68 Insight | Space vehicles This has come primarily through supplying its fibre optic gyroscopes (FOGs) for GEO and LEO (low Earth orbit) applications, although many of its quartz (silicon dioxide) accelerometers are used in orbital satellites as well, and both sets of products also deliver inertial data in launchers and interplanetary space missions. Exail’s clients in the space industry include Arianespace’s Ariane 5 and 6 launcher rockets, the ESA’s Aeolus, Sentinel 2, Solo and MTG programmes, and the ExoMars mission planned for launch in 2028. Exail says that through these various projects, its FOGs and accelerometers have amassed more than 6 million hours of orbital flight without incident. Over the past 20 years, the Astrix 90 and 200 FOGs have been supplied for space applications by Airbus through its partnership with Exail (with both solutions embedding Exail’s technologies). More recently however, Exail has developed its own space gyroscope, the Astrix NS, to address the new challenges faced by the space industry. The Astrix NS is aimed at newer spacecraft like SmallSats being produced at volume. It weighs 1.4 kg, consumes up to 7 W until end of life (EOL) with an angular random walk (ARW) of 0.005o/√hour, as well as an EOL bias stability of 0.2o/hour (less than 0.02o/hour at start of life). All of Exail’s FOGs are interferometric systems, and their operating principles are the same as its terrestrial FOGs, albeit each part being qualified for use in space according to ECSS (European Cooperation for Space Standardisation) rules. The qualification requirements are applied to the company’s optical fibres, diodes, pottings and integrated optical chips, along with its PCBs and all other electrical, electronic and electromechanical components as defined by the ECSS. “To prevent problems arising from the vacuum of space, we have a team dedicated to the thermal design of the system,” de Toldi says. “One big problem in vacuum conditions is that we can’t dissipate heat from our components, as there’s no air, so we need to transfer it through the base mounting plate. “The other main problem is outgassing, which is really bad for Earth observation satellites as gases emitted from components can have an impact on their optical elements, but the gyros need to be near the optical payloads to minimise interference in attitude readings. Our circuit boards and everything used in potting and adhesives are most at risk of outgassing, so a lot of work goes into selecting the right components for that application.” To achieve product lifetimes for the required number of years, reliability analyses are performed in-house to identify ways to prolong their MTBFs, including installing triple redundancies in components and using more than one inertial unit on each craft. “More recently though, we’ve been running analyses using the FIDES method,” de Toldi adds. “This is a guide for calculating reliability estimates for electronic systems, and based October/November 2023 | Uncrewed Systems Technology Exail’s FOG inertial sensors are used on the Ariane 6 rocket, among other spacecraft (Courtesy of Arianespace)
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