Estimating Continuity of GNSS

James Fernow and Daniel O'Laughlin

Abstract:	In this paper we describe a method for estimating the continuity of service (a measure of the probability of loss of service during a given exposure time) provided by the GNSS signal-in-space including GPS and Satellite-Based Augmentation Systems (SBASs) such as FAA’s Wide- Area Augmentation System (WAAS). Many organizations have done considerable work estimating availability of service provided by GNSS, but comparatively little work has been done to estimate their continuity. Yet, even if service provided by a particular implementation of GNSS has high availability, the service may not be operationally useful if continuity performance is not sufficiently high. Just as availability differs for different defined types of service (e.g., en route positioning, nonprecision approach integrity monitoring) provided by a single system, continuity also differs for different types of service. Requirements on service performance may be specified in terms of a minimum instantaneous continuity or on average continuity. Requirements may be specified on the contribution to continuity risk (one minus continuity) from unpredictable events such as random satellite outages and alerts by integrity monitor functions. Requirements may also be specified on the frequency of loss of service due to predictable events such as predictable satellite motion and scheduled satellite outages due to maintenance activities. The latter performance measure is not a probability, strictly speaking. A computer model has been developed that is similar to service volume models developed by several organizations including MITRE, but whose output includes different types of continuity in addition to availability. The model is being used to identify the dominant contributors to continuity in the Full LPV Performance (FLP) phase of WAAS and to help ensure that WAAS parameters are set to values consistent with continuity requirements. Some results are presented.
Published in:	Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004) September 21 - 24, 2004 Long Beach Convention Center Long Beach, CA
Pages:	2113 - 2123
Cite this article:	Fernow, James, O'Laughlin, Daniel, "Estimating Continuity of GNSS," Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004), Long Beach, CA, September 2004, pp. 2113-2123.
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