GNSS integrity assessment of an Integrated PNT-Unit in a signal degraded inland water environment

Kurzfassung

Within the framework of the project “Precise and Integer Localisation and Navigation in Rail and Inland water Traffic” (PiLoNav) an integrated Position, Navigation and Timing (PNT) Unit for inland waterways has been developed and implemented through the combination of Global Navigation Satellite Systems (GNSS) receivers and an inertial measurement unit (IMU), where the IMU is used for short-term stabilization and to assure continuous PNT solution in GNSS signal blockage areas. The aim of this integrated navigation system is to provide precise navigation capability with defined levels of accuracy, integrity and continuity. Integrity in GNSS stands for the evaluation of the trust that can be placed in the correctness of the information supplied by a navigation system with or without augmentations. This trust can be increased by using only non-faulty GNSS signals for positioning estimation. GNSS integrity monitoring should on one hand perform self-consistency checks on redundant measurements, and on the other hand, trigger timely and valid warnings to users when it must not be used for the intended operation. Therefore, the implementation of a Receiver Autonomous Integrity Monitoring (RAIM) algorithm as a part of the PNT-Unit assures GNSS integrity. RAIM was first introduced in the aviation sector for only using reliable satellites during safety critical landing approaches. The need of RAIM in the maritime sector emerges from the requirements for maritime radio navigation systems as specified by the International Maritime Organization (IMO). Due to the limited manoeuver space on inland waterways, the requirement on positioning accuracy and integrity is very demanding. This work summarizes the difficulties of a classical snapshot-RAIM algorithm for a non- augmented GPS-based navigation system in operational degraded signal environments. Such environments include inland waterways with bridges, locks and other natural and artificial obstacles that cause GPS signal blockage and multipath. As the RAIM algorithm is based on pseudorange redundancy, the number of visible satellites is crucial for the availability of integrity information. It will be shown that the environment described above often compromises RAIM availability and performance. First results, based on several hours of collected GPS-data during a measurement campaign which took place at the river Moselle in Koblenz in August 2012, are given.