ALEGRO – A step towards maritime Ground Based Augmentation Systems

Abstract

The project ALEGRO is one of the initial projects of the “Forschungshafen Rostock” initiative (Research Port Rostock) geared towards the development and setup of maritime Ground Based Augmentation Systems (GBAS). The development of differential positioning methods was accelerated by the civilian community of GPS users at the beginning of the 1990s in order to effectively reduce accuracy problems with single-frequency GPS positioning. At the same time, IALA DGNSS networks were set up for maritime applications in order to reliably ensure accuracy to within 10 m in coastal areas, in compliance with the International Maritime Organization’s (IMO) requirements for GPS-based positioning systems. Meanwhile, the performance requirements set by the IMO for GNSS-based localizations have risen. To ensure accuracy in the decimetre range while simultaneously monitoring integrity in port areas, augmentation systems are still required, though with more modern techniques. According to initial studies by IALA (International Association of Marine Aids to Navigation and Lighthouse Authorities), augmentation systems suitable for this are those which use pseudolites or phase-based differential methods. To fulfil the safety requirements, users of GNSS/GBAS-based localization systems must be informed within a few seconds in case of the occurrence of system errors and signal interference that result in losses of accuracy greater than the tolerable positioning error. The challenge here is to develop integrated systems allowing high-precision localizations combined with integrity monitoring under all conditions. This technology is the prerequisite on the road to automated berthing manoeuvres. The primary goal of this project was the development of hardware and software for a phase-based GBAS (Ground Based Augmentation System) experimentation system at the Port of Rostock. The key element in the GBAS processing system is the "GNSS Performance Assessment Facility". It derives pseudorange domain and position domain alarms from the incoming receiver data streams to provide a first real-time integrity monitoring of the used GNSS. The decision process for the selection of measured values, to be broadcast from the reference station to the users is based on specific test algorithms. To obtain quality estimations these algorithms infer from the relationship between reference values measured in real time, station-specific reference values, and positioning with the DIA process (Detection, Identification and Adaptation) at the established location.