Integrity Assessment of a Maritime Carrier Phase Based GNSS Augmentation System

Kurzfassung

The beacon system of the International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA) has been the standard maritime GNSS (Global Navigation Satellite System) augmentation system for many maritime applications since the 90th. The stations provide differential corrections to GNSS receivers in order to improve navigation accuracy and monitor the quality of GNSS satellite transmissions. Depending on the rover-to-reference station separation and the age of the corrections being applied, the horizontal accuracy of the system is below 10 metres at more than 95% of the time. To ensure integrity, the integrity monitoring station (IM) of the IALA beacon system monitors the GNSS signals, verifies the availability of the reference station (RS) and immediately notifies users to disregard the signal or, in the worth case, to use another station. According to IALA Recommendation R-135 [3] there are potential alternatives to the IALA beacon system for the distribution of safety related differential services. One considered option designed to achieve position accuracies below one decimetre (e.g. for port and inland waterways) is carrier phase based GNSS positioning, so called Real Time Kinematic (RTK). As in Differential GNSS (DGNSS) the basic concept of a single base RTK system involves a reference station with a well known position, but instead of pseudorange corrections, raw measurements are transmitted to a rover receiver via a communication link (here we do not consider network RTK). The data processing at the rover site includes ambiguity resolution of the differenced carrier phase data and finally the estimation of the rover position. Single base RTK is also limited by the distance between reference receiver and the rover receiver due to the spatial decorrelation of distance-dependent biases such as orbit error, and ionospheric and tropospheric signal refraction. But one significant drawback of RTK, in comparison to the DGNSS approach, is that the maximum service area of the RTK reference station is restricted to 10 to 20 kilometres. Nevertheless for high precise port operations [4] and locking of ships, single base RTK can be an option, provided that also integrity is assured. The intention of the work presented here was, to study integrity aspects that would appear, if one enhances the DGNSS service of an IALA beacon system by an additional RTK service. The major concern has been to assess if shore based integrity monitoring stations, performing a number of tests on the GNSS signals, may be sufficient to assure the required integrity for high precise, safety critical maritime applications using RTK.