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Session B3: Future Trends in GNSS Augmentation Systems

EGNOS V3 Demonstration of SBAS Signal Transmission with Increased Strength and Robustness Performances
Arnault Sfeir and Abdenasser Zaidi, Airbus Defense and Space; Marc Solé-Gaset, Miguel-Ángel Suárez-Gopar, David Lázaro-Leiva, Joan Clua-Lara, Indra; Sergi Locubiche-Serra, José A. López-Salcedo, Gonzalo Seco-Granados, ESA; Cyrille Boulanger and Peter Claes

This paper will present improvements to EGNOS broadcast signal performances, as demonstrated during the first live experimentation campaign with the new uplink station NLES (Navigation Land Earth Station) through a Eutelsat satellite.
EGNOS is the EU SBAS in charge of providing GNSS users with navigation corrections and an integrity service, broadcasted from geostationary (GEO) satellites.
Airbus Defense and Space, together with its key partners, is currently developing the second generation of EGNOS (EGNOS V3) for the European Space Agency (ESA) and for the European Union Agency for the Space Programme (EUSPA) cfr. [RD.1].
This new generation of SBAS will operate on a multi-frequency (L1/L5, E1/E5), multi-constellation basis (GPS, Galileo) embedding security protection against cyber-attacks. Thus, EGNOS V3 will improve both GPS and Galileo performances, in order to provide satellite-positioning services to the most safety-critical applications such as aircraft navigation, as specified in aeronautic SARPS standards [RD.2] [RD.3].
In addition, and in order to provide new services and features such as dissemination in E5b band and signal broadcast through non-GEO satellites, ESA, European Union (EU) Space institutions and industry have initiated preliminary definitions assessments for the evolution of EGNOS service beyond Version 3, under the EGNOS Next Generation studies frame, part of the EU R&D Horizon Europe program.
As part of the EGNOS V3 development, Airbus, with its key partners Indra, IEEC/Universitat Autònoma de Barcelona(UAB), and Eutelsat, have successfully completed the first test signal broadcast of the European Satellite-Based Augmentation System (SBAS) EGNOS V3 respecting the Single Frequency L1 and Dual Frequency Multi Constellation (DFMC) L5 formats.
The EGNOS V3 test signal campaign engaged the new version of EGNOS NLES developed by Indra as well as the GEO satellite E5WB’s dual-frequency SBAS payload developed by Airbus. This first NLES to E5WB integration aimed at verifying the Long Loop Algorithm (LLA) ensuring each signal is rightly steered at the satellite output by adapting in real time the uplink signal from the ground.
The test campaign secured the critical design of EGNOS V3 NLES before final implementation and full factory qualification. It additionally confirmed that its design increases strength and robustness in continuity of the signal, and it checked the adaptation capability to unexpected satellite on board local oscillator drift. Moreover, the LLA design enables an easy introduction of new signal independent steering.
In this context, the scope of the proposed paper is:
A. to describe the first EGNOS V3 test signal broadcast, being the first European SBAS signal transmission respecting both L1 SF and L5 DFMC formats.
B. to present the good performances measured during this successful LLA demonstration campaign in real conditions {Indra NLES + satellite E5WB + Eutelsat RF station};
C. to detail the enhanced robustness and increased continuity offered to the users of the coming future EGNOS V3 L1 & L5 signal broadcast.
D. to introduce the flexibility provided by NLESV3 LLA for potential evolutions of EGNOS Next Generation.
A. A KEY EGNOS V3 DEVELOPMENT MILESTONE
This first EGNOS V3 NLES to satellite E5WB integration aimed to verify an EGNOS V3 key algorithm ensuring each signal is rightly steered at the satellite output by adapting in real time the uplink signal from the ground. This was an important milestone to secure the critical design of EGNOS V3 NLES before the final implementation and its full factory qualification. This NLES LLA demonstration campaign was also the first European SBAS transmission fully respecting the Single Frequency L1 and DFMC L5 formats. The tests were conducted during Q4 2021 from the Rambouillet teleport using E5WB satellite.
For the first time in a European SBAS system, an upper range PRN (150) was used. This has been possible thanks to the frame provided by the DFMC SARPS (SBAS L1 SARPS [RD.2] and L5 SARPS [RD.3]). In addition to that, the use of upper range PRN provided an additional safety barrier in order to prevent current EGNOSV2 safety-of-life (SoL) users to use an EGNOS V3 test signal. From the EGNOS V3 point of view, the objective of this campaign in real conditions was to demonstrate the correctness and suitability of the NLES LLA design as well as to check the correct interfacing between NLES and RF Station/GEO satellite. Further information about the NLES configuration used during the demonstration will be provided in the paper.
