High-Integrity and Low-Cost Local-Area Differential GNSS Prototype for UAV Applications

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