Tightening DGNSS Protection Levels Using Direct Position-Domain Bounding

O. Osechas, J. Rife

Abstract:	The corrections computed by operational space-based augmentation systems (SBAS) consistently achieve a high quality [1] that, the authors believe, has the potential to support GAST-D operations without requiring a local ground facility. We envision a system that would use differential corrections computed by SBAS, but with enhanced fault detection provided by specialized autonomous monitors (such as [2]) and collaborative monitors (such as [3]). If feasible, such a system would have two major advantages over ground-based augmentation systems (GBAS): It would reduce the installation costs generally associated with GBAS, and it would be more robust to radio-frequency interference (RFI) than GBAS. The approach proposed in this paper would make due without a local ground facility. In this sense, the proposed approach is different that the notion of a Local Airport Monitor (LAM), which has previously been explored as a means to leverage SBAS corrections for precision landing applications [4]. In LAM, SBAS corrections were processed at a local ground station, checked for integrity with local ground-based monitors, and re-broadcast to users via the VDB. Instead, in the proposed system users would use SBAS-computed differential corrections and perform their own integrity checks on board, autonomously for fast faults, or via networked collaboration for slow faults. The challenges involved in supporting GAST-D service with SBAS differential correction stem from two requirements: Time-to-Alert (TTA) and Vertical Alert Limit (VAL). SBAS currently provides LPV-200 service, which operates with a TTA greater than 6 s, while GAST-D requires a 2 s time-to-alert. The Vertical Alert Limit for LPV-200 service is 35 m; by contrast, GAST-D service requires a VAL of 10 m. Availability would be abysmal if SBAS were to be used for GAST-D approaches without modification, as typical SBAS protection levels very often exceed 10 m. This paper will introduce an alternative concept for obtaining a vertical protection level (VPL) that is more compatible with GAST-D operations. Our approach is based on two capabilities: local monitoring for off?nominal events and tight bounding of nominal errors. Local monitoring is essential to meet TTA requirements and to achieve higher spatial resolution (e.g. for ionosphere anomaly detection) than is possible with SBAS alone. Presuming that off-nominal events can be excluded through supplemental monitoring, a much tighter protection level for the SBAS-corrected position solution is possible, particularly if the position-bound is formulated directly in the position domain. (By contrast, typical protection levels are based on root-sum-squared range-domain error bounds, an approach which introduces significant excess conservatism.) The notion of achieving GAST-D using SBAS corrections is not unreasonable given actual SBAS performance under nominal conditions. According to [1], WAAS is able to provide a service with errors below 2 m 95% of the time. Assuming a truly Gaussian distribution and extrapolating out to a 10^-9 integrity bound implies a protection level of approximately 6 m bound. Thus, margin exists relative to a 10 m VAL to cover inflation needed, for instance, to cover heavy distribution tails [5]. Though supported by operational SBAS data, the notion of satisfying a 10 m VAL seem very aggressive relative to the protection levels provided by current SBAS systems. In effect, our proposed method would remove conservatism in existing protection level calculations needed for the following reasons. (a) To account for a long time-to-alert: A significant reduction in TTA is required for GAST-D operations and will reduce the potential for faults to develop over time before detection. (b) To account for ionosphere undersampling: A networked ionosphere monitor can provide sensitivity to anomalies on a smaller scale than SBAS monitoring networks [3]. (c) To account for range-domain overconservatism: Overbounding range-domain errors introduces significant overconservatism in computing a position-domain bound when heavy tails are taken into consideration [6], an effect which is mitigated in our approach by the introduction of a direct position-domain VPL calculation. A major focus of this paper will be on the idea of direct position-domain VPL calculation. The bounding model will be based on an analysis of real-world data, drawn from trusted, publicly available sources, such as IGS and EGNOS SISNet. The development of this error model will require a thorough analysis of all quantities currently used for VPL calculations, as well as some quantities not currently considered. In order to establish a model with predictive power, these influences will likely be categorized and grouped according to their influence on the position error, and a binning scheme will have to be designed to provide the best possible repeatability. Ultimately our VPL calculation will be based on a look-up table, rather than a formula (as is currently the case for existing SBAS users). An important concern will be to keep this look-up table as compact as possible, so that it can be applied practically in a real-time system. Preliminary results suggest a compact look-up table may be possible, because satellite geometry is shown to have a relatively weak effect on error variance over a wide range of typical conditions. The potential benefit of our concept is enticing: GAST-D service on a continental scale without requiring ground facilities. This would represent a dramatic reduction in the costs involved in providing GAST-D service to airports within SBAS coverage. References: [1] Federal Aviation Administration. (2012) Wide Area Augmentation System Performance Analysis Report # 43. [2] O. Osechas, P. Misra, and J. Rife. (2012) Carrier-Phase Acceleration RAIM for GNSS Satellite Clock Fault Detection. NAVIGATION, 59(3), pp. 221-235.. [3] O. Osechas, J. Rife. (2012 ) Distributed Ionosphere Monitoring by Collaborating Mobile Receivers. Proceedings ION GNSS. [4] J. Rife, S. Pullen, T. Walter, E. Phelts, B. Pervan, P. Enge. (2006) WAAS-Based Threat Monitoring for a Local Airport Monitor that Supports Category I Precision Approaches. Proceedings ION PNT. [5] J. Lee, S. Pullen and P. Enge. (2009) Sigma Overbounding using a Position Domain Method for the Local Area Augmentation of GPS, IEEE Transactions on Aerospace & Electronic Systems 45(4), pp. 1262 - 1274 . [6] J. Rife and B. Pervan (2012). Overbounding revisited: discrete-error distribution modeling for safety-critical GPS navigation. IEEE Transactions on Aerospace and Electronic Systems, 48(2), pp. 1537-1551.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	1329 - 1340
Cite this article:	Osechas, O., Rife, J., "Tightening DGNSS Protection Levels Using Direct Position-Domain Bounding," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 1329-1340.
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