May 14, 2012
ESA Fixes Proba-1 Satellite
Lee Rannals for RedOrbit.com
Proba-1 has been in hibernation since last winter, and has been working and orbiting around the Earth for more than a decade.
The space agency said that new software from the Technical University of Denmark (DTU) allows the satellite to distinguish between genuine star constellations to measure its pointing direction versus clusters of radiation-induced "hotspots."
“The result is that Proba-1´s startracker is practically rejuvenated,” Frederic Teston, head of ESA´s Proba program, said in a press release. “The mission is back in business.”
Proba-1 was designed for autonomous operations when it entered low-Earth orbit on October 22, 2001.
Grounder controllers at ESA only need to supply geographical coordinates of a ground target, and the micro satellite will steer itself into the appropriate position.
Proba's agility derives from its Advanced Stellar Camera, an autonomous CCD-based startracker that has a computer which uses star constellations to assess the satellite's altitude and rotation.
Years of continuous use eventually lead the micro satellite to build up bright points on the CCD, which began to camouflage actual stars.
“Such hotspots correspond to single pixels, while stars extend beyond single pixels exhibiting lens effects,” Troelz Denver, associate professor at DTU, said. “The challenge comes as radiation hits accumulate, having two, three or more adjacent pixels impaired. Proba-1 ended up perceiving three to four times more hotspots clusters than stars.”
The radiation effects show up even stronger on the CCD as the temperature increases, so when Proba-1 when into hibernation, the intensity of sunlight on the satellite increased by 10 percent because of the Earth's position in orbit around the Sun.
The satellite was originally developed as a technology demonstrator with a two-year lifetime, but it has evolved into an operational Earth observation mission. So, after the satellite's hibernation scientists around the world were inconvenienced due to its faulty startracker.
Denver said ESA performed maneuvers to help decrease the startracker temperature, while they also supplied imagery from the instrument for testing.
“The new algorithms developed made use of the fact that Proba-1´s ASC has two camera heads —preventing the Sun or Earth blocking out views of the stars along a single direction," he said. “When one head has difficulties isolating the stars from the hotspot clusters, it´s now being assisted by the other.”
The algorithms are also available for use on ASC versions that have flown on ESA's Smart-1 Moon mission, the Sun-monitoring Proba-2, the gravity-mapping GOCE and the upcoming Swarm satellite constellation.