Proba-3’s first autonomous formation flight

Computer screens line all sides of the control room at ESEC, ESA’s European Space Security and Education Centre in Redu, Belgium, displaying long lists of numbers, lines of code and complex graphs seemingly incomprehensible to an outside observer.

Monitoring the screens with undivided attention are engineers of Proba-3’s flight control team, carefully analysing the data in front of them, ready to jump into problem-solving mode if any issues arise.

In precisely timed intervals, they send commands to the two Proba-3 satellites in space according to the flight plan – a series of steps scheduled for specific points in the spacecraft’s orbit.

Proba-3 is the world’s first precision formation flying mission. In their most precise configuration, its two spacecraft – the Coronagraph and the Occulter – will fly 150 metres apart in perfect formation, simulating a single giant spacecraft.

Once per orbit, the two spacecraft will align with the Sun so that the 1.4-m large disc carried by the Occulter casts a 5-cm shadow onto the optical instrument on the Coronagraph, allowing it to study the faint solar corona.

While several existing missions rely on formation flying technologies to operate, Proba-3’s two spacecraft will be the first to maintain their relative positions down to a few millimetres for hours at a time, without any control from the ground.

Earlier this week, the team in Redu executed the first stage of the complex flight operations needed to achieve the desired formation.

This first stage was dedicated to ground-commanded manoeuvres that brought the spacecraft closer to each other, from about 600 metres to only 144 metres apart. At the same time, the two were tasked with adopting their target alignment, in which the Coronagraph enters the shadow cast by the Occulter.

From here on, it was up to the spacecraft to autonomously adjust their position and maintain it for several hours.

Proba-3 mission manager Damien Galano describes how this crucial milestone was achieved: “First, we used GPS information to determine the precise location of the two satellites in space.

“Then we commanded the thrusters to eject small amounts of propellant to get the spacecraft as close as possible to the desired formation, to about 144 metres apart.

“Once the on-board autonomy was activated, the spacecraft measured and controlled their relative positioning using the Visual Based System, which consists of a wide-angle camera on the Occulter tracking a set of flashing LED lights on the Coronagraph, supplemented by a narrow-angle camera that enables a more precise positioning.”

“From here on, the spacecraft were maintaining their position autonomously, using the intersatellite link to exchange vital positioning information with each other,” notes Proba-3 systems engineer Teodor Bozhanov.

“Through the link, the Occulter spacecraft can also send instructions to its partner. If the positioning software detects a misalignment, the propulsion system can make small adjustments to get the two aligned again. At this stage, we were not interfering, only monitoring.

“For the Coronagraph spacecraft to move into the shadow cast by the Occulter, the system uses a set of shadow-detecting sensors that are located around the coronagraph instrument. This allows the satellites to stay in one line with the Sun.”

Proba-3 systems engineer Esther Bastida Pertegaz adds: “Over the past two days, we collected a wealth of data which we will now use to finetune the systems. In the coming weeks, we will do more testing to achieve the desired precision, making Proba-3 the world’s first-ever precision formation flying mission.

“The key words here are ‘precise’ and ‘autonomous’,” says Noelia Peinado of ESA’s General Support Technology Programme (GSTP), through which all of the positioning technology onboard Proba-3 was developed.

Many of the instruments that allow Proba-3 to autonomously perform precise formation flying are technology demonstrations – meaning this mission is devoted to proving their suitability for future space applications.

“It is the combination of all these instruments and software working together that make Proba-3 unique. A decade ago, none of these technologies were available – now we are about to accomplish what no one has before, enabling many more ambitious missions to come.”

Esther comments: “An endeavour like this brings together people from different areas of expertise. Systems engineers, flight dynamics engineers, spacecraft operators and subsystem experts all meet here in the control room to work together towards a common goal – to make ESA’s first autonomous precise formation flight happen. It’s a great privilege to be a part of it, witnessing two decades of efforts finally coming to fruition.”

Through In-Orbit Demonstration experiments, the General Support Technology Programme (GSTP) enables ESA to pioneer new technologies. Many of these demonstration missions have launched recently or are due to launch in the coming years, like the Agency’s first standalone deep space CubeSat HENON or the LUMIO CubeSat that will explore meteoroid impacts on the far side of the Moon.