Integrity concepts for future maritime Ground Based Augmentation Systems

Kurzfassung

Global Navigation Satellite Systems (GNSS) require augmentation to achieve integrity and accuracy performance for high-precise safety of life applications. The current standard maritime GNSS augmentation system is a differential GPS (DGPS) beacon system, which provides correction data and integrity information according to the IALA-standard [IALA-R-121]. They are broadcasted in the 300 kHz radio-navigation band in accordance with ITU-R Recommendations [DIN EN 61108-4]. Even if such systems, also called Ground Based Augmentation Systems (GBAS), increase the accuracy and integrity of GNSS substantially, the performance reached by these systems is not sufficient to meet all International Maritime Organization (IMO) requirements, especially those for critical traffic areas like ports and for e.g. automatic docking manoeuvres [IMO A.915(22)]. In order to support the applicability of satellite navigation in such areas, the German Aerospace Centre (DLR) has started to develop a maritime GBAS that meets all IMO requirements. While the current IALA (International Association of Marine Aids to Navigation and Lighthouse Authorities) GBAS is a Code-based Differential GNSS (C-DGNSS), what means it broadcasts information concerning code corrections, our developments aim for multi-frequency Phase-based Differential GNSS (P-DGNSS). For this purpose DLR has installed an experimental maritime GBAS in the port of Rostock (Germany) enabling algorithm development in the ground and user subsystem as well as their validation. The ground subsystem consists of two independent stations. The first station is operating as reference station and the second one as integrity monitoring station. This is similar to the hardware architectural design of the current IALA Beacon DGNSS architecture [IALA-R-121], whereby the GBAS uses high-rate receivers to enable a fast signal assessment in real time. Moreover, the proposed software architecture consists of real time processor chains that enable a hierarchical assessment from single data types via satellite signals up to the used GNSS with respect to the supported P-DGNSS service. Each of the implemented processors provides quality parameters like code and phase noise, Signal to Noise Ratio (SNR), Horizontal Positioning Error (HPE). These are considered as suitable input data for the GBAS integrity monitoring and the conditional provision of augmentation data and integrity flags. Thus Performance Key Identifiers (PKI) must be specified for each quality parameter which allows distinguishing between the nominal and the disturbed behaviour of GNSS and GBAS according to different positioning performances. The GBAS is complemented by a statistical analysis, which is deriving statistical performance parameters with respect to real time quality parameter collected during the previous 24 hours. The statistical performance parameters are used in the first instance to gradually improve the measuring models by an auto-adaptive system and to specify PKIs described by valid value ranges and thresholds. Then they are employed to detect outliers in real time and to estimate protection levels. The proposed quality parameters and related PKIs have been derived from 20 Hz GPS raw data of four GBAS stations in Germany (Research Port Rostock, DLR in Neustrelitz, Braunschweig) and France (Toulouse). Based on examples it will be shown that the nominal signal behaviour at the reference station can be employed to detect signal disturbances during GBAS operation in real time. In addition to the investigation of the single performance key identifiers, special attention is paid to the description of dependencies between the various performance key identifiers.