Resilient, Low-Cost Navigation in Urban Environments with GNSS Precise Point Positioning, Inertial Measurement Unit and Precise Clock Sensor Fusion

Abstract

The Precise Point Positioning (PPP) measurement processing technique for Global Navigation Satellite Systems (GNSS) is widely applied in scientific and commercial applications that require sub-metre level accuracy, particularly in areas with few obstructions. PPP is expanding into mass-market applications at the consumer level. However, the technique suffers from the inherent disadvantages of GNSS-based technologies. Obstructed environments, such as urban canyons, downgrade the solution. The study presents a novel solution that provides stronger resilience and improved performance in challenging environments by implementing clock-aided algorithms and augmenting the GNSS receiver with both an inertial measurement unit (IMU) and an external clock as a frequency reference. The PPP algorithm was augmented with clock-specific modifications, including the receiver clock coasting (RCC) and receiver clock modelling (RCM) algorithms. The study began with building a prototype triple-sensor fusion using higher-end hardware and was successfully tested by collecting and processing real-world, kinematic GNSS data. Horizontal rms errors have been reduced from 1.49 m with the traditional dual-sensor solution to 1.14 m with the novel solution. With multiple simulated outages, the horizontal rms errors were reduced from 2.07m to 1.36m using the RCM solution and 0.55musing the RCC solution. To address the requirement of consumer-grade applications, a mass-market-friendly variant using low-cost hardware was built and tested in real urban canyons. The positioning accuracy over time was found to improve by 10–40% inmost datasets when compared to traditional PPP/IMU fusion using identical hardware. In both fusion configurations, three-satellite GNSS solutions were verified. Finally, the study delved into position-domain IntegrityMonitoring (IM) and showed an improved integrity performance of the novel triple-sensor solution. The number of unavailable epochs under the IM criteria was reduced by half. The percentage of safe operations in the cross-track and along-track components, as determined by the criteria, increased from 97.20% and 82.34% to 99.45% and 92.67%, respectively, indicating a more trustworthy solution for safety-of-life applications. The result has proven the novel triple-sensor solution as a viable option in improving the performance and integrity for liability-critical applications such as intelligent vehicles. Further research is recommended to optimise the characterisation of the external oscillators and to achieve deeper integration among the three sensors.