SBAS L1/L5 Enhanced ICD for Aviation: Experimentation Results

J. Fidalgo, M. Odriozola, M. Cueto, A. Cezón, C. Rodriguez, D. Brocard, J.C. Denis, E. Chatre

Abstract:	The SBAS Interoperability Working Group (IWG) was formed by the different SBAS service providers to ensure that SBAS systems are compatible and to plan future enhancement. Nowadays its main goal is to contribute to the elaboration of a new standard able to augment up to 4 GNSS constellations simultaneously and offering new services for SBAS dual frequency (L1/L5) users. For that purpose, an Enhanced SBAS L1/L5 ICD is currently under consolidation in the frame of the IWG. Two ICD families were firstly under consideration by IWG: • UDRE (User Differential Range Error) ICD family, aimed at defining a SBAS L1/L5 ICD model as similar to the current SBAS L1 ICD as feasible. • DFRE (Dual Frequency Range Error) ICD family, proposed in Walter et al. (2012) whose main objective was to simplify the number of messages and their structures as much as possible, eliminating the fast corrections and sending a single correction message per satellite and a MT6-equivalent message with DFRECI (DFRE Change Indicator) and updates of DFREIs every 6 seconds. After a period of analysis and trade-off, see Cueto et al. (2013) an agreement has been reached at IWG mid-2013 in order to work on a unified SBAS L1/L5 ICD, called from herein SBAS L1/L5 Enhanced ICD, providing the combined advantages of both candidates ICD’s: • Advantages of UDRE ICD for dynamic (i.e. multiple DFREI’s update) upon system event, allowing mitigating the risk of continuity events at user. • Advantages of DFRE ICD for simplicity, deterministic scheduling state conditions. This paper provides a summary of the experimentation results on the robustness and expected performances of the SBAS L1/L5 Enhanced ICD implemented in the frame of PROSBAS (Prototyping and Support to Standardization of SBAS L1/L5 Multi-Constellation Receiver) project’s related activities. This project, funded by the European Commission, has been technically managed by ESA (EGNOS Project Office), and the PROSBAS Industry Consortium has been led by GMV. Despite the fact that PROSBAS project covers aviation and maritime activities, this paper is focused on the aviation activities developed in the project. The aforementioned results have already been shared with IWG whose feedback has been taken into account in the analysis developed. The strategy followed to properly define and validate the SBAS L1/L5 Enhanced ICD, as well as to develop a proper experimentation to study the behaviour of this ICD, has been based on the following main steps: • Definition of an enhanced ICD: a definition’s summary of the SBAS Enhanced L1/L5 ICD was provided in Fidalgo et al. (2014). An iterative process for defining the SBAS L1/L5 ICD messages between ESA (EGNOS Project Office), Stanford University and PROSBAS consortium (GMV) has been carried out. The SBAS L1/L5 Enhanced ICD considered in this paper has been the result of such iteration process. • Development of the PROSBAS Service Provider and Receiver Prototypes including the implementation of the SBAS L1/L5 Enhanced ICD. These prototypes allow to extensively studying the behaviour of the Enhanced ICD in terms of robustness and expected performances while allowing flexibility in configuration. • Definition of the test plan, in terms of Key Performance Indicators (KPIs) in order to analyse the robustness and expected performances of SBAS L1/L5 Enhanced ICD. PROSBAS Prototypes provide as outputs KPIs related to the message sequencing in order to analyse the Enhanced ICD Robustness as well as Receiver KPIs performances such as accuracy, availability, integrity and continuity. Different experimentation tests and scenarios have been prepared in order to analyse the defined KPIs. • Analysis of the results of the different tests. • Provision of recommendations for the SBAS L1/L5 ICD following the results´ analysis. The following aspects of the SBAS L1/L5 Enhanced ICD are analysed and presented in this paper: • Sensitivity Analysis: The impact on performances of the Key Implementation Parameters (such as Delta_FC, UDRE statistics…) is analysed. • Fulfilment of SBAS L1/L5 Mission Availability, Continuity and Accuracy by analysing the compliance of LPV-200 requirements of the SBAS L1/L5 Enhanced ICD over ECAC service area. • Cat I VAL = 10m. compliance with two