Browsing by Author "Sun, Kerry"
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Item Characterizing Phase-Center Motion of GNSS Antennas Used in High-Accuracy Positioning(Center for Transportation Studies, University of Minnesota, 2019-06) Dave, Aditya; Saborio, Ricardo; Sun, Kerry; Sainati, Robert; Gebre-Egziaher, Demoz; Franklin, RhondaEmerging transportation applications require positioning solutions with accuracy of a few centimeters. Current Global Navigation Satellite Systems (GNSS), such as GPS, GLONASS, and Galileo are, in some instances, capable of providing this level of accuracy. Real-Time Kinematic (RTK) techniques can generate solutions accurate to a few centimeters in a given locale. Precise Point Positioning (PPP) techniques promise to deliver RTK-level performance on a global scale. Even though low-cost, RTK-capable GNSS receivers are available today, antennas are a key component affecting quality of the positioning solution. Unless coupled with a high-quality (thus, more expensive) antenna, a low-cost receiver may not provide the centimeter-level accuracy needed for a safety-critical transportation application (e.g., autonomous vehicle, driver assist systems, etc.). Stability of the antenna phase-center is dependent on the antenna quality and can potentially move on the order of tens of mm if not centimeters. The purpose of the work reported here was to characterize the nature of this motion as a function of antenna quality. Anechoic chamber tests were performed using one high-cost and another low-cost GNSS antenna. The selected antennas represented “book ends” on the cost spectrum. These experiments showed that phase-center motion on the low-cost antenna can be a factor of four times larger than on high-quality antennas. Since anechoic chamber tests are not practical for each antenna installation in transportation applications, methods for antenna-specific, in-situ, phase-center motion calibration (modelling) methods have been suggested. Preliminary results suggested efficacy of these in-situ methods.Item Observability and Performance Analysis of a Model-Free Synthetic Air Data Estimator(Journal of Aircraft, 2019) Sun, Kerry; Regan, Christopher D; Gebre-Egziabher, DemozThe performance, accuracy, and observability of a model-free angle of attack and angle of sideslip estimator are presented. The estimator does not require an aircraft dynamic model; rather, it only relies on measurements from a GPS receiver, an inertial measurement unit, and a pitot tube. The estimator is an inertial navigation system (INS)/GPS extended Kalman filter augmented with the states to account for wind and an additional measurement from a pitot tube. It is shown that the estimator is conditionally observable. Conditions (maneuvers) that enhance its observability are identified. A bound on the angle of attack and angle of sideslip estimate uncertainties is derived. The effect of INS/GPS, horizontal and vertical wind uncertainty on the accuracy of angle of attack and angle of sideslip estimate is assessed. Simulation and flight-test results of the method are presented. The results show that the 1−σ bound on a small, slow-flying unmanned aerial vehicle for angle of attack and sideslip angle estimates are about 5 and 3 deg, respectively.Item Reliable Air Data Solutions For Small Unmanned Aircraft Systems(2020-07) Sun, KerryThis dissertation examines the problem of increasing Air Data System (ADS) reliability for small Unmanned Aerial Systems (UAS). A reliable ADS is required for the safe operation of aircraft; traditionally, a hardware redundant ADS design has been used. However, hardware redundancy is not feasible in small UAS since reliable ADS are expensive, and many small UAS have more stringent size, weight, and power constraints. The impracticality and limitations of hardware redundancy have motivated research in the last decade to identify alternatives to traditional ADS. In particular, estimating air data quantities, often denoted as Synthetic Air Data System (SADS), has become a viable strategy of interest. This dissertation examines the use of SADS to increase ADS reliability for small UAS. The key challenges associated with increasing ADS reliability in UAS are examined. First, calibrating low-cost air data sensors for small UAS is not well addressed in the open literature. Most existing calibration techniques do not work well with small UAS operating at low airspeeds, especially when the effects of wind cannot be ignored. This dissertation develops a method for calibrating a 5-hole probe sensor applied on small UAS using only using data from an IMU and GNSS. Second, many SADS use the aerodynamic parameters to help estimate air data quantities, but the accurate aerodynamic model is often unavailable. This dissertation proposes a model-free SADS which allows estimating angle of attack and sideslip without the need for an aerodynamic model. The performance and observability of this SADS are tested using both simulation and flight data. In addition, the problem of ADS integrity is addressed by systematically designing and analyzing the performance to ensure that it satisfies probabilistic continuity and integrity certification requirements. An ADS Fault Detection and Isolation (FDI) algorithm to detect and mitigate the effect of realistic Pitot tube failure modes is designed. The approach used is the Integrity Monitoring framework, which has been used successfully with GNSS-based precision landing systems for commercial aircraft. The FDI algorithm is validated with a flight data set in which a Pitot tube failed due to water blockage.