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\t\t\t\t\t\t10\/12\/2024<\/span> Satellites in orbit can become absentminded. Space radiation randomly flips onboard memory bits, potentially corrupting mission data and functionality. To better understand how this invisible onslaught works in practice, a team systematically analysed how radiation affected the memories of ESA\u2019s three Swarm spacecraft during a decade of mapping Earth\u2019s magnetic field.<\/p>\n<\/div>\n The resulting paper, from an ESA and Airbus\u00a0team, was awarded Best Data Workshop paper at the recent European radiation effects conference, RADECS 2024. Its findings also hold relevance for other missions employing the same mass memory hardware, including Copernicus Sentinel-6, also in Earth orbit, and BepiColombo, headed to Mercury through deep space.<\/p>\n The space beyond Earth is filled with radiation: high-energy particles originating from the Sun, belts of protons, electrons and ions trapped within Earth\u2019s magnetic field and an exotic menagerie of \u2018cosmic rays\u2019 \u2013 also particles, despite their name \u2013 which are shot inwards from far beyond the Solar System.<\/p>\n<\/p><\/div>\n As these particles traverse through satellite components, they generate transient electric charges which can lead in turn to \u2018Single Event Upsets\u2019 \u2013 single or multiple memory bit flips \u2013 as well as more serious interrupts, \u2018stuck-bits\u2019 and ultimately \u2018latch-ups\u2019 \u2013 a destructive runaway short circuit.<\/p>\n ESA is a leader in the field of radiation effects and mitigation. Experts design in countermeasures such as radiation shielding for key components and \u2018error detection and correction\u2019 systems which regularly check for any disruption then put it right.<\/p>\n<\/p><\/div>\n \u201cDown on Earth\u2019s surface everyday computers are mostly protected from these effects, although big data centres do have to consider them because their sheer scale makes some radiation effects statistically more likely,\u201d explains ESA research fellow Marco Pinto, working in the Agency\u2019s Radiation and Component Reliability Section.<\/p>\n \u201cFor space, our work to mitigate these effects is traditionally guided by radiation software tools \u2013 such as ESA\u2019s own Spenvis, SPace ENVironment Information System\u00a0\u2013 as well as radiation monitors that are flying on various ESA missions in Earth orbit and beyond, extending for instance to the BERM monitor on BepiColombo\u00a0and RADEM, flying on the Juice mission to Jupiter.<\/p>\n<\/p><\/div>\n \u201cThe starting point for our work on Swarm came through our looking into other sources of data, in a project called Conrad, for CONtinuous feedback of RADiation effects in flight.<\/p>\n \u201cSingle event effects do not typically interfere with normal mission operations, but it turns out they are all logged in the mission raw data. So why not investigate them, to compare them to our pre-mission modelling and see how effective our radiation hardness measures have been in practice?\u201d<\/p>\n<\/p><\/div>\n Launched in 2013, ESA\u2019s Swarm mission is not one but three identical spacecraft, flying in formation to acquire three-dimensional maps of variations in Earth\u2019s magnetic field. The study focused on three memory components of each satellite\u2019s On-Board Computer, known to be sensitive to radiation effects, but designed to overcome them.<\/p>\n \u201cSwarm proved a good subject for study because its power and temperature levels remained stable throughout the decade under study, and sustained radiation damage on the component materials \u2013 known as Total Ionising Dose \u2013 remained low throughout. So we could concentrate here on Single Event Upsets throughout the 3327 days under study.\u201d<\/p>\n<\/p><\/div>\n The good news is that the observed error rate turned out to be lower than the pre-flight estimates, although these were based on a worst-case scenario approach.<\/p>\n And the source of these effects appears to differ from current models, with the limited geo-location suggesting the majority of upsets have been triggered by protons encountered within the South Atlantic Anomaly. This is an orbital region possessing higher than usual radiation levels because Earth\u2019s magnetic field dips downward here. The second contribution comes from heavy ions from galactic cosmic rays.<\/p>\n<\/p><\/div>\n \u201cIn fact the highest satellite shows far few effects than the others,\u201d adds Marco. \u201cThis might be due to part-to-part variability \u2013 even in the same batch of components there can always be some variations that might impact performance \u2013 or else local non-uniformities in radiation levels.<\/p>\n \u201cWhat is exciting is we can now apply the lessons learned to the management of Sentinel-6 and BepiColombo, for a start, and we can look into the operational archives of other missions, as a whole new source of data.<\/p>\n \u201cThe most challenging part \u2013 to really turn the data into well understood information \u2013 has been to liaise with the mission operators and check in with the industrial teams that put Swarm together, really working backwards to get a close-up understanding of how the relevant systems work. But the results so far are well worth it!\u201d<\/p>\n<\/p><\/div>\n
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