SBAS in Equatorial Regions

The current picture of satellite navigation systems includes global (GPS, GLONASS, Galileo), regional (SBAS, QZSS, Compass, IRNSS) and local systems (GBAS, hybrid systems combining GNSS and other sensors). The use of GNSS for safety critical applications and in consequence as 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. The analysis of the current trends observed in the navigation community suggests 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 and MSAS are operational SBAS systems. Additionally, there are other SBAS systems that are currently under development (as for example SDCM in Russia and GAGAN in India) or under study as it is the case of SACCSA program in Latin-America (ICAO Regional Project RLA/03/902 – Solucion de Aumentacion para el Caribe, Centro y Sudamerica / Augmentation Solution for the Caribbean, Centro and South America). One of the major challenges in the aviation community is the implementation of the Performance-based Navigation (PBN) concept. GNSS technologies play an important role to achieve this objective. 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. It is important to recall that most GNSS augmentation systems have been initially designed for Mid-latitude ionospheric conditions and therefore important differences at performance level are expected. During high solar activity (next expected solar maximum for 2013-2014), GPS mono-frequency receivers have very degraded accuracy performances in equatorial regions. Using a SBAS solution would significantly improve the performances of GPS receivers (most mask-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. 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 GPS positioning. The objective of this paper is to provide a general view of the experience gained by GMV in equatorial regions in SBAS systems. The paper will describe the main limitations for SBAS technologies in those regions and the strategy followed by GMV within SACCSA project to analyze their effects in Latin-American region. Examples of the kind of effects analyzed in this paper are the impact of the iono mapping function, the equatorial anomaly (with its spatial and temporal gradients), ionospheric depletions and scintillations, as well as the analysis of the current applicability of MOPS to equatorial regions. Special attention will be paid to extrapolate Latin-American conclusions and to derive recommendations to other regions of the Word with similar ionospheric characteristics as for example Africa, Middle-East, South Asia, etc. From a previous analysis shown in paper “Characterization of Equatorial Ionospheric Features on the verge of the Next Solar Cycle Maximum” (ION 2012), it can be deduced that even if all equatorial regions suffer from more or less the same equatorial ionospheric behavior, there are differences between regions with different longitude that could slightly modify the final SBAS approach. Thanks to the GMV powerful platform magicSBAS, important advances have been achieved in equatorial regions. magicSBAS is an SBAS Augmentation System demonstrator that collects GPS and GLONASS data (measurements and ephemeris) from a regional network of reference stations, computes satellite orbits and clocks, ionospheric and integrity information in accordance with ICAO SARPS standards. The paper will present a performance analysis, using magicSBAS, for different equatorial regions in the World (Africa, Middle-East, South Asia) and for different ionospheric scenarios (low – high solar activity). The regions to be considered will depend on the availability of data. The paper will also present the real-data results of different GMV platforms running in real time all around the world with special emphasis in equatorial regions. Additionally, the paper will include the comparison of real data performances with and without SBAS technology (GPS only) in terms of accuracy for different scenarios (low and high solar activity in different latitudinal ionosphere regions) to analyse the improvement in performances gained with a SBAS-like technology for multimodal users.