The EUMETSAT Polar System-Sterna (EPS-Sterna) mission will provide microwave observations to support global and regional Numerical Weather Prediction (NWP) and Nowcasting (NWC). Designed to complement the EPS-SG, NOAA JPSS, and CMA Fengyun polar-orbiting satellites, EPS-Sterna aims to minimize the time required to achieve 90% global coverage through a constellation of small satellites. These platforms, based on the ESA Arctic Weather Satellite (AWS) Proto-Flight Model (PFM), will operate in Sun-Synchronous Orbits (SSO) at a reference altitude of 595 km across three distinct orbital planes (15:30, 19:30, and 23:30 LTAN). While the use of Electric Propulsion (EP) enables these small platforms to reach their operational slots, the low-thrust nature of the transfer introduces significant complexity into the Orbit Acquisition phase, which can extend up to several months.
 
Unlike impulsive chemical maneuvering, low-thrust trajectories require long-duration firing arcs where the orbital elements are highly coupled. The optimization problem is further constrained by the specific limitations of the EPS-Sterna platform, including maximum firing duration, the necessity of performing thruster swaps, and mandatory pause durations for battery recharging. Furthermore, the operational concept must account for ground segment constraints, specifically the frequency of maneuver telecommand uplinks. This requires the trajectory solution to be robust against execution errors over long propagation arcs ranging from days to weeks.
 
This paper presents the optimization framework developed for the implementation of EPS-Sterna orbit acquisition. The core algorithm targets the precise acquisition of Semi-Major Axis, Eccentricity, Inclination, and Local Time of Ascending Node (LTAN). The solver minimizes a cost function weighted between total transfer duration and propellant consumption, utilizing three distinct firing modes: (1) Pure In-Plane Thrusting for altitude adjustment; (2) Pure Out-of-Plane Thrusting centered around orbital nodes for inclination correction; and (3) Combined Coupled Thrusting, a hybrid mode employing an optimized yaw steering profile to simultaneously correct Semi-Major Axis and Inclination. A critical component of this strategy is the management of the drifts of the Argument of Latitude and the LTAN. The optimizer utilizes a "Wait & Drift" strategy at intermediate points to passively correct them, thereby minimizing propellant use. The algorithm iterates on the sequence, order, and duration of firing and coasting modes to ensure that all target orbital elements are achieved simultaneously.
 
We present results for all three operational orbital planes. We employ a grid-search methodology to generate "Strategy Maps" - visualizations that identify the optimal firing and coasting sequence based on the initial injection state. These maps reveal the sensitivity of the transfer duration and propellant budget to launcher injection dispersion. Specifically, the analysis demonstrates how the initial dispersion in Semi-Major Axis and Inclination drives the selection of the optimal strategy. For scenarios with large Semi-Major Axis and Inclination dispersions, results indicate that combined yaw-steered maneuvers significantly reduce the total time to station compared to sequential pure strategies. Conversely, for scenarios with low dispersions, additional firing phases are included to accelerate the drift of orbital phase and local time, thus reducing the total acquisition duration to an acceptable level. Finally, the paper discusses the selection of the optimal "biased" injection orbit. By intentionally targeting an injection state that accounts for expected launcher dispersions, the natural drift between separation from the launcher until firing can commence, and the expected drift during the long orbit transfer, we demonstrate how the nominal acquisition duration can be minimized.