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57 unmanned systems inside August/September 2016 ugust/Septe August/September 2016 August/September 2016 be August/September 2016 0 August/September 2016 6 August/September 2016 ENGINEERING. PRACTICE. POLICY. feedback control can be used to stabilize an aircraft even under extreme fault scenarios, such as a single functional control surface. Re- searchers at the UAV Lab are currently work- ing toward demonstrating safe landing with one functional control surface. Analytical Redundancy in Navigation System Similar to what was done for the actuation part of the control system, navigation and attitude determination systems can be redesigned so they are robust to failed sensors. This has been the subject of prior publication by the authors of this article. Readers wishing to learn more about the details for how to design such a navi- gation system should refer to the papers listed at the end of this article. Accurate attitude (or three-dimensional ori- entation) estimates are key for control of small UAS. One of the standard approaches used for attitude determination in small UAS applica- tions is that of integrated IMU/GPS system. In these systems, a low cost IMU (consisting of a triad of rate gyros, accelerometers and magne- tometers) is used to mechanize an inertial navi- gation system (INS). This INS provides attitude, position and velocity estimates at a high rate (> 10 Hz). The drift errors inherent to the INS solu- tion are kept in check by periodic position up- dates form GPS. The reliability study described earlier showed IMU are the "weak link" for Type 2 failures. On the other hand, while the GPS receiver/antenna pair were more reliable, they had more different failure modes. Thus, in what follows we consid- er GPS failures. Whenever GPS is unavailable or becomes unreliable (e.g., temporary multi- path errors due to ref lections from buildings close by) the attitude errors will grow. To deal with such outages an alternate solution that relies on Attitude Heading Reference System (AHRS) can be mechanized. This alternate solution fuses the attitude estimates generated by integrating a triad of rate gyros with aiding measurements from an air data system, accel- erometer and magnetometer triad. The INS/GPS and AHRS solutions are two parallel or analytically redundant attitude so- lutions. Figure 7 is a block diagram showing the overall architecture of an analytically re- dundant attitude solution that was tested on the Goldy FCS shown in Figure 5. The redun- dant attitude solutions are denoted Attitude Solution #1 and #2 in Figure 7. Thus, in lieu of having two GPS receivers, a single receiver with "smart" algorithms can be used to switch between these two solutions and, thus, provide analytical redundancy for attitude. Conclusion If small UAS are going to be ubiquitous in urban and rural skies, they will have to be very reliable. Failures of the avionics used to guide, navigate and control them can lead to collision. Collisions with other UAS or infrastructure will have, at a minimum, economic consequences. More impor- tantly, however, they can be hazardous to persons in and around the UAS when a collision occurs. The traditional approach to increasing avionics reliability by using physical redundancy is not al- ways a viable solution in SWAP constrained UAS. This means a combination of physical and analyti- cal redundancy must be used. References 3. J. Chen and R. J. Patton, Robust model-based fault diagnosis for dynamic systems, Kluwer academic publishers, 1999. 4. I. Lakshminarayan, Model based fault detection for low-cost UAV actuators, Thesis, University of Minnesota, 2016. 5. R. Venkataraman and P. Seiler, "Safe Flight Using One Aerodynamic Control Surface," in AIAA Science and Technology Forum and Exposition, San Diego, 2016. 6. T. Layh and D. Gebre-Egziabher, "A Fault-Tolerant, Integrated Navigation System Architecture for UAVs," in ION International Technical Meeting, Dana Point, CA, 2015. 7. T. Layh, J. Larson, B. Taylor and D. Gebre-Egziabher, "A Recovery System for SUAV Operation in GPS-Denied Environments Using Timing Advance Measurements," in ION International Technical Meeting, Dana Point, CA, 2015.