The Copernicus Sentinel-1 missions, part of the European Copernicus programme, consist of a pair of satellites in Low Earth Orbit operating in sun-synchronous frozen orbits with a repeating ground track to ensure periodic monitoring of globally distributed regions. To maintain payload performance, the ground track must remain close to its reference, while the eccentricity vector stays near the frozen value. For Copernicus Sentinel-1A, this requirement has become increasingly challenging because retrograde thrusting is not permitted under current operational constraints. As a result, orbit-maintenance manoeuvres are necessarily prograde, with the manoeuvre size fixed by the required ground-track correction and the eccentricity correction determined solely by the argument of latitude at which the manoeuvre is executed. Under these conditions, traditional ground-track and eccentricity-control strategies—which rely on the ability to apply both prograde and retrograde impulses—are no longer effective in guaranteeing compliance with the eccentricity limits.

In principle, the most sustainable strategy for eccentricity management is to exploit passive control: identify natural drift trajectories along which the eccentricity vector remains within mission thresholds for as long as possible, thereby avoiding the need for retrograde correction. When the eccentricity-vector control region is wide, the natural motion of the eccentricity vector can be approximated as a uniform circular rotation about the nominal frozen eccentricity vector. In this regime, the displacement of the vector’s true centre of rotation—caused by predictable perturbations such as solar radiation pressure and lunisolar gravity—remains small relative to the size of the control region and can therefore be treated as stationary without loss of accuracy. For Copernicus Sentinel-1, however, the allowable control region is narrow, and these perturbations produce a non-negligible and time-varying displacement of the centre of rotation. Accurately determining the instantaneous location of this centre of rotation is therefore essential for identifying natural drift trajectories that keep the eccentricity vector within limits without requiring retrograde manoeuvres.

This work presents a robust and operationally lightweight methodology to compute the instantaneous centre of rotation (ICR) of the eccentricity vector under these conditions. Rather than relying on detailed analytical SRP formulations or closed-form perturbation models, the method reconstructs the ICR geometrically. Starting from the reference orbit, two small eccentricity offsets are applied to generate two initial state vectors. These are then propagated under the full perturbation model, and, by exploiting the rigid-body rotation property of the eccentricity vector in near-frozen orbits, the trajectory of each offset represents a point on the same rotating body. The instantaneous centre of rotation and the mean angular velocity of the eccentricity vector can therefore be determined directly from the relative motion of the two propagated vectors, without requiring analytical expansions or assumptions about the structure of the perturbations.

The geometric approach incorporates predictable forces naturally, handles SRP and lunisolar effects without added model complexity, and remains robust to drag-induced short-term deviations. Operational experience with Copernicus Sentinel-1 demonstrates that the resulting ICR time history reliably predicts the evolution of the eccentricity vector in flight and enables the derivation of a natural drift trajectory that remains compatible with mission thresholds for extended periods. During periods of low solar activity, when atmospheric densities are reduced and ground-track corrections require only small prograde ΔV, the spacecraft can remain close to the targeted drift trajectory by selecting the argument of latitude appropriately. Even during high-drag periods, when larger corrections are required, the same objective can be achieved by splitting the manoeuvre into two burns at well-chosen arguments of latitude, allowing the net eccentricity-vector rotation to remain aligned with the desired natural drift. This drift trajectory therefore serves as a practical reference for scheduling prograde ground-track corrections so that the induced eccentricity rotation supports, rather than opposes, the desired evolution. In doing so, the method provides an effective planning tool for missions where retrograde manoeuvres are not available, improving eccentricity-control robustness without increasing manoeuvre frequency or complexity.