Flying two spacecraft is harder than one

What’s harder than flying a single satellite in Earth orbit? Flying two—right beside each other, at proximities that would normally trigger collision avoidance maneuvers.

This is the plan for ESA’s Proba-3 double-satellite mission, which will take off from India on Wednesday 4 December. During active formation flying, the pair will hold position at about 150 meters from each other, to a precision equal to the thickness of the average fingernail. So how are they going to manage it?

Picturing precise formation flying success

“ESA has flown formation flying missions before, but the distances involved have been measured in the tens of kilometers or more,” explains Damien Galano, Proba-3 mission manager. “Proba-3 is very different because our satellites will be flying just one and a half football fields away from each other during active formation flying. And their relative positions will be maintained precisely to just a single millimeter for six hours at a time.

“And we won’t just be proving our success with telemetry, but through something everyone can interpret instinctively. By lining up with the sun, one spacecraft will cast a precisely controlled shadow onto another, to cover the sun’s brilliant disk entirely, so that the million-times-fainter solar corona will become visible for sustained observation. This will either work or it won’t: That is the challenge we have set ourselves.”

High orbit for mission success

Key to overcoming that challenge is to select an environment where success becomes feasible. A standard low Earth orbit was quickly ruled out because of all the influences that would affect the pair: the stronger pull of gravity plus perturbations due to Earth’s imperfect shape and air drag up at the top of its atmosphere, along with Earth’s reflected light.

“Early simulations showed we would need to make so many positioning adjustments with our thrusters that our propellant would be exhausted swiftly; the mission would have been over in about half an hour!” recalls ESA’s Frederic Teston, who has overseen the Proba family of missions. Instead the pair needed to go where perturbations are minimal, and the pull of gravity is much lower—meaning it takes less propellant to shift position.

An ideal location would have been around one of the sun-Earth Lagrange Points surrounding our planet, where gravitational fields are cancelled out, but it would have been too costly for such a budget mission to reach them. Instead, a highly elliptical—or elongated—orbit was selected, which starts at an altitude of 600 km and reaches all the way up to 60,500 km during each 19 hour 36 minute orbit.

Picture it as a rollercoaster loop: around the bottom of the orbit, the spacecraft move much faster, but slow down as they climb higher, from 10 km/s down to 1 km/s—and because of this decreased velocity, spend more time at the “apogee” of the orbit than at the bottom. For the lower part of the orbit, the pair fly freely along safe paths—although ready to react if a collision risk emerges. Then, as they move up toward apogee, the signal is given for them to begin moving into active formation, which takes about two hours.

Driverless spacecraft

Any human oversight of Proba-3’s formation flying would be impractical, not least because of the distances involved—any radio signal would take a fifth of a second to reach to the top of their orbit, an uncomfortably long pause when dealing with orbital velocities.

Instead, the satellite pair will line themselves up with the sun on a fully autonomous basis, akin to terrestrial driverless cars. Following a comparable approach, no single positioning system is sufficient by itself to achieve the necessary precision. The mission instead combines a suite of absolute and relative positioning technologies ranging from GPS receivers and radio links to optical cameras and LEDs, a laser link and finally shadow position sensors.

Sequence of positioning actions

To begin with, startrackers—computer-linked cameras that recognize the constellations around them—chart each spacecraft’s “attitude,” or current pointing direction in space. For the lower part of their orbit, satnav receivers aboard both spacecraft compute relative positions to a high level of accuracy, although GPS signals are only used operationally below the 20,200 km altitude of the GPS satellite constellation. The Proba-3 pair also continuously exchange ranging information and other data through radio inter-satellite links.

More is needed to achieve active formation flying, starting with Proba-3’s Vision Based Sensor system. A wide-angle camera is used to track an LED pattern on the other satellite, providing relatively coarse “first glimpse” information on the satellites’ distance from each other, as well as supplementary information on their attitude. This is supplemented by a narrow-angle camera which locks onto a second, much smaller LED pattern, providing relative positioning information down to a scale of about a single centimeter.

By itself this is not enough however. Still finer positioning comes via the Fine Lateral and Longitudinal Sensor (FLLS) on Proba-3’s “Occulter” spacecraft. This shines a laser towards a corner cube retro-reflector on the face of the “Coronagraph” spacecraft, which is reflected back in turn to the Occulter. This FLLS provides relative positioning down to millimeter accuracy.

Finally, to ensure a steady lock, a Shadow Positioning Sensor system—based on photo detectors arranged around the Coronagraph telescope’s 5-cm diameter aperture lens—ensures the Occulter’s approximately 8-cm diameter shadow remains cast correctly on all sides. Any discrepancy triggers a correction.

To help hold them as steady as possible, the pair of spacecraft possess no moving parts whatsoever, other than a rotating filter wheel aboard the Coronagraph.

Flight leader and wingman

For maneuvering, the mission employs a flight leader and wingman approach. The Coronagraph spacecraft is the master, equipped with a hydrazine-based newton-scale propulsion system that it uses to break and acquire formation while also ensuring a safe “perigee” formation. The Occulter follows the Coronagraph’s lead by employing a 10 millinewton cold gas thruster system, emitting small puffs of nitrogen akin to fractions of a single human breath.

“During the active formation flying phase, the cold gas thrusters will make small pulses every 10 seconds,” explains Proba-3 systems engineer Raphael Rougeot.

“The remaining perturbations we have to contend with are solar radiation pressure—which is the small but steady push from sunlight itself—and the small difference in gravity from the pair not being at the same point. These amount to a few millimeters per second. In practice we are a bit more sensitive to sideways displacement than lateral back-or-forward displacement. To give an idea, if the moon is a few kilometers closer or further away from Earth it doesn’t change a solar eclipse much, but if it moves sideways a similar amount, then you’d start seeing more sunlight!”

Fall back toward Earth

After six hours, the two spacecraft are released from their active formation to fall back towards Earth on parallel but safe orbits—although a collision avoidance maneuver would be automatically triggered if one spacecraft drifts too close to the other, or if one were to become faulty.

To avoid such an eventuality, both spacecraft have fully redundant systems, and their computational loads are distributed across both platforms to avoid any risk of slow down—so, for instance, while the Coronagraph spacecraft oversees the demanding coronal observations, the Occulter performs the relative GPS calculations which help keep the spacecraft safe around perigee as well as the maneuvers to make and break active formation.

Proba-3 is a technology demonstration mission first and foremost, with coronal observations only one type of formation flying it will attempt, along with resizing its baseline length, retargeting its orientation, and close rendezvous.

In the end, the limiting factor for the mission is expected to be propellant, with a two-year lifetime forecast. The two spacecraft’s low 600 km perigee means that they are forecast to burn up in the atmosphere a scarce five years after that.