A Safe Option to Pave the Way for the Implementation of a Satellite Based Augmentation System

J. Ostolaza, M. López, J. Autrán, J.J. Lera, D. Pérez, D. Hill, V. Boissinot

Abstract:	Satellite Based Augmentation Systems (SBAS) complement existing global navigation satellite systems (GNSS) in terms of accuracy, integrity, continuity and availability. Their applications are present in a wide range of do-mains as aviation, precision agriculture or maritime and their advantages are undeniable. Nevertheless, whilst Europe has benefitted for a number of years from the European Geostationary Navigation Overlay Service (EGNOS) and North America from the Wide Area Augmentation Service (WAAS), the countries of Africa have not had the same opportunity to benefit both economically and socially from improved navigation services. Many airports across Africa lack the infrastructure that enables aircraft to use precision landing approaches. This makes landings less safe, reduces their mainstream commercial use and constrains regional economic development. The deployment of a SBAS system in this region would allow publishing precision approaches without any complex infrastructure at the airports, encouraging the air traffic and commerce increase. SBAS-Africa is about deploying an end-to-end SBAS testbed to demonstrate the potential benefits of implementing SBAS capabilities in Southern and Eastern Africa. The initiative is co-funded by the United Kingdom Space Agency (UKSA) through its International Partner-ships Space Programme (IPSP) and made possible by collaboration between the South African National Space Agency (SANSA) and the UK Space Agency (UKSA). The project objectives are to introduce an SBAS in Africa for the support of the Satellite Navigation in different sectors, to transfer the technical SBAS knowledge to relevant African organizations in each region for the implementation of the preliminary backbone infrastructure, but also to show the economic benefits from the use of satellite or space technology in African countries. Pursuing the objectives explained before, the SBAS-Africa project has delivered a live SBAS signal in space serving the southern part of the African continent. The system generates SBAS messages using GMV’s mag-icSBAS tool suite (a state-of-the-art real time and post processing operational SBAS testbed developed by GMV to offer non-safety critical SBAS augmentation to any interested region or organization) with input data from a network of GPS Ground Monitoring Stations (GMS) developed by NSL which are deployed across South Africa and neighbouring countries. Communications for the GMS make use of the high throughput AVANTI satellite Hylas 2 to bring the GPS raw data in a timely manner to magicSBAS. The messages are broadcast via the ARTE-MIS GEO satellite, former EGNOS operational GEO satellite now owned by Avanti. After some deployment tests, the system broadcast SBAS-like messages during the trials campaign in order to prove the benefits in different sectors. ARTEMIS satellite was used together with Inmarsat’s 3F2 and 3F5 between 2003 and 2005 in test configuration for EGNOS system qualification testing. Up to 2015, it was used as an operational SBAS GEO satellite in order to provide EGNOS service over Europe. One of the main challenges encountered during the testbed integration has come from the fact that ARTEMIS has a highly inclined orbit (up to 12º in 2016). Even though no GEO-ranging capabilities were foreseen to be provided by the system, a simple but robust control of the signal was needed to permit a continuous and safe tracking by SBAS users. Indeed, Doppler effect causes different frequency shifts in the carrier and code components of the signal (approximately direct function of the frequency), requiring a real-time feedback control to ensure a sufficiently stable code-phase coherence as well as consistent Doppler measurement according to the relative speed of ARTE-MIS. In a first stage, the control algorithm developed to compensate these effects starts by computing an estimation of the range and relative speed of the Artemis with respect to the uplink station using the orbit data from a TLE file and the Doppler measured in a receiver. In a second stage, the control actions are computed based on the previous estimation. Note that ionospheric and tropospheric effects have been neglected for the first phase of the testbed. Nonetheless, besides the technical challenge of configuring and deploying such a system, there was a major Safety concern: the end-to-end SBAS test-bed was going to broadcast SBAS-like messages and these messages could potentially represent a threat to any Safety-of-Life operation user of any other SBAS service. This is remarkable for a testbed system running in South Africa broadcasting SBAS messages through ARTEMIS GEO satellite, whose footprint overlaps with EGNOS service area. A failure in the content of a message or an incorrect PRN configuration could induce a safety risk which can’t be afforded even more when the testbed is broadcasting live messages over EGNOS service area, using a former EGNOS satellite. A rigorous safety analysis was performed to identify the necessary barriers to mitigate above mentioned risks to acceptable levels. The objectives were twofold: ensure that messages content was correct and that they were sent to ARTEMIS using the correct PRN. The subsystem that plays the key role providing the ultimate level of confidence on the generated SBAS messages in the SBAS Africa project is the Uplink Safety Monitor (USM). The USM is a hardware/software element that acts as a safety firewall blocking the output stream when any defined barrier is activated. It receives two types of inputs, the SBAS-like messages before being uplinked, and the so called feedback channels, containing SBAS-like messages previously broadcasted by the test-bed and received by reference receivers. On the one hand, the USM thoroughly checks that no message will be uplinked if it can create a hazardous event to a Safety-of-Life user. On the second hand, the USM closes the loop by checking that SBAS-like messages received through the loop feedback channels are consistent and have the expected content. This paper will explain with detail the end-to-end SBAS test-bed, focusing on the control algorithm and the safety approach implemented in the Uplink Safety Monitor. It will also show how the trials and tests were done in a safe manner, unleashing the end-to-end test bed to demonstrate the capabilities of a SBAS system without jeopardizing Safety, neither for the SBAS-Africa user nor for the any other SBAS SoL user.
Published in:	Proceedings of the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2016) September 12 - 16, 2016 Oregon Convention Center Portland, Oregon
Pages:	3249 - 3257
Cite this article:	Ostolaza, J., López, M., Autrán, J., Lera, J.J., Pérez, D., Hill, D., Boissinot, V., "A Safe Option to Pave the Way for the Implementation of a Satellite Based Augmentation System," Proceedings of the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2016), Portland, Oregon, September 2016, pp. 3249-3257. https://doi.org/10.33012/2016.14558
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