DEPENDABLE DESIGN OF AN EDUCATIONAL CUBESAT

The heritage of launched educational missions involving CubeSats shows a non-negligible number of failures that causes low level of success of these missions. The workforce is generally inexperienced, the budget is limited, short development time, the use of components off the shelf (with high performance but not designed for hostile operative environment) and reduced and inadequate verification campaigns are the main root causes of this unsuccessful trend. All these factors contribute to a high percentage of dead-on-arrival spacecrafts: 20% of launched CubeSats have never been operative, not even for the first contact with a ground station. Moreover, the infant mortality is relevant. To mitigate the problems, CubeSat developers and integrators have worked on the quality of components and the improvement of the activities made in any phase of the product life cycle. At system level, the trend is to adopt high quality EEE elements, with higher reliability but higher cost. However, high cost, sophisticated instrumentation and tools and long environmental test campaigns are enemy of the educational programmes. One of the solutions to maintain COTS based project, with low budget and fast delivery is to adopt, at system level, dependable architectures. It means introducing redundancies at hardware, data, software level in the design phase and tailoring the procedures and rules for the testing phase. The thesis aims at proposing a set of tools and methods that favour the design and verification of a dependable CubeSat, showing the effectiveness of this solution through a real case study, the new CubeSat 3U developed by the CubeSat team of Politecnico di Torino. The result is an effective process that can be applied even by students to enhance the reliability of their CubeSats. The first guideline is to integrate dependability activities in the early phases of a mission, as they can support the design and required trade-offs. As soon as the system functional analysis is complete and a first architecture is defined, an FMEA (Failure Modes and Effects Analysis) analysis can be performed. The FMEA's purpose is to identify the main ways the system can fail, define the degree of severity of such failures, and assert detection methods and compensating measures for them. The FMEA should help developers to identify the critical functions that the system shall perform to fulfill its mission objectives. The major FMEA drawback is the impossibility to evaluate the effect of combinations of failures. Then, a Fault Tree Analysis (FTA) should be executed. The FTA helps identify the minimum set of combined failures needed to make an undesirable event (such as the loss of a critical function) happen. At this point, the developers can decide which mitigation strategies to apply to reduce the severity and probability of these failures. For COTS, the probability of failure occurrence shall be estimated by experience. Then, it is possible to build risk matrices for the system before and after the mitigation strategies. Reliability Block Diagrams (RBD) are another way to assess the mitigation strategies' impact and to prove the increased system reliability by estimating the reliability of the system before and after applying mitigation strategies. This method is applied to a real educational mission currently under development in the CubeSat team Polito, SILVA, and the resulting dependable design is discussed.