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The European Space Agency\u2019s Proba-3 mission has achieved its ambitious goal when its two spacecraft, the Coronagraph and the Occulter, flew 150 metres apart in perfect formation, simulating a single giant spacecraft.<\/p>\n
Earlier this year, the first step of the mission was successfully completed. The operations team, consisting of engineers from ESA and its closely collaborating industrial partners, came together at the Agency\u2019s European Space Security and Education Centre in Redu, Belgium.<\/p>\n
Using a set of positioning instruments, they were able to align the two spacecraft in formation, and monitor them as they maintained their relative position autonomously.<\/p>\n
Now, following more finetuning and testing, the team has achieved the desired precision, making Proba-3 the world\u2019s first-ever precision formation flying mission.<\/p>\n
The mission relies on several innovative technologies, many of which are technology demonstrations developed through ESA\u2019s General Support Technology Programme (GSTP). \u201cTo do something that has never been done before, we needed to develop new technologies,\u201d notes Esther Bastida Pertegaz, Proba-3 systems engineer.<\/p>\n
\u201cThe formation flying is performed when the spacecraft are more than 50\u00a0000 km above Earth,\u201d explains Raphael Rougeot, Proba-3 systems engineer.<\/p>\n
\u201cHere, the Earth\u2019s gravity pull is small enough, so that very little propellant is needed to maintain the formation. Then the formation is broken and needs to be acquired again over the next orbit, in a repeated cycle.\u201d<\/p>\n
The ultimate goal is for the two spacecraft to 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.<\/p>\n
Teodor Bozhanov, formation flying system engineer, explains further: \u201cThe initiation of this formation-flying repetitive sequence is performed by the ground control centre, with the operations team obtaining position information to determine the exact location of the two satellites in space. The mission\u2019s thrusters are then used to bring them closer together.<\/p>\n
\u201cAll the rest is done autonomously. The spacecraft measure and control their relative position using the Visual Based System, which includes a wide-angle camera on the Occulter tracking a set of flashing LED lights on the Coronagraph.<\/p>\n
\u201cOnce the satellites get close enough to each other, a narrow-angle camera locking onto the same set of lights enables a more accurate positioning.\u201d<\/p>\n
Raphael describes the last step needed to close the precision gap: \u201cAlthough we were previously able to perform formation flying using only the camera-based systems on board, we were still missing the desired precision.<\/p>\n
\u201cTwo major achievements have been key to unlocking it. First, it was the calibration of the on-board laser instrument, and its integration into the full formation flying loop.\u201d<\/p>\n
\u201cThis laser instrument, called the Fine Lateral and Longitudinal Sensor (FLLS), enables relative positioning down to a millimetre accuracy,\u201d adds Jorg Versluys, Proba-3 payloads manager. \u201cIt consists of a laser beam fired from the Occulter spacecraft and reflected in the Coronagraph\u2019s retroreflector back to the Occulter, where it is detected.\u201d<\/p>\n
\u201cThe second crucial achievement was successfully using the shadow position sensor,\u201d Raphael continues. \u201cAn on-board algorithm based on the measurement of light intensity around the coronagraph aperture ensures that the Coronagraph spacecraft stays in the shadow cast by the Occulter spacecraft.\u201d<\/p>\n
Esther notes: \u201cCombining all these sensors, and thanks to the on-board software managing all the spacecraft systems and providing Navigation, Guidance and Control functions, the formation is stable beyond expectations.\u201d<\/p>\n
Damien Galano, Proba-3 project manager, concludes: \u201cWe are talking about millimetric accuracy in range, and sub-millimetric in the lateral position. We can\u2019t wait to see the completion of the instrument calibration and the first processed image of the Sun\u2019s corona.\u201d<\/p>\n<\/p><\/div>\n