Preparing a spacecraft for its first days in orbit requires a large team of spacecraft operations engineers, flight dynamics specialists, ground operators, software experts and project teams, which have supported the design, production and testing of the satellite. This team is supported by control rooms, ground stations and communications networks.

For the launch of FLEX and Sentinel-3C, ESA is preparing all of this for two missions at once.

Once in space, FLEX and Sentinel-3C will operate in closely related orbits, creating additional operational considerations during launch and early operations.

“Launching two satellites is not the difficult part. We’ve done similar things before, such as ESA’s Herschel and Planck missions in 2009, but those were managed by a combined team,” says Franco Marchese, ESA’s Flight Operations Director for the Sentinel-3C launch and early orbit phase (LEOP). “What makes this launch different is that we have two teams operating in parallel and that the satellites fly in basically the same orbit. That’s the real challenge.”

“ESOC is one of the few mission control centres in the world that can do that,” he adds.

A team of teams

Every launch campaign relies on a "team of teams" consisting of specialists across many disciplines working as one operational community.

Libe Jauregui, ESA’s Flight Dynamics Manager for Sentinel-3C, captures the feeling: “During a LEOP, everyone is trying to meet a common goal. You really feel it.”

For Sentinel-3C and FLEX, however, two complete mission teams are preparing to operate side by side. Each has its own flight control team, ground stations operations team, flight dynamics team and project teams focused on their respective spacecraft.

While the teams of the two missions prepare and operate independently, their work intersects at key points. Some resources and support teams are shared, while certain activities require close coordination between the two missions.

This means preparing not only for the needs of each individual mission, but also for situations in which both teams may need to act, respond or access shared resources at the same time.

“Although the mission control teams remain separate, some activities are carried out jointly, such as shared processes and documentation,” says Pablo Asenjo Garcia, Ground Operations Manager for Sentinel-3C at ESA. “Each team still has to validate its own procedures, but there are clear synergies that enhance collaboration at ESOC. We can train together, share support teams and learn from each other throughout the preparation process.”

These points of connection are at the heart of the joint simulation exercises.

Simulations: living launch day before the launch

Long before a spacecraft reaches the launch pad, mission teams prepare for LEOP through a simulation campaign: a series of highly realistic rehearsals that allow the team of teams to practise responding to both routine and unexpected situations.

The rehearsals often run for days at a time – just as LEOPs do. Teams practise nominal and contingency scenarios, including spacecraft anomalies, orbit determination issues, ground segment failures and other operational surprises. Behind the scenes, ESA’s specialised simulation officers design and run these exercises, monitoring multiple voice loops, introducing simulated failures and adapting events in real time as teams respond.

But simulations are about more than testing systems and procedures.

“Simulations are not just about the technical side of things,” says Emilie Brannan, a Simulation Officer for Sentinel-3C at ESA. “There’s also a human side to it.”

They give teams the opportunity to practise communicating, making decisions under pressure and coordinating across disciplines before doing so with real spacecraft in orbit.

Twice the spacecraft, twice the challenge

While most simulations are conducted separately, the joint exercises concentrate on something more unusual: situations that could affect both missions at the same time.

“Teams have to remember that whatever happens to their spacecraft may have implications for the other mission,” explains ESA FLEX Simulation Officer Gustavo Baldo Carvalho. “What happens if they suddenly need the same resource at the same time? Those are exactly the kinds of situations we challenge teams to think through during the simulations.”

Simulation planners rehearse the nominal launch sequence, including the two spacecraft separations, a staggered acquisition-of-signal timeline and the collaboration required as both spacecraft enter closely related orbits. This phase requires particularly close coordination between the flight operations directors and flight dynamics teams, who exchange orbit information and refine trajectory predictions as the launch unfolds.

Of course, the teams also rehearse contingency scenarios.

During one dual-mission exercise, an unexpected launcher injection resulted in orbital parameters that differed from pre-launch calculations. While teams worked to determine the satellites' positions and assess potential proximity risks, additional spacecraft anomalies were introduced, pushing both control rooms to full capacity. Later in the simulation, a fire drill added another layer of complexity, requiring the teams to adapt while maintaining operations.

“This ‘big contingency’ simulation put a lot of strain on the teams, but it’s great to push our limits and be ready for whatever comes our way,” says Pere Ramos Bosch, ESA’s Flight Dynamics Manager for FLEX.

A team of teams – times two

The Sentinel-3C and FLEX launch campaign may appear to be simply two spacecraft sharing a ride into space.

Inside ESOC, however, the picture looks different.

The missions remain operationally independent but preparing them side by side creates opportunities to share experience, solve common problems and strengthen the connections between teams.

“Often, individuals from both teams are working together to solve a shared problem,” explains Pere. “The team feeling is enhanced because we are in the same place. The person you need to speak to is often literally down the hall.”

“When everyone is training together, it’s a community feeling,” adds Emilie.

On 15 September, Sentinel-3C and FLEX will leave Earth together before beginning their separate missions in orbit.

At ESOC, the challenge is not simply to prepare two spacecraft for their first days in space. It is to prepare two independent mission teams to operate side by side – coordinating in synergy whenever their paths, resources or decisions intersect.