B. NLES/LLA TECHNICAL PERFORMANCE RESULTS DURING EGNOS V3 TEST SIGNAL BROADCAST
L1 and L5 SARPS [RD.2] [RD.3] define the SBAS signal characteristics. In L5, 2 levels of signal quality are defined for GEO ranging or no GEO ranging signals. GEO Ranging is not used in EGNOS V3 and the performances are then aligned to the related requirements.
In the LLA demonstration campaign, the very first steering attempt by NLES and its LLA passed immediately with the steering performances reached from the first minutes onwards, demonstrating very fast convergence. The Indra tool capacity for measuring in real time the {NLES+RFE+GEO} steering performance without the need to interrupt the signal broadcast, enabled continuously receiving and recording on the first attempt during 3 days without any NLES issue.
Most of the steering requirements in [RD.1] and [RD.2] are defined at the output of the GEO satellite when obviously no access to direct measurement is possible in a real situation and therefore before accumulating downlink range and Doppler when measured from the RF station ground antenna. Typically, measuring these performances requires either to stop signal transmission and/or to wait some days to potentially get GEO orbit data provided by the operator to perform the assessment of the requirements for the transmitted signal. Within the frame of EGNOS V3, a new approach has been implemented in the NLES LLA. This approach consists on having the capacity to perform the assessment of steering performances in real time without stopping the NLES RF signal transmission. The LLA team has developed two different techniques to achieve this objective: (1) the first one bases its assessment on the estimations done by the LLA internal Kalman filter [RD.5], while (2) the second is fully based on computations independent from the NLES LLA. Both techniques make use of the Long Loop pseudorange and carrier phase measurements.
The paper will provide more details about EGNOS V3 steering requirements from SARPS [RD.2] [RD.3]. The results of the 3 days LLA test will be provided in the full paper, demonstrating the compliance to EGNOS V3 requirements, with additional features such as the capability to measure the GEO satellite local oscillator (LO) drift in quasi real time.
C. ENHANCED ROBUSTNESS AND INCREASED CONTINUITY OFFERED TO THE USERS OF THE COMING FUTURE EGNOS V3 L1 & L5 SIGNAL BROADCAST
For reliability purposes and to improve the EGNOS continuity, there are 2 NLES per GEO satellite in warm redundancy (1 active, 1 passive), to reduce dependency against all ground equipment failure. However when the active NLES switches to the passive one, it generates a continuity event. These switches have to be as rare as possible and can be due to the NLES susceptibility to external events such as GEO to NLES short RF interruptions (masking, RFE equipment hot switching when failure, …) or the lack of other RF signals used by the LLA (GNSS time computation). In the frame of NLES for EGNOS V3 development, its Long Loop Algorithm development has been thought to strongly improve the NLES robustness before the need for an NLES switch, so it is expected to offer a better EGNOS continuity performance than required.
NLES LLA uses four different signals to perform signal steering [RD.5]:
GNSS (GPS and Galileo) signals used to keep signals stick to GNSS time. The NLES unit tracking RF signal from omnidirectional antenna continuously provides the measurements needed for determining the clock offset between the GNSS time and the NLES clock. In that sense, these are fed to a filtering block within the estimation module in order to filter out the measurement noise and provide an accurate estimate of such offset. This estimate is then used by the LLA for clock steering of the SBAS signals to GNSS time by means of NLES Signal generator corrections.
Long Loop (LL) signal encompasses the signal coming from the signal generator that is transmitted to the GEO satellite and broadcasted back to the NLES. After passing through the RF antenna signal conditioning block (input filter, LNA…), it is fed into the LLA. It includes, but not only, all the uplink and downlink propagation effects, which are to be estimated within the LLA for further compensation. In addition, it includes the corrections applied at the signal generator.
Middle Loop (ML) signal being the return path back to the NLES. It is used to subtract the signal generator corrections from the long loop path, in order for the Kalman filter to purely perceive the propagation effects to be estimated, irrespective of the controls that are actually being applied by the LLA.
Short Loop (SL) signal coming from the return path back to the NLES without including the effect of the RF antenna signal conditioning block. This signal is fed back to the LLA and it is used as a source of information about the corrections that have been applied at the signal generator so far at a given time instant, and thus determine the corrections that remain to be applied.
