MTG-S1 was launched by a Falcon 9 rocket into a Super Synchronous Transfer Orbit on July the 1^st, 2025 and then transferred to geosynchronous orbit. It is the second in-orbit spacecraft of the Meteosat Third Generation (MTG) Programme, a cooperation between EUMETSAT and ESA, with EUMETSAT being responsible for operating the satellites during Commissioning, Routine and Disposal Phases. MTG-S1 enhances the capabilities of the Meteosat fleet by carrying the first European sounder payload in geostationary orbit, in addition to the Copernicus Sentinel-4 Mission. The sounder payload will complement the optical imaging service and the new lightning detection service provided by MTG-I1, the first MTG satellite, launched in December 2022. All MTG satellites share a common platform which is equipped with a chemical bi-propellant propulsion system for orbit control and reaction wheels for three-axis stabilized attitude control.

Following a successful Commissioning Phase and a relocation campaign, MTG-I1 entered its operational service at the end of year 2024. Ever since, it is being controlled in a slot near 0 degrees longitude and in an inclination region below 1 degree. On July the 1^st, 2024 the first combined manoeuvre was executed: thanks to the precise attitude control provided by the platform, a single manoeuvre is exploited to counteract both the out-of-plane and in-plane disturbances by means of attitude biases, while achieving the offloading of the reaction wheels. Each manoeuvre provides a Delta-V in both the North-South and East-West directions, at the same time eliminating any undesired radial component caused by thrusters’ cross-coupling effects. This allows for propellant savings with respect to separated North-South and East-West controls. The reduced number of manoeuvres allows to significantly decrease the operational workload on the control teams, as well as to minimize the instrument outages. However, these benefits come with a few drawbacks. The location of manoeuvres along the orbit becomes constrained by the out-of-plane control (relative position with respect to node crossings), impeding the use of the standard Sun-pointing-perigee strategy for optimal eccentricity control. To address this, the out-of-plane manoeuvres direction (northwards or southwards) and therefore the manoeuvres location along the orbit is selected depending on the day of the year, to achieve an eccentricity evolution below its required value. Another drawback, which has been fixed by introducing a change in the overall operational concept, was the loss of Ka-Band antenna communication due to the commanded attitude bias during manoeuvres execution. The first year of MTG-I1 orbit control achieved with combined manoeuvres is presented in this paper, highlighting advantages and limitations of such strategy.

MTG-S1 is currently in Commissioning Phase, located in a longitude slot around 3.4 degrees West. Semi-major axis and eccentricity are controlled with stand-alone in-plane manoeuvres. Inclination and RAAN are let to free drift until they reach the assigned control region. At that point in time, currently predicted for the second quarter of 2026, MTG-S1 orbit control via combined manoeuvres will begin, similarly to MTG-I1. At the end of the Commissioning Phase, in Summer 2026, the satellite will be relocated to the 0 degrees longitude slot, in co-location with MTG-I1. The co-location of the two satellites will be based on eccentricity/inclination separation, a commonly adopted strategy for operating multiple geostationary spacecrafts in the same longitude slot, which guarantees safety of the co-location in terms of minimum inter-satellite distance. This standard approach has been tailored to reduce both the payloads and star-trackers field-of-view crossings of one satellite with respect to the other. The tuned parameters are the co-location angle and the separation in longitude. The co-location angle (defined as the angle between the relative eccentricity and relative inclination vectors) has been selected as a trade-off between maximum safety and minimum number of optical interferences. The effects of a superimposed longitude separation have been analysed for both constant and variable longitude offsets. Particular attention has been given to the eccentricity control, which becomes more challenging when no Sun-pointing-perigee is applicable due to the execution of combined manoeuvres. The final strategy selected to co-locate MTG-I1 and MTG-S1 is presented in detail in this paper.