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  • The proposed FDIR technique is presented in the

    2019-09-20

    The proposed FDIR technique is presented in the following format: Section 2 contains a survey of typical orbits of interest with regard to the cytochalasin d environment found in them, which is contrasted with a LEO orbit. Additionally, how the requirements for the satellite system can be estimated is presented by examining this radiation environment. Section 3 contains a detailed description of the proposed FDIR scheme, including the designs of the current limiters, watchdog timers, special consideration for logic design, and the implementation of more complex FDIR schemes on top of the one presented. Section 4 then analyzes how radiation affects the specific components of the implementation and how the proposed FDIR policy copes with these effects. Finally, Section 5 presents some of the more important performance characteristics, obtained by measuring an implementation of the proposed FDIR policy.
    Space beyond low Earth orbit Space is a harsh environment for any electronic system, but not all parts of space are equally hostile to electronics. For this reason, it is extremely important to be aware of the radiation environment present in the orbit into which a satellite is launched. For this purpose, the SPENVIS online tool (http://www.spenvis.oma.be/) was used for estimating the amount of radiation that would be present in each of the three distinct orbits, which could present the next step for nanosatellites. The estimates were compared to a reference 600km, 97.8° inclination Sun Synchronous Low Earth Orbit (SSO), which is one of the more popular orbits where nanosatellites are presently being launched into at the time of writing of this paper. The three orbits evaluated were: a 0° inclination Geostationary Orbit (GEO), a Geostationary Transfer Orbit (GTO), which was simplified as an elliptical 0° inclination orbit with a perigee of 300km and an apogee of 35,786km, and a Lunar Transfer Orbit (LTO), which was also approximated as an elliptical 0° inclination orbit with a perigee of 300km and an apogee of 384,400km. A Lunar Orbit (LO) was not directly evaluated due to lack of simulation tools and the fact that the radiation present around the Moon is only slightly higher than a LEO with a high radius [5]. Though the transfer orbits\' approximations are not directly applicable for a nanosatellite, due to limited propulsion options a part of a nanosatellite transfer orbit to either Lunar Orbit (LO) or GEO would have to travel through a similar radiation environment [6], at least for part of the way. Other SPENVIS parameters were left at their default settings, with the Total Ionizing Dose (TID) analysis performed on a model with finite aluminum slab shields. It can be seen immediately that, when compared to the reference SSO orbit, all three orbits had drastically increased radiation profiles. Since the proposed approach aimed at retaining one of the primary advantages of nanosatellites – their reliance on COTS components, electronic components on board such a nanosatellite must be shielded by aluminum in order to operate over extended periods of time. How much shielding is required is heavily dependent on the required life-time of the satellite as well as its design. Carefully selected COTS components can survive up to 30krad [7], meaning that for a 3 year mission, a cut-off of 10krad per year is selected. Following the results presented in Fig. 1: at least 1.75mm of aluminum for a LTO, 2.25mm of aluminum for GEO, and 4mm of aluminum for GTO would be required. One method of achieving this aboard a nanosatellite is, during the designing of nanosatellite electronics, to generate 3D models of the electronics, which are then used to mill out a block of aluminum to the required width, which is then fastened to the Printed Circuit Board (PCB) which also houses the electronic components. Though this could adversely impact the mass budget, it should be noted that a 4mm shield of size 10cm by 10 cm weighs only 108g (Fig. 2, Fig. 3). Further, due to the stackable nature of nanosatellite electronics, only the two boards at the extreme ends of the satellite would need to be shielded by the full amount. The details of how the internal nanosatellite electronic boards are affected are presented in [8], [9], [10], where it can be seen that the internal boards receive the full amount of radiation only at the edges, and even there it is reduced when compared to the boards in the extreme positions, meaning that less shielding is needed for them.