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Session B3: Future Trends in GNSS Augmentation Systems

EGNOS V3 Legacy Service – Preliminary Performances
Roland Braun, Airbus Defence & Space Gmbh; Marc Boyer, Airbus Defence & Space SAS; Mickael Dall’Orso, Nicolas Giron, Carlos López de Echazarreta, EGNOS Project Office - European Space Agency (ESA)

EGNOS is the European Satellite Based Augmentation System (SBAS). Its mission consists in providing an augmentation signal to the GPS and GALILEO, improving the accuracy of their navigation service and monitoring their integrity over Europe. The EGNOS Missions have been specified to fulfil the needs for a wide number of applications, notably those for which safety is a critical element (e.g. civil aviation or maritime). EGNOS V3 Legacy services are:
- Open Service (OS), available to the public over Europe,
- Safety of life Service (SoL), which provides the most stringent level of signal in space performance to aviation Safety of life users over Europe,
- EGNOS Data Access Service (EDAS) for customers requiring the provision of EGNOS products for commercial or professional use
Airbus Defence and Space, together with its key industrial partners, is currently developing the second generation of EGNOS, called EGNOS V3, under a working contract with the European Space Agency who acts as design authority and the system development prime on behalf of the European Commission (cf [RD.1]).
EGNOS V3 will ensure seamless continuation of the current EGNOS V2 SF Legacy services, i.e. it will augment GPS using the L1 Coarse/ Acquisition (C/A) civilian signal in a first step delivering the so called Single Frequency or Legacy Service mode. This version of EV3 (named EV3.1) is intended to include the usage of Galileo ranging data for the ionosphere monitoring, called the SF Enhanced Service mode. In a second implementation step (release EV3.2) it will also augment GPS and Galileo constellations in the L1/E1 and L5/E5a bands in the so called Dual Frequency Multi-Constellation (DFMC) service mode. The implementation of the DFMC SBAS will be opening a new era, improving the robustness and the performances of the SF Legacy service over Europe for any user capable of DFMC. The Extended Safety of Life navigation services will be usable for European civil aviation needs but also for maritime and land-based users over the European service area.
Through its multiple active services, EGNOS V3 provides the following additional information to users (augmentation data):
- Satellites orbits and satellites clocks corrections for GPS (in SF and SF Enhanced mode, and in GPS-only DF mode) and GALILEO navigation information (in GPS+GAL DFMC mode),
- In GPS SF Legacy and Enhanced mode, modelling of ionosphere current state that enables users to remove ionosphere impacts from their navigation solution,
- Integrity information that enables users to bound its final navigation residual error with respect to its real position within a warranted range at any time,
- Timing information that enables users to always correlate the time resulting from their navigation solution to UTC(OP) time
This publication will present results on the initial performances of the EGNOS V3 Single Frequency (SF) Legacy Service for the two available sub-modes for GPS SF augmentation established within the EGNOS V3 Central Processing Facility SF (CPF-SF). These two sub-modes are concerning the computation of the ionospheric delay corrections and associated integrity bounds which are:
(1) “Legacy mode” (only GPS measurements are used for ionosphere monitoring) or
(2) “Enhanced mode” (both GPS and Galileo measurements are used for ionosphere monitoring).
In the first part of the publication it will be provided an overview of the main features of the EGNOS V3 CPF-SF service algorithms for both the Legacy and Enhanced mode. It is recalled that EGNOS V3 CPF contains major evolutions, such as the provision of MT28 instead of MT27 used in current operational EGNOS, or the implementation of a Bayesian filtering technique for the ionosphere monitoring which can process either GPS-only or GPS+Galileo ionosphere observation data from the RIMS network (pending on whether the CPF is configured in “Legacy” or “Enhanced” mode).
In the second part of the publication it will be presented the achievable EGNOS V3 Single-Frequency performances under the Fault-Free conditions (i.e. in the absence of specific GNSS SIS threat cases or system internal failures). Results are produced using CPF prototype SW (called CPF Algorithmic Models - CAM) played into the Service Performance Validation Platform (SPV-P) using representative synthetic and real data scenarios. Notwithstanding that CPF Algorithm Models have not reached their CDR level yet, their level of maturity is considered sufficient to provide a preliminary view of the EGNOS V3 Legacy Service performances.
Overview of the EGNOS V3 Algorithms
The Central Processing Facility (CPF) is the navigation heart of EGNOS V3 that estimates the SBAS broadcast parameters based on a set of measurements collected at the deployed Ranging Integrity and Monitoring Station (RIMS). The CPF is composed of two independent chains. The first chain is the Prediction Chain Facility (PCF) which estimates the SBAS broadcast parameters (i.e. corrections and bounding parameters), taking into account the fact that they have to be valid during their entire validity period (in compliance to ICAO SARPs and RTCA DO-229 MOPS requirements). The second chain is the Integrity Chain Facility (ICF) monitoring the safety of all the broadcast parameters all along their validity period. The prediction and integrity chains are fed by observables generated by two different RIMS chains located at the same geographical site, respecting a minimum distance between antennas to ensure a decorrelation of the measurement errors. This RIMS chains are called RIMS-PRED for the PCF and RIMS-INT for the ICF.
