The Agency for Air Navigation Safety in Africa and Madagascar (ASECNA) awarded a contract to carry out the preliminary architecture definition study (so-called phase B) of the Satellite Based Augmentation System for Africa & Indian Ocean (A-SBAS) initiative it is leading. This initiative aims to enhance navigation and surveillance operations during all phases of flight, and thereby to significantly improve flight safety and efficiency in Africa. In addition to SBAS services in support of air navigation, ASECNA also explores the provision of additional services on dedicated Signal-In-Space (SIS) channels, such as Precise Point Positioning (PPP) and Emergency Warning Service to populations (EWS). The SBAS services, expected from 2024, will increase the positioning performance provided by GPS satellite navigation constellation, by decomposing all possible range error sources and distributing corrections and/or alerts to its users by means of geostationary satellites. The phase B study aims to provide ASECNA with the definition of the best architecture option that will support the incremental deployment and provision of SBAS services in terms of levels and areas of service, a full visibility of the following phases of development, deployment and operation, and the capability to provide a pre-operational service. This paper presents the selected baseline system architecture and the associated SBAS performances regarding availability, accuracy, integrity and continuity on the first incremental services steps, and the pre-operational service as well. As the African continent encompasses equatorial areas, between both equatorial fountains, the phase B study has paid particular attention to the ionosphere behavior and the way to define an accurate and integer internal model in order to monitor the IGPs (Ionospheric Grip Point) of interest for the targeted service areas. Thales Alenia Space had been relying on the SAGAIE study, based on data collection from 2013 (closed to high solar peak of cycle 24) to 2017, to define two very distinct regimes in the ionosphere to be considered for the services availability : a so-called "severe" regime corresponding to periods of time around the equinoxes and a so-called "nominal" regime elsewhere. The proportion of time spent in either of these two regimes depends on the solar cycle. The SBAS navigation Kernel has been designed and developed by Thales Alenia Space taking benefit of TEC and gradient amplitudes, equatorial plasma bubbles phenomenon as per scintillations characterization over the targeted service areas according to SAGAIE experience. The internal ionosphere model processes both GPS and Galileo data to compute and broadcast GIVD (Grid ionosphere Vertical Delay) and GIVE (Grid ionosphere Vertical Error) on the IGP grid. This design has demonstrated its capability to provide performances for the future SBAS for Africa and Indian Ocean (A-SBAS) in line with the Standards and Recommended Practices of the International Civil Aviation Organisation (ICAO).