NLES has been designed to be able to support interruptions on the reception of these signals. Based on the type of signal interruption, there are two main scenarios to be considered:
the case of short masking, burst interferer, equipment hot redundancy switch, affecting uplink or downlink antenna during or after LLA convergence, so the LLA must sustain in this condition up to 3 seconds. The LLA implemented within EGNOS V3 incorporates a prediction mode that internally propagates the state-space model of the signal parameters to be tracked and the controls without relying on input measurements. This becomes a self-standing approach that ensures system stability and service continuity when the Medium Loop or Long Loop signals are unavailable for short duration.
The case of large interruption(s) of GNSS (timing) and Short Loop signal. This might be caused by an interferer close to NLES omnidirectional antenna for instance. NLES implementation within EGNOS V3 incorporates a mechanism being able to estimate NLES-GNSS time clock offset which allows keeping NLES signals aligned to GNSS time. This mechanism withstands large interruptions (more than 12 hours) of either GNSS or SL signal, without impacting the NLES LLA performances.
Both scenarios have been forced and checked during the LLA demonstration campaign. The final paper will provide further details about the results of the real tests and the implemented techniques.
Another source of NLES switching depends on the implemented design regarding the integrity check of the SBAS broadcasted at DAL B level. Unlike other SBAS uplink station might engage an NLES switch as soon as 3 successive received downlink messages differ from the 3 NLES uplink ones, or immediate switch when at least 1 message is missing or truncated because of RF short interruptions, the EGNOS V3 NLES design has been optimized as following:
NLES in active state switches to passive state (SiS cut) as soon as (at least) 3 successive discrepancies occur; a discrepancy is no or a truncated L1/L5 downlink message received from SBAS satellite or a L1/L5 downlink message different from the selected L1/L5 SBAS uplink message.
As needed for the SiS integrity check, the NLES calibration signals, linked to uplink part, might be subject to also RF interruptions as per above potential short RF interruptions due to RF station HPA or LNA switch.
These interruptions do not lead to EGNOS V3 GEO broadcasted integrity issue because integrity remains warranted thanks to the NLES integrity function implemented design in the EGNOS V3 system. This at the same time increases SiS continuity.
D. OTHER EGNOS V3 / NLES DESIGN FEATURES AND ITS FLEXIBILITY PAVING THE FUTURE
As mentioned above, the EGNOS V3 NLES is able to measure the satellite on board local oscillator drift driving the absolute frequency translation from the received C-band uplink to L-band downlink. Therefore, via a configuration maintenance task, the NLES can pre-correct it with the closest discrete value while keeping both satellite’s transponders’ frequency coherency. In this way, this budget is partly removed from the Doppler shift budget encompassing satellite movement (inclination, manoeuvers), but also this C uplink to L downlink translation factor associated to this satellite master reference unit performance in local oscillator drift. The correction capacity from ground by the NLES has been demonstrated during the campaign and will be deeply described in the paper.
The EGNOS V3 NLES LLA design enables an easy introduction of new signal independent steering such as adding an E5b signal. In addition, these terrestrial users need from high elevation broadcasts that could be offered by non-geostationary orbits like LEO, MEO, HEO or IGSO, but these orbits request higher Doppler range capacity and robustness for the NLES to initially track in high Doppler shift satellite conditions and then to steer the signal.
Like the other SBAS uplink stations, the EGNOS V3 NLES LLA tracks both L1 and L5 signals to steer both broadcasted signals at the satellite output. To pave the way for EGNOS Next Generation architecture and taking advantage of relaxation of steering requirements in L5 frequency, the EGNOS V3 NLES LLA architecture is ready to decouple and independently steer L1/L5/E5b signals.
The EGNOS V3 NLES has been designed in order to be able to handle an extended C-band frequency range of [5.7; 7] GHz allowing NLES to be used with a wider range of geostationary satellites.
The paper will explain how the current EGNOS V3 NLES architecture is already paving the way towards the EGNOS Next Generation for which three European studies have already started.

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] Standards and Recommended Practices (SARPS) Annex_10_amdt91, 2018
[RD.3] DFMC SBAS Standards and Recommended Practices (SARPS), 2020
[RD.4] Architecture and Performance of the Long Loop Algorithm for EGNOS V3 NLES Stations, ION GNSS+ 2021



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