The key challenge of EGNOS V3 in the prediction chain is the ionosphere parameters estimation at the required level of integrity in situations of under-sampling with a poor number of RIMS measurements(edge of the service area). The EGNOS V3 solution is based on a covariance method, which allows precise predictions of expectation values for the ionospheric delays and error bounds well suited to non-uniform spatial sampling. This adaptive interpolation method intrinsically models the estimation uncertainty of the ionosphere allowing for tighter integrity bounds in the ionosphere gradient conditions encountered by EGNOS.
EGNOS V3 prediction chain uses Message Type 28 (MT28) for an optimized availability, continuity and integrity over the GEO footprint. MT28 is used to provide increased availability through accurate description of the satellite orbit and clock error covariance suited per user Line-of-Sight. By broadcasting the exact shape of this error bound ellipsoid, the user receives the optimal information of the augmented satellite for the user’s specific location. This way the UDRE/MT28 process allows further improving performance versus the alternative MT27 approach, with the added benefit of declaring the service over the entire GEO Broadcast Area (GBA) without restriction.
The integrity chain inside the CPF is a critical element in the system for monitoring all active SBAS messages and the new SBAS messages proposed to be broadcast by the PCF. To that end the ICF is composed of a set of complementary barriers that continuously monitors the integrity of the to-be-broadcast and already-broadcast SBAS parameters. To ensure a complete independence with the prediction chain, it is fed by an independent stream of RIMS measurement and is based on diversified algorithms relative to the prediction chain.
First EGNOS V3 achieved performances for the Legacy Service – LPV 200 Precision Approach
Based on EGNOS services, LPV-200 (Localizer Performance with Vertical guidance) enables aircraft approach procedures (operationally equivalent to CAT I instrument landing system (ILS) procedures) allowing lateral and angular vertical guidance during the final approach without requiring visual contact with the ground until reaching a decision height (DH) of 200 feet above the runway.
Within this paper it is presented first results for LPV 200 Availability, Accuracy, Continuity Risk and Integrity performances for EGNOS V3 in SF Legacy mode. These results are calculated from end-to-end simulations executed on the simulation platform, and using:
• The prototype of the EGNOS V3 CPF software, integrating Prediction chain and Integrity Chain software, called CPF Algorithm Models (CAM). The CPF-CAM is fully representative of the final software that will be implemented into the operational CPF
• Both, synthetic scenario and real-data scenarios: The synthetic scenarios have been built using highly representative modelling of all relevant GNSS signal characteristics including environmental conditions like the ionosphere gradient and scintillation behaviour, troposphere, as well as local effects as multipath and RFI at the specified EGNOS V3 RIMS network (44 RIMS incl. 1 RIMS for ENT-UTC determination). On the level of the satellite constellations it used the Expandable 24-slot constellation defined in GPS SPS PS 2008 [RD.4] with the three slots B1, D2 and F2 expanded (for a total number of 27 satellites) and the Galileo OS SDD constellation of 24 satellites [RD.5] (used for ionosphere monitoring only in case of SF Enhanced mode). With respect to ionosphere gradient conditions it is applied the prescribed Ionosphere Gradient Models provided by ESA which comprises nominal, severe and extreme conditions to test EV3 CPF performance in all possible conditions (ionosphere gradient models based on data analysis since 2001 and comprising representative degraded conditions from previous solar-cycle #23 and #24). On the level of real data scenarios, EGNOS V2 RIMS data have been collected, and selected for nominal and severe ionosphere conditions.
The obtained results clearly demonstrate the efficiency and the robust design of the EGNOS V3 CPF algorithms to operate in all types of ionosphere gradient conditions with a high level of accuracy for ionosphere corrections and satellite parameters, good margin in the integrity bounding performances as well as the benefits on increasing service coverage area using MT28 and the new concept for ionosphere modelling.

REFERENCES
[RD.1] European GNSS Agency, “Airbus awarded EGNOS V3 contract”, February 2018.
https://www.gsa.europa.eu/newsroom/news/airbus-awarded-egnos-V3-contract
[RD.2] ICAO Standard and Recommended Practices (SARPs), Annex 10, Part 1, Amendment 90.
[RD.3] Minimum Operational Performance Standards (MOPS) for airborne navigation equipment (2D and 3D) using the Global Positioning System (GPS) augmented by the Wide Area Augmentation System (WAAS) - RTCA MOPS DO-229 C/D/E.
[RD.4] Global Positioning System Standard Positioning Service, Performance Standard GPS SPS PS 2008, 4th Edition, Sept. 2008
Galileo OS Service Definition Document, Issue 1.2, 16 December 2021



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