Inside Unmanned Systems provides actionable business intelligence to decision-makers and influencers operating within the global UAS community. Features include analysis of key technologies, policy/regulatory developments and new product design.
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62 unmanned systems inside June/July 2016 AIR UAS RELIABILITY I n manned aircraft, what is considered the maximum acceptable rate is derived from requirements established by the certifying agencies. For example, in the United States the Federal Aviation Administration established the requirements for certification of manned aircraft systems. Even though it is an over sim- plification, for the discussion here, it is safe to say the requirements for the maximum acceptable catastrophic failure rate are between 10 -9 and 10 -6 failures per flight hour. This means a GNC system on a manned aircraft must be reliable enough to ensure a catastrophic failure of the entire aircraft occurs with a probability of less than once in 10 6 and 10 9 hours of flight. These numbers are below the lifetime of a commercial aircraft. It is not clear whether these numbers should be applicable to UAS. This is a question that will have to be determined or decided by all the stake holders in UAS operations. What we are interested in here is to determine how to de- sign GNC systems once we know what the tar- get reliability figure or acceptable failure rate is. This, of course, is the answer to the second ques- tion we posed earlier. Reliability Via Physical Redundancy Designing a GNC system with overall reliability better than 1 failure per 10 6 to 10 9 f light hours implies the components making up the system have reliability figures better than that. Find- ing sensors and components whose reliability is such that when they are integrated into a larger GNC system, the overall failure rate will be less than 10 6 or 10 9 is difficult. Sensors with the requisite level of reliability would be exor- by Inchara Lakshminarayan, Raghu Venkataraman, Daniel Ossman, Peter Seiler and Demoz Gebre-Egziabher DESIGNING RELIABILITY INTO SMALL UAS AVIONICS: part 1 Introduction If current projections of the utility of small Unmanned Aircraft Systems (UAS) (or "drones" as they are referred to in the popular press) are cor- rect, then they will be common place sights in urban and rural skies. They will be in the suburban skies delivering packages to homes or as couriers in the urban downtown areas. In rural settings they will be around as precision agriculture platforms or for remote inspection of infrastruc- ture such as power lines or railroad tracks. The projected ubiquity of the vehicles has important social and legal implications that will have to be addressed. While a significant portion of the discourse associated with UAS has focused on these social and legal issues, their implication on the engineering design of drones is a topic that has not received as much attention. The purpose of this article is to explore, albeit brief ly, one im- plication ubiquity has on the design of guidance, navigation and control (GNC) systems of small aerial vehicles (class 1 and 2 UAS). For convenience and without a loss of generality, let us anchor our discus- sion to one particular application of UAS: package delivery in urban settings. A small UAS used in this application will be expected to navigate in and above urban canyons. It will have to do this without colliding with build- ings, power lines or other UAS. Faults in the guidance, navigation or control systems can lead to collision. These faults can either be the result of hard- ware failure or the result of software and algorithmic shortcomings. Any engineered system will not be fault-free and, thus, it is unrealistic to expect 100% mishap free operations of UAS. However, we can and should try to minimize the likelihood of GNC system faults that can lead to collisions to a level considered acceptable by society at large. To do this we need to answer, at least, the following three questions: (1) What is the maximum acceptable failure rate?, (2) How do we design GNC systems to meet such a failure rate requirement?, and (3) How do we prove a given GNC system design meets the failure rate requirement? These are not new questions and designers of GNC system used on manned aircraft or large aerospace systems (e.g., mis- siles, satellites, etc.) have addressed or dealt with them in the past. Therefore, before proceeding to answer these questions for UAS it might be instructive to see how these questions have been answered or dealt with by those who design GNC systems for manned aircraft. Reliability in Manned Aircraft