isolate unexpected effects in the semi-major axis, Mean Local Time of Ascending Node (MLTAN) and in the in-plane evolution.

The semi-major axis exhibits oscillations with an amplitude of about six meters, which we linked to deviations in the RAAN rate due to Lunisolar gravity, that forces the control to indirectly move the orbit between different realizations within the same repetition cycle family, in order to preserve ground track repetition. The observed deviation in MLTAN, about one minute over the mission, can be attributed mainly to third body and tidal effects, with a minor contribution from a slight imperfection in the original enforcement of Sun-Synchronicity. The in-plane evolution anticipates the nominal expectation by approximately 3 degrees, which we decomposed as a combination of the orbital plane deviation, irregularities in Earth rotation, and equator evolution as a consequence of precession and nutation movements; all these require adjusting in-plane positioning in order to keep the node crossing.
 

 

To perform the analysis we developed dedicated analytical solutions, avoiding the need for numerical propagation or systematic Earth path repetition checks. These methods, are straightforward to implement, computationally efficient, and match closely with results obtained through more comprehensive numerical approaches. Furthermore, we also compare the results when the methodology is applied to TLE data, demonstrating comparable accuracy.

The insights obtained are useful for mission control optimization, to better characterize models of LEO orbital dynamics, and for the design of future Earth observation missions. Moreover, our approach offers a practical methodology to extract critical orbital information from readily available TLE data, enhancing opportunities for operational monitoring and historical analysis.