SBAS Performance Analysis in Equatorial Regions

Current satellite navigation systems include global (GPS, GLONASS, Galileo), regional (SBAS, QZSS, Compass, IRNSS) and local systems (GBAS, hybrid systems combining GNSS and other sensors). The use of Global Navigation Systems for safety critical applications, and therefore for the implementation of the PBN (Performance Based Navigation) requires certain levels of confidence on the positioning obtained by the user equipment. This is possible by complementing the core GNSS signals with other systems or techniques to produce a solution with the needed level of integrity. Taking into account the current trends observed in the navigation community, it is expected that, for the coming years, the GNSS integrity solutions may rely on SBAS, GBAS, RAIM or new techniques including integration with other sensors. In this global GNSS picture, SBAS appears as a feasible solution to regionally augment the GNSS constellations to provide increased accuracy with integrity. WAAS, EGNOS, MSAS and GAGAN are operational SBAS systems. Additionally, there are other SBAS systems that are currently under development, such as SDCM in Russia, or under study as it is the case of SACCSA (Solucion de Aumentacion para el Caribe, Centro y Sudamerica / Augmentation Solution for the Caribbean, Centro and South America) program in Latin-America (ICAO Regional Project RLA/03/902). SBAS technology provides benefits not only to aviation users but also from a multimodal user perspective. Even if aviation community is clearly the motto of SBAS technology, most part of the users of SBAS systems are multimodal users (i.e. non-aviation community). At equatorial latitudes, the ionosphere can become a significant problem on GNSS and in particular on SBAS and GBAS technologies in comparison with other regions of the world. The ionosphere affects electromagnetic signals broadcast by GNSS satellites by delaying the propagation of the code and in some cases by losing the track of the satellite signal. The main problems associated to the equatorial region are: 1) the presence of the equatorial anomaly, that complicates the ionospheric modelling for systems like SBAS; 2) the presence of small scale electron density irregularities in the F region that cause scintillations on the received signals; and 3) the presence of ionospheric bubbles or depletions, which are strong reductions in the ionospheric F-region plasma density due to the appearance of a Rayleigh-Taylor instability in the post-sunset, producing severe radio signal disruptions when crossing them. During high solar activity GPS mono-frequency receivers provide degraded accuracy performances in equatorial regions. Using a SBAS solution would significantly improve the performances of GPS receivers (most mass-market receivers with SBAS capability) providing a high benefit for different user communities such as agriculture, farming, aviation (airports), maritime (including fluvial navigation), rail, oil and energy industry (including off-shore), road transport (AVL systems, dangerous goods transport) and LBS (Location Based Service). Additionally to the expected improvement in accuracy and availability, SBAS technology can provide reliability allowing the user the capability of computing Protection Levels that could be used as a guarantee of the SBAS positioning. It is also important to recall that most GNSS augmentation systems have been initially designed for mid-latitude ionospheric conditions and therefore a significant worsening at performance level is expected when applying the mid-latitude algorithms and configurations to equatorial regions. The main objective of this paper is to present a performance analysis using real data and the algorithms developed by GMV for low latitudes and for different equatorial regions in the World (South America, Africa, Middle-East and South Asia). Different ionospheric conditions (low – high solar activity) are to be taken into account, with the aim also of taking advantage of the current solar maximum conditions and the increasing number of stations available. The regions and ionospheric activity conditions considered will depend on the availability of data. The performance analysis has been done using a platform developed by GMV, magicSBAS, which is an SBAS Augmentation System demonstrator that collects multiconstellation GNSS data (measurements and ephemeris) from a regional network of reference stations where available, computes satellite orbits and clocks, ionospheric and integrity information in accordance with ICAO SARPS standards. Thanks to this powerful platform, important implementation advances have been achieved in equatorial regions. In order to analyse the performances achieved at user level in equatorial regions for different solar conditions, two different kinds of analysis have been performed, using as input the information generated by magicSBAS: • SBAS performance assessment in both pseudorange and position domains: Errors in pseudo-range and user domains will be estimated using GMV´s eclayr tool: SREW (Satellite Ranging Error at Worst user location), GIVDerror (Grid Ionospheric Vertical Delay Error) and Position Error (HPE and VPE). UDRE (User Differential Range Error), GIVE (Grid Ionospheric Vertical Error) and Protection Levels (HPL and VPL) will be also estimated and these boundaries will be compared with their corresponding errors and alarm limits to check integrity and availability. • Comparison of performances obtained in presence of high and low ionospheric activity and with/without the augmentation of the SBAS: in order to analyze the improvement in performances gained with a SBAS-like technology for multimodal users, GMV´s magicGEMINI tool will be used. The SBAS messages generated with magicSBAS and the corresponding GPS observation files will be used as input to magicGEMINI, which is a GNSS performance assessment and monitoring tool specifically designed to meet the needs of air navigation service providers and airspace users. Finally, the paper also compares the performance results achieved for the different regions analysed in this paper and provides a summary of the main limitations for SBAS technologies in equatorial regions.