Contents lists available at ScienceDirect Acta Astronautica journal homepage: www.elsevier.com/locate/actaastro Nanosatellites in LEO and beyond: Advanced Radiation protection techniques for COTS-based spacecraft David Selčan ⁎ , Gregor Kirbiš, Iztok Kramberger Faculty of Electrical Engineering and Computer Science, University of Maribor, Smetanova ulica 17, 2000 Maribor, Slovenia ARTICLE INFO Keywords: Nanosatellites FDIR COTS components SEE Fault tolerant logic Over-current protection ABSTRACT This paper presents an approach for implementing radiation protection FDIR (Fault Detection, Isolation and Recovery) techniques designed especially for nanosatellites, capable of ensuring reliable operation in harsh orbits using COTS (Commercial off the Shelf) components. The radiation environment, as encountered by nanosatellites utilizing Flash-based FPGAs in orbits higher than Low Earth Orbit, is analyzed, primarily focusing on SEE (Single Event Effects). In order to assure reliable operation, the FDIR policy is split into two levels: the Low Level FDIR which ensures that no permanent damage occurs to the satellite's electronics, which then allows the use of a High Level FDIR tasked with maintaining high availability. A hierarchical approach, consisting of three types of current limiters in combination with watchdog timers and fault tolerant logic implemented inside a flash-based FPGA is proposed for the Low Level FDIR. The impacts of various radiation- induced faults are analyzed with respect to how the FDIR techniques mitigate them. The proposed current limiters and watchdog timers have been implemented and evaluated for suitability of use with the hierarchical FDIR policy. In order to decrease the impacts on the size and weight footprints, the current limiters were implemented as stacked 3D modules. 1. Introduction In recent years a new trend has emerged in the design and verification of spacecraft. Rapid advances within the field of integrated electronics have enabled the use of inexpensive and highly performant electronic components, which have also found their way into the space industry. These so-called Commercial off-the-Shelf (COTS) compo- nents allow the constructions of spacecraft, specifically satellites, at significantly lower costs and development times. This has allowed small, interwoven teams (as typically found within university environ- ments) to design nanosatellites from the initial concept stages to the final stages within drastically reduced time frames. These nanosatel- lites typically have a mass of less than 10 kg and occupy a volume of less than 8.4 dm 3 . Indeed, the popularity of this approach cannot be denied, with more than 90 different nanosatellites being launched in 2013 alone, with the number expected to increase even further in the following years [1]. Though their roots lie in university-based education and technology demonstration missions, their usages since their inceptions have evolved into including science, remote sensing, telecommunica- tion, and even commercial interests [2]. Indeed, perhaps the most important aspect of the nanosatellite approach is the possibility of launching a multitude of nanosatellites as a single satellite constella- tion, whereby these tasks previously thought of as too expensive could be accomplished (e.g. on-demand remote sensing, global monitoring, other real-time satellite applications). Unfortunately, due to the history of nanosatellite development (university based mission with limited funding) and their format (severe mass and size constraints for most parts), little effort has been invested increasing their reliability. Specifically, most nanosatellites today feature little redundancy. Additionally, due to heavy uses of COTS components, they are typically unprepared for operating within an environment that includes higher amounts of radiation. The operations of nanosatellites within a relevant environment are also not usually verified thoroughly. Though these limitations have already resulted in the failure of a couple of nanosatellite missions [3], their use in primarily Low Earth Orbit (LEO) meant that most missions proceeded without major problems even with the potential lack in reliability. However, talks are already underway to bring the nanosatellite platforms along even further by using them for interplanetary missions. In order for nanosatellites to still be practical beyond LEO, where the radiation environment and operational constraints are much harsher, one method would be to modify their designs to be more in line with how http://dx.doi.org/10.1016/j.actaastro.2016.11.032 Received 6 October 2015; Accepted 21 November 2016 ⁎ Corresponding author. E-mail address: david.selcan@um.si (D. Selčan). Acta Astronautica 131 (2017) 131–144 0094-5765/ © 2016 IAA. Published by Elsevier Ltd. All rights reserved. Available online 27 November 2016 crossmark