EDAS (EGNOS Data Access Service): Alternative Source of Differential GPS Corrections for Maritime Users

E. Lacarra, J. Vázquez, M.A. Sánchez, L. Bonacasa, T. Horváth

Abstract: EDAS (EGNOS Data Access Service) is the EGNOS ground-based service (through the Internet), which provides free of charge access to all the data generated and collected by the EGNOS infrastructure in real time and also through a historical archive. EDAS gathers all the raw data coming from the GPS, GLONASS and EGNOS GEO satellites collected by all the receivers located at the EGNOS Ranging and Integrity Monitoring Stations (RIMS), which are mainly distributed over Europe and North Africa. Once the data is received, EDAS disseminates this GNSS data (GPS/GLONASS satellite navigation and observation data, EGNOS messages) in real time to EDAS users and/or service providers that distribute the data locally or to specific set of applications. In addition, and based on this GNSS data, EDAS computes and disseminates differential corrections to support DGNSS positioning. 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. ESSP also manages the EGNOS Helpdesk, which provides technical support to users answering any potential question or clarification about EDAS, along with the registration of new EDAS users. The continuous provision of EDAS Services started in 2009. Currently, the Services provided by EDAS are the following: • Main Data Streams: EGNOS data is provided through the Internet in real time in ASN.1 format (Service Level 0) and RTCM 3.1 format (Service Level 2), covering raw GPS/GLONASS/EGNOS GEO observations and navigation data received by all EGNOS stations, along with the EGNOS augmentation messages. • Data Filtering: Filtering capabilities to allow receiving EGNOS data from only certain subsets of RIMS stations when connecting to EDAS Service Level 0 and 2. • SISNet Service: EGNOS messages provided in real time using the SISNet protocol defined by ESA. • FTP Service: Historical EGNOS data available through an FTP site, including: EDAS SL0, SL2 raw data, GPS/GLONASS navigation and observations (RINEX format), EGNOS messages (ems + RINEX-B formats) and IONEX data files. • Ntrip service: RIMS raw measurements are provided in real time using the Ntrip protocol. The EDAS Ntrip service also provides RTK messages and DGNSS corrections, taking the EGNOS RIMS as reference stations. Firstly, the paper will introduce the EDAS system and its architecture, informing about the main types of information that are available through its real-time and off-line services. It will present the online information available in relation to this Service: • EDAS Service Definition Document (EDAS SDD) that describes its characteristics, conditions of access and committed performance (in terms of availability and latency) • EGNOS User Support website where a user can apply for the registration to EDAS and can access the real-time status of the services. Recent EDAS performance in terms of availability and latency will be shown, presenting trending evolution versus time and statistics. • EGNOS Monthly Performance report, publicly available, which includes the EDAS performance of the reported period • EGNOS Helpdesk, which is the main point of contact available to EGNOS users. After introducing the different techniques that could be applied to improve GPS standalone positioning accuracy using EDAS data, this paper will focus on the potential use of EDAS to support a DGNSS navigation service for maritime users. EDAS itself is providing differential corrections to the GPS and GLONASS satellites through its Ntrip service, taking the EGNOS RIMS stations as reference stations. Performance results using EDAS differential corrections will be presented for different locations in Europe. From the point of view of the maritime community, the above scheme suffers from at least three important drawbacks: the potential limitations imposed by the existing GPRS coverage, the geographical distribution of the EGNOS RIMS stations which is not driven by maritime user needs and the compatibility with the deployed user equipment. In order to assess the impact of the first two limitations (GPRS coverage and EGNOS RIMS configuration), a coverage analysis will be performed with the aim to integrate the Internet coverage information available from European states together with the baseline restrictions which are implicit to DGNSS solutions. As an alternative to overcome the third limitation, linked to the fact that transmission is not done through the existing transmitting infrastructure, and for which user equipment is widely deployed, the potential use of the EGNOS messages from EDAS to generate DGNSS corrections will be presented. 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. The principle of 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 referenced to the locations of interest for maritime users (e.g. beacons locations), named as EGNOS Virtual Reference Stations (EGNOS-VRS). Transmission to final users could be done through the existing infrastructure (IALA beacons, AIS stations) hence ensuring compatibility with the deployed user equipment. In order to illustrate the potential of the abovementioned alternative, a pilot test was conducted in Hungary in 2014, where EDAS-based RTCM corrections were generated for an AIS base station location by the river Danube. To assess the achievable EGNOS-VRS positioning performance, raw observation data of GNSS monitoring stations was processed together with the converted RTCM SC-104 corrections using the DGNSS monitoring software of Alberding GmbH. The results indicated that EGNOS-VRS differential corrections meet the accuracy requirements of inland waterway navigation in Central Europe. To extend the performance evaluation, additional positioning tests were carried out in 2015 at different European locations. The derived position solutions were compared to EGNOS RIMS based DGPS solutions over short, medium and long baselines.
Published in: Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015)
September 14 - 18, 2015
Tampa Convention Center
Tampa, Florida
Pages: 953 - 966
Cite this article: Lacarra, E., Vázquez, J., Sánchez, M.A., Bonacasa, L., Horváth, T., "EDAS (EGNOS Data Access Service): Alternative Source of Differential GPS Corrections for Maritime Users," Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015), Tampa, Florida, September 2015, pp. 953-966.
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