Title: High-Integrity and Low-Cost Local-Area Differential GNSS Prototype for UAV Applications
Author(s): Dongwoo Kim, Jinsil Lee, Minchan Kim and Jiyun Lee, Sam Pullen
Published in: Proceedings of the 30th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2017)
September 25 - 29, 2017
Oregon Convention Center
Portland, Oregon
Pages: 2031 - 2054
Cite this article: Kim, Dongwoo, Lee, Jinsil, Kim, Minchan, Lee, Jiyun, Pullen, Sam, "High-Integrity and Low-Cost Local-Area Differential GNSS Prototype for UAV Applications," Proceedings of the 30th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2017), Portland, Oregon, September 2017, pp. 2031-2054.
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Abstract: As civilian use of unmanned aerial vehicles (UAVs) increases, safe operation of UAVs while preventing collisions with either humans or ground structures has become a significant concern. To perform autonomous UAV missions Beyond Visual Line-Of-Sight (BVLOS) or in low-altitude airspace safely, achieving high accuracy and reliability of navigation solutions is required. This motivates the development of a cost-effective local-area UAV network that utilizes a Local-Area Differential Global Navigation Satellite System (LAD-GNSS) navigation solution [1, 2]. LAD-GNSS achieves a level of integrity comparable to that of Ground Based Augmentation System (GBAS) Category I operations by monitoring navigation faults at the ground station and by broadcasting integrity information to the UAV [3]. The architecture of this system involves space-conserving hardware configurations and several simplified GBAS integrity monitoring algorithms to reduce both the cost and the complexity of the system. LAD-GNSS is designed to support UAVs with a minimum operating altitude of either 50 ft plus obstacle height (within 5 km of the ground facility) or 200 ft (within 20 km of the ground facility) by providing an accurate position solution and a tight uncertainty bound on its position error. A prototype of LAD-GNSS has been developed and evaluated for both accuracy and integrity performance. The key ground and airborne functions of this system are shown in Figure 1 below. One notable characteristic of this prototype is that it utilizes a two-way datalink between the ground facility and the airborne user, which provides a major improvement in system flexibility. The two-way datalink enables the system not only to allocate integrity risk to each fault hypothesis dynamically to obtain the minimum safe protection level [4] but also to simplify the geometry screening needed to mitigate ionospheric anomalies by computing the maximum error in vertical position (MIEV) only for the satellites known to be tracked by each UAV. Specifically, each UAV continuously sends its GNSS measurements to the ground station, so that error corrections and integrity information can be generated by the ground station just for this known satellite geometry. This information is then broadcast back to the UAV to allow it compute its position solution. The integrity status of each UAV, including its current protection levels, is maintained by the ground facility and is used to guide each vehicle while maintaining safe separation from nearby obstacles and other UAVs. The LAD-GNSS prototype is composed of two parts: a ground module and an onboard module. The ground module operates in a manner similar to a GBAS ground facility. Most of the computations regarding integrity monitoring are performed in the ground module. The onboard module computes position solutions using the corrections broadcast from the ground module. The position solutions are then fed into a flight controller, which is the 3DR Pixhawk. The hardware components of the prototype for the ground module and the airborne module are described as follows. For the ground module, the equipment includes a single pair of NovAtel OEM-V3 receivers and a NovAtel GPS 703 GGG antenna (choke ring type) that can receive L1, L2 and L5 GNSS signals. An Intel NUC mini-PC is used to perform integrity monitoring calculations. For the onboard module, the same receiver model as one of the ground module is mounted on the octocopter UAV platforms. A NovAtel compact GNSS antenna is used for the onboard antenna. Due to the limited UAV payload capacity, a small single-board Raspberry Pi processor, which is capable of performing just the positioning calculations, is loaded on the UAV instead of a mini-PC. An Xbee Pro1 S1 modem, which can support communications over a 1-mile range, is provisionally used for the communication link. The software for both the ground and airborne modules runs in the Linux C language. Flight tests were conducted to evaluate the performance of this LAD-GNSS prototype in Yeongwol. Yeongwol has been designated by the government of South Korea as a permitted area of 95 km^2 for UAV flight tests. The flight paths chosen were designed to cover areas of interest considering practical applications of UAVs, such as agriculture, surveying, mapping, and reconnaissance. The distance of the designed path above the sparsely populated area is 1.5 km. The true trajectory of the UAV was derived by post-processing based on double differenced carrier-phase measurements. The Navigation System Error (NSE) of the system was then computed as the difference between the true UAV path (from post-processing) and the path estimated by LAD-GNSS. While LAD-GNSS can be the primary source of navigation for UAVs, other navigation sensors are required to provide reliable navigation in case of GNSS signal interference or blockage. In this study, the prototype has been provisionally extended to incorporate multiple sensors to assure complete UAV navigation system safety. With the inclusion of multiple sensors, a new fault hypothesis, which is a multi-sensor failure, is added to the existing LAD-GNSS integrity fault tree. The onboard module integrates Inertial Measurement Unit (IMU) sensor output with the LAD-GNSS solution using a Kalman filter (KF). A KF innovation-based fault detection algorithm is developed to protect the UAV from IMU sensor faults. In addition, optimal protection levels are computed by allocating the newly introduced multi-sensor integrity risk dynamically together with the LAD-GNSS integrity risk. This allocation to potential multi-sensor failures would change depending on sensor type and quality. Thus, optimal allocation is essential and would be effective for the proposed multi-sensor integrated system. The extended prototype was also tested at the Yeongwol test site, and its performance was compared with the flights that used LAD-GNSS only. References: [1] S. Pullen, P. Enge, and J. Lee, "High-Integrity Local-Area Differential GNSS Architectures Optimized to Support Unmanned Aerial Vehicles (UAVs)," Proceedings of ION ITM 2013, San Diego, CA, Jan. 28-30, 2013. [2] M. Kim, K. Kim, J. Lee, and S. Pullen, "High Integrity GNSS Navigation and Safe Separation Distance to Support Local-Area UAV Networks," Proceedings of the 27th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2014), Tampa, Florida, September 2014, pp. 869-878. [3] M. Kim, J. Lee, D. Kim, J. Lee, and S. Pullen, “Design of Local Area DGNSS Architecture to Support UAV Networks: Optimal Integrity/Continuity Allocations and Fault Monitoring,” Proceedings of ION PNT 2017, Hawaii, May 2017. [4] B. Pervan, S. Pullen, and J. Christie, “A Multiple Hypothesis Approach to Satellite Navigation Integrity,” Navigation: Journal of the Institute of Navigation, Vol. 45, No. 1, Spring 1998, pp. 61-84.