Modelling of Performance Key Identifiers for Characterisation of Current Capabilities on Maritime Augmentation Services

Kurzfassung

In recent years, much effort has been devoted to ensure the reliable and resilient provision of navigational data for the safe and efficient operation of maritime vessels. Given the current status of Global Navigation Satellite Systems (GNSS), mariners use GNSS’ favourable availability to determine positioning, velocity and timing data (PVT) on vessels during berth-to-berth operations. However, vulnerabilities on the GNSS performance towards signal hazardous environments, reinforce the need to count with on-shore systems to contribute meeting the accuracy requirements during strategic and tactical navigational tasks. In consequence, the German Aerospace Centre (DLR), as an associated member of the International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA), has implemented an augmentation system with the ability to provide correction data for the improvement of the positional accuracy for precise safety-of-life applications, following the concept of an IALA Beacon DGNSS service. This system, known as Maritime Ground-Based Augmentation System (MGBAS), applies a two-station concept where a reference station (RS) is responsible for the provision of corrections data and the integrity monitoring station (IMS) answers for the evaluation of provided corrections. Recent approaches have shown that a MGBAS must satisfy two fundamental characteristics: First, the system should have the ability to perform functions according to specified conditions -requirements- during a certain period of time, and second, the system must detect and compensate error sources on its sensors and services. Therefore, evaluate the real-time performance of the components of the system and of its individual products for an improved error modelling constitutes a key element for the mitigation of threats during safety-critical applications. To determine the advantages and disadvantages of the system, DLR’s approach for the analysis of augmentation data is divided into four sub-facilities: 1. Data processing in the GNSS signal domain, where plausibility and consistency tests are applied to investigate the usability of satellite-specific code and carrier phase measurements for service provision and/or GNSS based positioning. 2. Data processing in the GNSS position domain, where the feasibility of GNSS based positioning (e.g. number of usable satellites, dilution of position), the achieved reliability (e.g. availability of position and current position error) is verified taking into account the different positioning techniques. 3. Data processing in the service signal domain, where residual errors regarding single corrections as well as the estimation of decorrelation effects are considered to validate the service provision. 4. Data processing in the service position domain, where the IMS acts as an artificial user to demonstrate the usability of the GNSS augmentation service for positioning and the achievable performance taking into account the supported position techniques. However, the absolute determination of the level of performance on the system in not always possible in real-time. Therefore, it is required to define a a set of performance key identifiers (PKI) to estimate the current achieved performance level. Applicable performance indicators are, amongst others, range errors, code and phase noise, carrier-to-noise ratio (CNR), and positioning errors. For these, adequate thresholds should be etermined to enable the differentiation between usable/unusable observations. Performance indicators create the basis to decide whether the provision of GNSS augmentation service is feasible or not. How the selection and modelling of appropriate PKI can improve the error assessment on the system is therefore the justifiable question discussed in this work.