DFMC SBAS Prototype in Africa

Abstract

Four remarkable events are currently concurring to make possible the establishment of a very first demonstration of a preliminary DFMC SBAS service in Africa: • Galileo and GPS constellations near completion and/or replenishment ensure the provision of a significant number of operational dual frequencies (L1 & L5) navigation satellites; • The collaborative work of the Eurocae-RTCA WG-62 is well underway and DFMC MOPS are close to their finalization, while DFMC SARPs are endorsed and will be applicable by November 2023; • TAS (Thales Alenia Space) has developed an efficient DFMC navigation kernel compliant to the latest versions of those DFMC SBAS related standards; • And last but not the least, ASECNA (Agency for Air Navigation Safety in Africa and Madagascar) has officially launched its Augmented Navigation for Africa (ANGA) initiative, recognised by the International Civil Aviation Organisation, that intends to provide a full Legacy SBAS SoL service in the coming years but that is already broadcasting a demonstration service over African sub-saharian regions. More specifically, the Galileo constellation comprises 24 operational E1-E5a capable satellites since the last satellites launched in December 2021 are operational. The modernization of GPS space and ground segment is also in progress, and with the newest block III satellite operational since February 2023, the GPS constellation now comprises 18 operational L1-L5 capable satellites. GPS and Galileo have not reached the full operational capability for L1-L5/E1-E5a services, still they now offer a wide range of DFMC observability and measurements anywhere on the ground. The joint work of EUROCAE and RTCA is expected to give birth to a MOPS DFMC L5 Revision A (ED259A) in mid-2023. However, many successive work/draft versions have been produced up to now and we have based the results of this study on the latest available versions. Based on its long experience on various SBAS such as EGNOS or ANGA, TAS has developed a DFMC SBAS navigation kernel compliant with the work of the WG-62. As its Legacy SBAS L1 counterpart, this DFMC navigation kernel can be used to feed various SBAS performance studies with relevant and valuable augmentation messages. Moreover, it can also run in real time with actual GNSS stations measurements to provide an initial non safety-of-life SBAS service, very similarly to an operational SBAS system. The first part of the paper will deal with simulation studies in Africa. Under a CNES (Centre National d’Etudes Spatiales) contract, TAS has evaluated the performances of its DFMC navigation kernel using real GNSS data over a few representative African scenarios. The scenarios cover nominal and also degraded conditions (such as the loss of monitoring stations, or a depleted constellation). Two of those DFMC SBAS scenarios will be presented in the paper. They both augment GPS and Galileo constellations, they use the same network of 15 reference stations, but they differ on the time periods (December 2021 for the first scenario, and March 2022 for the second one) to evaluate the impact of the ionosphere activity on the DFMC SBAS performances. The selected network of reference stations includes stations from various agencies participating in the International GNSS Service (IGS) and stations from the SAGAIE network (Stations ASECNA pour l’Etude de l’Ionosphère Equatoriale). The stations are mainly located in the African equatorial region, and their observables have been collected from IGS or CNES servers. The preliminary analyses show very good navigation performances. The pseudorange integrity is met with high margins. The horizontal position error (95%) is below 0.9m and the vertical position error (95%) is 1.6m, with maximum safety index lower than 0.47. The 99% availability area for APVI and LPV200 services largely covers Africa, from 40°W to 80°E, and from 60°S to 60°N. Furthermore, CAT-I availability, with a 10m vertical alert limit, is available more than 90% of the time on the entire African continent. In the last part of the paper, we will describe the real time DFMC SBAS testbed that has been set up in view of the demonstration that started this Spring 2023. This testbed will use the ANGA signal broadcasted by ASECNA’s demonstration infrastructure to produce both valuable demonstration L1 and L5 augmentation messages. The Signal in Space (SiS)will be broadcasted by Nigcomsat 1-R GEO satellite. To our knowledge, this will be the first DFMC SBAS demonstration SIS to be broadcasted in Africa through NIGCOMSAT-1R SBAS GEO, and one of the first in the world. In conclusion, the paper will present the performance analysis of the ANGA DFMC demonstration signal regarding positioning accuracy, availability, continuity and integrity margins over the service area. The results will illustrate the level of navigation performances expected for a corresponding DFMC operational service over a wide service coverage area characterized by complex and challenging ionosphere conditions.