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\t\t\t\t\t\t02\/12\/2024<\/span> What\u2019s harder than flying a single satellite in Earth orbit? Flying two \u2013 right beside each other, at proximities that would normally trigger collision avoidance manoeuvres.\u00a0<\/p>\n<\/div>\n This is the plan for ESA\u2019s 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 around 150 metres from each other, to a precision equal to the thickness of the average fingernail. So how are they going to manage it?<\/p>\n<\/p><\/div>\n Picturing precise formation flying success <\/b><\/p>\n \u201cESA has flown formation flying missions before, but the distances involved have been measured in the tens of kilometres or more,\u201d explains Damien Galano, Proba-3 mission manager. \u201cProba-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 millimetre for six hours at a time.<\/p>\n \u201cAnd we won\u2019t 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\u2019s brilliant disc entirely, so that the million-times-fainter solar corona will become visible for sustained observation. This will either work or it won\u2019t: that is the challenge we have set ourselves.\u201d<\/p>\n<\/p><\/div>\n High orbit for mission success<\/b><\/p>\n 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\u2019s imperfect shape and air drag up at the top of its atmosphere, along with Earth\u2019s reflected light.<\/p>\n \u201cEarly 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!\u201d recalls ESA\u2019s 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 \u2013 meaning it takes less propellant to shift position.<\/p>\n<\/p><\/div>\n 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 \u2013 or elongated \u2013 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.<\/p>\n Picture it as like a rollercoaster loop the 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 \u2013 and because of this decreased velocity spend more time at the \u2018apogee\u2019 of the orbit than at the bottom. For the lower part of the orbit the pair fly freely along safe paths \u2013 although ready to react if a collision risk emerges. Then as they move up towards apogee the signal is given for them to begin moving into active formation, which takes about two hours.<\/p>\n<\/p><\/div>\n Driverless spacecraft<\/b><\/p>\n Any human oversight of Proba-3\u2019s formation flying would be impractical, not least because of the distances involved \u2013 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.<\/p>\n 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. Instead the mission 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.<\/p>\n<\/p><\/div>\n Sequence of positioning actions<\/b><\/p>\n To begin with, startrackers \u2013 computer-linked cameras that recognise the constellations around them \u2013 chart each spacecraft\u2019s \u2018attitude\u2019, 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.<\/p>\n More is needed to achieve active formation flying, starting with Proba-3\u2019s Vision Based Sensor system. A wide-angle camera is used to track an LED pattern on the other satellite, providing relatively coarse \u2018first glimpse\u2019 information on the satellites\u2019 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 centimetre.<\/p>\n<\/p><\/div>\n By itself this is not enough however. Still finer positioning comes via the Fine Lateral and Longitudinal Sensor (FLLS) on Proba-3\u2019s \u2018Occulter\u2019 spacecraft. This shines a laser towards a corner cube retro-reflector on the face of the \u2018Coronagraph\u2019 spacecraft, which is reflected back in turn to the Occulter. This FLLS provides relative positioning down to millimetre accuracy.<\/p>\n Finally, to ensure a steady lock, a Shadow Positioning Sensor system \u2013 based on photo detectors arranged around the Coronagraph telescope\u2019s 5-cm diameter aperture lens \u2013 ensures the Occulter\u2019s approximately 8-cm diameter shadow remains cast correctly on all sides. Any discrepancy triggers a correction.<\/p>\n 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.<\/p>\n<\/p><\/div>\n Flight leader and wingman <\/b><\/p>\n For manoeuvring, 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 \u2018perigee\u2019 formation. The Occulter follows the Coronagraph\u2019s lead by employing a 10 millinewton cold gas thruster system, emitting tiny puffs of nitrogen akin to tiny fractions of a single human breath.<\/p>\n \u201cDuring the active formation flying phase the cold gas thrusters will make small pulses every 10 seconds,\u201d explains Proba-3 systems engineer Raphael Rougeot.<\/p>\n \u201cThe remaining perturbations we have to contend with are solar radiation pressure \u2013 which is the small but steady push from sunlight itself \u2013 and the small difference in gravity from the pair not being at the same point. These amount to a few millimetres 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 kilometres closer or further away from Earth it doesn\u2019t change a solar eclipse much, but if it moves sideways a similar amount then you\u2019d start seeing more sunlight!\u201d<\/p>\n<\/p><\/div>\n Fall back towards Earth<\/b><\/p>\n After six hours the two spacecraft are released from their active formation to fall back towards Earth on parallel but safe orbits \u2013 although a collision avoidance manoeuvre would be automatically triggered if one spacecraft drifts too close to the other, or if one were to become faulty.<\/p>\n 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 \u2013 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 manoeuvres to make and break active formation.<\/p>\n 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.<\/p>\n In the end, the limiting factor for the mission is expected to be propellant, with a two-year lifetime forecast. The two spacecraft\u2019s low 600 km perigee means that they are forecast to burn up in the atmosphere a scarce five years after that.<\/p>\n<\/p><\/div>\n
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