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64 unmanned systems inside June/July 2016 AIR UAS RELIABILITY systems and algorithms can make the problem of proving compliance with reliability requirements difficult. That is, the third question posed in the last paragraph of the introduction becomes much more difficult to answer when analytical redun- dancy is involved. The answer to the third ques- tion can be a subject of an entire separate article and, as such, will not be pursued further. Current UAS Reliability Before we proceed to provide examples of ana- lytical redundancy in the design of GNC systems for UAS, it might be useful to get a sense of the level of reliability of these vehicles today. That is we should answer the question "what is the state of reliability of current off-the-shelf UAS?"This is not a question that has a simple answer as it depends not only on the vehicles, but also the users, the operation in which they are used and many other factors. In the following paragraphs we will attempt to provide an answer to this question by presenting the results of an assess- ment performed to determine the reliability of a UAS. 1 While this is not the definitive answer to this question, it is one of few publically available studies we are aware of attempting to quantify off-the-shelf UAS reliability. The vehicle assessed was an UltraStick 120 aircraft, a photo of which is shown in Fig- ure 1. The Ultra Stick 120 is a commercial, off-the-shelf, radio-controlled aircraft with a wingspan of 1.92 meters and a mass of about 8 kilograms. It is a standard remote control aircraft which uses off-the-shelf electronics in the construction of its GNC system. It has the normal complement of aircraft controls, which includes: a pair of independent but dif- ferentially operated ailerons; a pair of indepen- dently operated f laps; a single rudder; a pair of elevators that are mechanically linked and op- erate in unison; and an electric motor and its associated electronic speed controller (ESC). The reliability analysis considered a line-of- sight operation where, if a failure occurs and is detected in time, a human operator can take over and land the UAS. This is a much less de- manding operation than beyond line-of-sight op- erations where the luxury of human intervention after a failure is less likely. Accordingly, system failures were broken into two categories. The first category comprised those faults that were deemed catastrophic and will likely result in loss or damage of the UAS and possibly other prop- erty. They were identified by the moniker "Type 1" faults. The second category are those faults which, if occurred, would prevent the UAS from accomplishing its mission but would not result in a catastrophic loss of the vehicle or damage to property. These were "Type 2" faults. An as- sumption being made with these Type 2 faults is that after they occur, a human operator will take over the operation of the UAS and perform a successful landing. Table 1 summarizes the results of this reliability assessment. As shown in Table 1, Type 1 failures are further divided into sub-categories of 1A and 1B. Type 1A failures are those where the UAS con- Table 1: Failure modes and likelihoods for the UltraStick 120 UAV. FAILURE TYPE RATE OF OCCURRENCE PER FLIGHT HOUR GNC SYSTEM COMPONENT CONTRIBUTING MOST TO FAILURE RATE Type 1A 2.17 x 10-2 Servos (Rudder & Elevator) Type 1B 2.14 x 10-2 Propeller/Motor Type 2 2.32 x 10-2 IMU and GPS Figure 2. A HITEC HS-225BB servo from the outside and inside. One common failure mode for these servos is an inoperable gear train due to wear or stripping of gear teeth.