constellations of the SBAS L1/L5 Enhanced ICD in double-frequency mode over ECAC service area. • L1 legacy comparison: the SBAS L1/L5 Enhanced ICD achievable performances in L5-only back-up mode are compared to the ones of L1 MOPS. • Receiver sensitivity: the receiver sensitivity in terms of VPL to the ionospheric and tropospheric residual errors as well as to the receiver noise and multipath has been analyzed. • Transition modes: the effect of losing L1 frequency is analysed in the context of the SBAS L1/L5 Enhanced ICD in L5-only back-up mode. • Smoothing Interval influence at receiver level. • Service Area Expandability, by analysing the fulfilment of SBAS L1/L5 Mission Availability/Continuity/TTA/Accuracy performances of SBAS L1/L5 Enhanced ICD in ECAC+AFI service area. • Degraded Conditions Analysis, by analysing the impact at user level of the presence of satellite or constellations alerts, losses and recovery, degraded ionospheric conditions, degraded environmental conditions (multipath) or message loss. The main objective of the experimentation presented in this paper is to provide the level of performances achieved with the SBAS L1/L5 Enhanced ICD implemented in PROSBAS. In particular, it will be shown that: the minimum DFRE that the SBAS system is able to broadcast is the key parameter for the VPL performances. LPV-200 compliance is feasible with 1 constellation in double-frequency mode in the context of SBAS L1/L5 Enhanced ICD. Cat I VAL = 10m. compliance is feasible with 2 constellations but it might be risky. The performances achievable with the SBAS L1/L5 ICD in double-frequency mode are significantly better than in the case of L1 MOPS. On the contrary, the performances achievable with the SBAS L1/L5 ICD in L5-only back-up mode are worse than the ones of L1 MOPS mainly due to the GIVE degradation in L5. Concerning the receiver sensitivity to the different error budgets, multipath error is the one that impacts more on the VPL performances followed by receiver noise and tropospheric errors, while iono-free residual error does not have a high impact. If L1 frequency is loss, L5-only back-up mode ensures the availability of the service while the performances are degraded. Nevertheless, if L5-only back-up mode is not implemented, the loss of L1 frequency implies unavailability of the service. Iono-free technique implies better performances than divergence-free mainly due to the fact that in the second case the ionospheric delay is not removed. In the context of iono-free technique, larger smoothing time constants provide better performances. The service area expandability is perfectly feasible without any limitation in double-frequency since no extra bandwidth is needed compared to the case of single-frequency that implies more bandwidth due to the fact that more ionospheric-related messages have to be broadcast. In particular, it is shown that LPV-200 is feasible with 1 constellation over ECAC+AFI service area taking into account EGNOS V3 reference conditions. The impact on VPL of the loss or alert of a satellite or constellation is considerable, in particular in points in the border of the service area. The impact of degraded ionosphere in L5-only back-up mode could be of the order of 1-2 meters in VPL in points in the centre of the service area. The impact on VPL of degraded multipath environment (20% over the nominal multipath) could be of the order of tens of centimetres. Furthermore, the degraded multipath typical of airport environments could degrade the VPL of the order of 1-2 meters. Finally, the degradation of VPL due to message loss at receiver level (loss of two consecutive “Satellite Corrections” message of two different satellites) is driven by the OBAD mechanism and could be of the order of tens of centimetres. Finally, in this paper some of the main open points to be addressed in the subsequent phases of the standardization work will be pointed out.
Published in:	Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015) September 14 - 18, 2015 Tampa Convention Center Tampa, Florida
Pages:	1764 - 1774
Cite this article:	Fidalgo, J., Odriozola, M., Cueto, M., Cezón, A., Rodriguez, C., Brocard, D., Denis, J.C., Chatre, E., "SBAS L1/L5 Enhanced ICD for Aviation: Experimentation Results," Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015), Tampa, Florida, September 2015, pp. 1764-1774.
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