EDAS for a DGPS Maritime Service: EGNOS Based VRS Performance with Pre-Broadcast Integrity Monitoring

J. Morán, E. Lacarra, J. Vázquez, M.A. Sánchez, F. Cantos, T. Horváth

Abstract: EDAS (EGNOS Data Access Service) is the EGNOS service providing free of charge Internet access to all the data generated and collected by the EGNOS infrastructure in real time and also through a historical archive. EDAS gathers the GNSS raw data collected by all the receivers located at the EGNOS Ranging and Integrity Monitoring Stations (RIMS), which are mainly distributed over Europe and North Africa. After processing the raw data collected by the RIMS, EDAS disseminates this GNSS information (GPS/GLONASS satellite navigation and observation data, EGNOS messages) to the EDAS users in different formats and protocols. Depending on the information provided, and also on the format and protocol used to transmit the GNSS data, the following services are distinguished: Main Data Streams, Data Filtering, SISNeT, Ntrip (real-time) and FTP (archive). In addition to the observation and navigation data broadcast by the GPS, GLONASS and EGNOS GEO satellites, EDAS, through its Ntrip service, computes and disseminates differential corrections and the required phase measurements and station data in RTCM format to support DGNSS and RTK positioning techniques. EDAS service provision is performed by ESSP, as EGNOS Services Provider, under contract with the European GNSS Agency (GSA), the EGNOS program manager. The European Commission (EC) is the owner of EGNOS system (including EDAS) and has delegated the exploitation of EGNOS to GSA. Firstly, the paper will introduce the EDAS system and its architecture, providing information about the data available through its real-time and FTP services. After introducing the EDAS services, this paper will focus on the potential use of EDAS as a source for the generation of differential GPS corrections based on Virtual Reference Stations (VRS), including the required integrity checks. The principle behind this solution would be to use the EGNOS augmentation message (RTCA format) delivered through EDAS as an input, to be then converted into RTCM SC-104 corrections (EGNOS-VRS) referenced to the locations of interest for maritime users (e.g. beacon locations). Transmission to final users could be done through IALA MF beacons or through AIS (Automatic Identification System) base stations (via AIS #17 message), hence ensuring compatibility with the deployed user equipment. It is worth highlighting that the main advantage of the DGPS solution based on VRS (taking the EGNOS messages from EDAS as input) with respect to standard DGPS is that corrections can be remotely generated for any location within the EGNOS coverage area. Hence, a physical reference station at or close to the transmission site would not be required for the delivery of DGPS corrections for maritime navigation. In this context, EDAS could be used as part of a cost-effective solution fully compatible with the user equipment already deployed, and even providing some room for the rationalization of the infrastructure, in the current context where some European countries are or will be facing obsolescence issues in the short term. In order to assess the potential benefits of the EGNOS-VRS concept for maritime navigation, the results of a performance analysis campaign conducted at different European locations were presented last year in the ION GNSS+ 2015 [19].This analysis showed that the accuracy and availability performance provided by an EDAS based VRS solution could be comparable to the results obtained with traditional DGNSS stations, indicating that the VRS corrections could meet the accuracy and availability requirements for general maritime navigation. However, it is important to remark that apart from the availability and continuity requirements, the radio-navigation system shall provide “integrity warning of system malfunction, non-availability or discontinuity” to users [20]. The IALA DGNSS Integrity concept is based on an Integrity Monitoring (IM) station that retrieves the corrections broadcast (or to be broadcast) by the Reference Station (RS) and verifies that the information is within tolerance (both at pseudorange and position domain levels) based on the fact that the position of the IM is known. On the other hand, two different integrity approaches are possible, depending on whether the integrity check is done before or after broadcasting the corrections to the users: Pre-Broadcast Integrity or Post-Broadcast Integrity. The present study will be based on the Pre-Broadcast Integrity concept, which is the recommended option for DGNSS service providers implementing the Virtual Reference Station concept (for more information, refer to [17]). Taking into account that integrity is an essential requirement for maritime radio-navigation systems, this paper will go one step further compared to the study presented last year, analysing the provision of integrity warnings in the EDAS-based VRS corrections and assessing the impact of the integrity check on the accuracy and availability performance. In order to do that, GPS measurements from multiple public reference stations in different European areas will be combined with EDAS-based VRS corrections generated for specific locations of interest and resulting in a diverse set of scenarios (different locations and baseline lengths have been considered). The main tool for the performance assessment to be presented in this paper will be the Alberding EuroNet application, a real-time DGNSS software with flexible and scalable design that is able to process the EGNOS corrections in RTCA format from an SBAS-enabled GNSS receiver or from the EDAS service and derive EGNOS-VRS corrections in RTCM 2.3 or AIS #17 messages ready to be broadcast via IALA radio beacons or AIS base stations. The integrity solution implemented is based on the pre-broadcast monitoring concept, checking the computed differential corrections before being broadcast to the users. Correction data availability and age, positioning accuracy, satellite specific PRC (Pseudorange Correction) and RRC (Range Rate Correction) values and residuals, data rate, etc. are monitored in real time. In case any of the monitored parameters exceed pre-defined threshold values for a given period of time (taking as reference the DGNSS Broadcast Site Settings proposed in [17]) the Pre-Broadcast Integrity Monitoring module automatically generates a warning message and the software sets the health status of the given satellite or the reference station to “unhealthy”.
Published in: Proceedings of the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2016)
September 12 - 16, 2016
Oregon Convention Center
Portland, Oregon
Pages: 3481 - 3493
Cite this article: Morán, J., Lacarra, E., Vázquez, J., Sánchez, M.A., Cantos, F., Horváth, T., "EDAS for a DGPS Maritime Service: EGNOS Based VRS Performance with Pre-Broadcast Integrity Monitoring," Proceedings of the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2016), Portland, Oregon, September 2016, pp. 3481-3493. https://doi.org/10.33012/2016.14811
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