A Novel Avionics Based GNSS Integrity Augmentation System for Manned and Unmanned Aerial Vehicles

摘要

This paper presents the main results of the research activities carried out by the Italian Air Force Flight Test Centre in collaboration with the University of Nottingham and Cranfield University in the area of GNSS Avionics Based Integrity Augmentation (ABIA). This research included design, integration and experimental flight test activities on the MB-339CD, TORNADO and TYPHOON aircraft, as well as the development of a novel approach to the problem of GNSS ABIA for mission- and safety-critical air vehicle applications and for multi-sensor avionics architectures based on GNSS. As soon as the validity of the ABIA concept was established, a prototype system was developed for use in flight test applications. This system is capable of alerting the pilot when the critical conditions for GPS signal loss are likely to occur, within a specified maximum time-to-alert. In this ABIA prototype, the aircraft on-board sensors provide information on the aircraft relevant flight parameters (navigation data, engine settings, etc.) to an Integrity Flag Generator (IFG), which is also connected to the on-board GPS receiver. The IFG can be incorporated into one of the existing airborne computers or can be a dedicated processing unit. Using the available data on GPS and the aircraft flight parameters, integrity signals are generated which are displayed on one of the cockpit displays and sent to an Aural Warning Generator (AWG). At the same time, the deviation from the ideal flight path is computed taking into account the geometry and the tracking status of the available GPS satellites, together with the flight test mission requirements and the information provided by the onboard avionic sensors. Current research is extending the results obtained from flight tests to the design of a more advanced ABIA system suitable for manned and unmanned aircraft applications. Mathematical models have been developed to describe the main causes of GNSS signal outages and degradation in flight, namely: antenna obscuration, multipath, fading due to adverse geometry and Doppler shift. Adopting these models in association with suitable integrity thresholds and guidance algorithms, the ABIA system is able to generate integrity cautions (predictive flags) and warnings (reactive flags), as well as providing steering information to the pilot and electronic commands to the aircraft/UAV flight control system. These features allow real-time avoidance of safety-critical flight conditions and fast recovery of the required navigation performance in case of GNSS data losses. In other words, this novel ABIA system addresses all three cornerstones of GNSS integrity augmentation in mission- and safety-critical applications: prediction (caution flags), reaction (warning flags) and correction (alternate flight path computation).