Proba-3: Flying two spacecraft is tougher than one

That is the plan for ESA’s Proba-3 double-satellite mission, which can take off from India on Wednesday 4 December. Throughout energetic formation flying the pair will maintain place at round 150 metres from one another, to a precision equal to the thickness of the typical fingernail. So how are they going to handle it?

Picturing exact formation flying success

“ESA has flown formation flying missions earlier than, however the distances concerned have been measured within the tens of kilometres or extra,” explains Damien Galano, Proba-3 mission supervisor. “Proba-3 may be very completely different as a result of our satellites will likely be flying only one and a half soccer fields away from one another throughout energetic formation flying. And their relative positions will likely be maintained exactly to only a single millimetre for six hours at a time.

“And we received’t simply be proving our success with telemetry, however by one thing everybody can interpret instinctively. By lining up with the Solar, one spacecraft will solid a exactly managed shadow onto one other, to cowl the Solar’s sensible disc solely, in order that the million-times-fainter photo voltaic corona will grow to be seen for sustained remark. It will both work or it received’t: that’s the problem we’ve got set ourselves.”

Excessive orbit for mission success

Key to overcoming that problem is to pick an atmosphere the place success turns into possible. A typical low Earth orbit was rapidly dominated out due to all of the influences that might have an effect on the pair: the stronger pull of gravity plus perturbations as a consequence of Earth’s imperfect form and air drag up on the prime of its environment, together with Earth’s mirrored gentle.

“Early simulations confirmed we would want to make so many positioning changes with our thrusters that our propellant could be exhausted swiftly; the mission would have been over in about half an hour!” remembers ESA’s Frederic Teston, who has overseen the Proba household of missions. As a substitute the pair wanted to go the place perturbations are minimal, and the pull of gravity is way decrease – that means it takes much less propellant to shift place.

A really perfect location would have been round one of many Solar-Earth Lagrange Factors surrounding our planet, the place gravitational fields are cancelled out, however it might have been too expensive for such a funds mission to achieve them. As a substitute a extremely elliptical – or elongated – orbit was chosen, which begins at an altitude of 600 km and reaches all the best way as much as 60 500 km throughout every 19 hour 36 minute orbit.

Image it as like a rollercoaster loop the loop: across the backside of the orbit the spacecraft transfer a lot sooner, however decelerate as they climb greater, from 10 km/s right down to 1 km/s – and due to this decreased velocity spend extra time on the ‘apogee’ of the orbit than on the backside. For the decrease a part of the orbit the pair fly freely alongside protected paths – though able to react if a collision danger emerges. Then as they transfer up in direction of apogee the sign is given for them to start shifting into energetic formation, which takes about two hours.

Driverless spacecraft

Any human oversight of Proba-3’s formation flying could be impractical, not least due to the distances concerned – any radio sign would take a fifth of a second to achieve to the highest of their orbit, an uncomfortably lengthy pause when coping with orbital velocities.

As a substitute the satellite tv for pc pair will line themselves up with the Solar on a totally autonomous foundation, akin to terrestrial driverless vehicles. Following a comparable strategy, no single positioning system is adequate by itself to attain the required precision. As a substitute the mission combines a collection of absolute and relative positioning applied sciences starting from GPS receivers and radio hyperlinks to optical cameras and LEDs, a laser hyperlink and at last shadow place sensors.

Sequence of positioning actions

To start with, startrackers – computer-linked cameras that recognise the constellations round them – chart every spacecraft’s ‘angle’, or present pointing route in area. For the decrease a part of their orbit, satnav receivers aboard each spacecraft compute relative positions to a excessive stage of accuracy, though GPS alerts are solely used operationally under the 20 200 km altitude of the GPS satellite tv for pc constellation. The Proba-3 pair additionally repeatedly alternate ranging info and different information by radio inter-satellite hyperlinks.

Extra is required to attain energetic formation flying, beginning with Proba-3’s Imaginative and prescient Based mostly Sensor system. A large-angle digicam is used to trace an LED sample on the opposite satellite tv for pc, offering comparatively coarse ‘first glimpse’ info on the satellites’ distance from one another, in addition to supplementary info on their angle. That is supplemented by a narrow-angle digicam which locks onto a second, a lot smaller LED sample, offering relative positioning info right down to a scale of a few single centimetre.

By itself this isn’t sufficient nonetheless. Nonetheless finer positioning comes by way of the Effective Lateral and Longitudinal Sensor (FLLS) on Proba-3’s ‘Occulter’ spacecraft. This shines a laser in direction of a nook dice retro-reflector on the face of the ‘Coronagraph’ spacecraft, which is mirrored again in flip to the Occulter. This FLLS supplies relative positioning right down to millimetre accuracy.

Lastly, to make sure a gradual lock, a Shadow Positioning Sensor system – primarily based on photograph detectors organized across the Coronagraph telescope’s 5-cm diameter aperture lens – ensures the Occulter’s roughly 8-cm diameter shadow stays solid appropriately on all sides. Any discrepancy triggers a correction.

To assist maintain them as regular as doable, the pair of spacecraft possess no shifting elements by any means, aside from a rotating filter wheel aboard the Coronagraph.

Flight chief and wingman

For manoeuvring, the mission employs a flight chief and wingman strategy. The Coronagraph spacecraft is the grasp, geared up with a hydrazine-based newton-scale propulsion system that it makes use of to interrupt and purchase formation whereas additionally making certain a protected ‘perigee’ formation. The Occulter follows the Coronagraph’s lead by using a ten millinewton chilly fuel thruster system, emitting small puffs of nitrogen akin to fractions of a single human breath.

“Through the energetic formation flying section the chilly fuel thrusters will make small pulses each 10 seconds,” explains Proba-3 techniques engineer Raphael Rougeot.

“The remaining perturbations we’ve got to deal with are photo voltaic radiation stress – which is the small however regular push from daylight itself – and the small distinction in gravity from the pair not being on the similar level. These quantity to some millimetres per second. In apply we’re a bit extra delicate to sideways displacement than lateral back-or-forward displacement. To provide an thought, if the Moon is just a few kilometres nearer or additional away from Earth it doesn’t change a photo voltaic eclipse a lot, but when it strikes sideways an identical quantity then you definitely’d begin seeing extra daylight!”

Fall again in direction of Earth

After six hours the 2 spacecraft are launched from their energetic formation to fall again in direction of Earth on parallel however protected orbits – though a collision avoidance manoeuvre could be routinely triggered if one spacecraft drifts too near the opposite, or if one have been to grow to be defective.

To keep away from such an eventuality, each spacecraft have totally redundant techniques, and their computational hundreds are distributed throughout each platforms to keep away from any danger of decelerate – so as an example whereas the Coronagraph spacecraft oversees the demanding coronal observations, the Occulter performs the relative GPS calculations which assist maintain the spacecraft protected round perigee in addition to the manoeuvres to make and break energetic formation.

Proba-3 is a expertise demonstration mission at the beginning, with coronal observations just one kind of formation flying it is going to try, together with resizing its baseline size, retargeting its orientation, and shut rendezvous.

In the long run, the limiting issue for the mission is predicted to be propellant, with a two-year lifetime forecast. The 2 spacecraft’s low 600 km perigee signifies that they’re forecast to deplete within the environment a scarce 5 years after that.