Formation Flying: Satellite constellations in perfect harmony
When I first started working in satellite mission design, the scenario was simple: one satellite, one mission. Each spacecraft carried everything it needed; sensors, radios, power – onboard. But times have changed.
Sergio
Sánchez Esteve, Product Owner of ORBIT FlightDynamics System at Terma
Today, space missions are not just composed of individual spacecraft. They are often collaborative, tightly orchestrated networks of satellite clusters where each satellite is either a link in a chain or plays a specific role. This shift, usually referred to as Satellite Formation Flying, is redefining how satellite operations function.
At its core, Satellite Formation Flying is about control. It’s about keeping satellites at a precise distance apart, moving together, and sharing the workload that a single massive satellite used to handle alone.
The benefits are clear: greater coverage, faster revisit times, more resilience against failures, and the flexibility to upgrade incrementally. If one satellite fails, the others fill the gap.
If you want to add a new sensor or an enhanced
camera, you launch just that, not a whole new billion-dollar satellite.
Making this vision work is far from straightforward. Getting two or more satellites to fly in sync while orbiting at 7.8 kilometres per second is a bit like threading a needle while racing round a racetrack. Any tiny error in propulsion or trajectory planning is magnified with each orbit, turning what seems like a minor deviation into a mission risk.
WHY FORMATION FLYING IS HARD
The traditional way of keeping a satellite in its orbit is called station-keeping. It’s a well-understood process: you track the satellite’s position, plan a manoeuvre, fire the thrusters, and keep it on course. Sounds easy, but it’s not.
Formation flying adds an extra layer of complexity. Now, it’s suddenly about keeping two or more satellites in the right spots relative to each other. If one satellite fires
Sergio Sánchez Esteve, Product Owner of ORBIT FlightDynamics System at Terma
ORBIT keeps the optimization and planning on the ground
its thrusters to correct its orbit, the relative distance to its partner changes. That means you are not just solving a navigation problem for one object, but for a dynamic system. A single error, a delay in firing or a bit more thrust than planned can cause the distance to drift, sometimes by kilometres. Over time, this drift can lead to collisions, lost formations, and consequently, mission failure.
That’s why at Terma, we designed our ORBIT FlightDynamics System with a different approach. Rather than relying on classical analytical models, which work great on paper but struggle with real-world uncertainties, we went fully numerical.
This means we compute the best manoeuvres in the operational environment and adjust for everything from gravitational fluctuations to solar radiation and drag. With this approach, we solve real equations for real satellites, in real space.
FORMATION FLYING STRATEGIES
In ORBIT, we use a strategy called Leader-Follower. It’s a concept borrowed from nature – like birds flying in formation or synchronized swimmers gliding through water. One satellite takes the role of the leader, setting the path and making the necessary corrections to stay on course. The follower, or followers, adjust their paths to maintain a precise distance from that leader.
This setup might seem simple, but in practice, it’s a technical ballet. When the leader fires its thrusters to correct its orbit, the follower must compensate almost instantly. Any change in velocity or position has to be mirrored with extreme accuracy, which is why ORBIT handles this synchronization through numerical optimization. It continuously recalculates the required manoeuvres, ensuring that every adjustment is accounted for and applied.
The beauty of this method is its resilience. If the leader needs to dodge a piece of space debris, it executes the manoeuvre, and ORBIT immediately recomputes the path for the follower. There is no lag, no manual recalculation, just seamless adaptation.
What’s more, this leader-follower design is scalable. While we are currently flying this architecture with
Photo courtesy Vit-Mar/Shutterstock
constellations of two satellites, the system is fully ready to take on more. Each follower maintains its track with minimal fuel use, and operators have full visibility and control from the ground.
GROUND-BASED SATELLITE CONTROL MATTERS
One of the most important decisions we made when building ORBIT was to keep the optimization and planning on the ground.
Some satellite missions choose to run these calculations onboard, which works well for very close proximity operations. But for the kind of missions we are targeting – those that maintain distances of ten, twenty, or even fifty kilometres – we found that keeping the complexity on Earth makes operations more reliable and more flexible.
There is a reason for this. When everything is computed on the ground, there’s no need for heavy onboard processors or complicated software uploads. Operators can monitor and adjust manoeuvres without waiting for data to trickle down from orbit. We simply compute, verify, and uplink. It’s straightforward, and it reduces risk.
REAL-SPACE APPLICATIONS AND THE ROAD AHEAD
We’ve seen this approach deliver real advantages for our customers where benefits include substantial improvements in both time and fuel efficiency. Manoeuvre planning that used to take hours was reduced to minutes. In addition, fuel consumption dropped significantly, thus extending mission life and reducing costs.
Looking forward, we are developing new architectures like ghost-leader formations – where satellites orbit around a virtual point instead of a physical leader. We are also creating hierarchical constellations that can scale into the hundreds along with formation control using the relative drag between satellites. These different control designs will push the boundaries of what constellations can do and enable missions that were previously impossible.
Formation flying is no future phenomena; it’s already in action above us. And with ORBIT, we are making it reliable, efficient, and ready for whatever challenges space throws our way.
Plot of the along-track distance evolution in time between leaderfollower satellite controlled within a dead-band
Formation Flying
