Global Positioning System Timing Criticality Assessment – Preliminary Performance Results

James Carroll and Kirk Montgomery

Abstract:	Precision timing and time interval (PTTI) capabilities are becoming more commonplace throughout the world. The increasing demand for and use of precise time comes with a potentially increasing risk of over-reliance on a single timing source. In addition, many precise time applications are considered “critical,” in that service disruption could result in severe consequences. The Global Positioning System (GPS) provides PTTI services that meet or exceed most user requirements, at increasingly lower cost. It is important to ensure that critical PTTI services are maintained during periods of disruption or loss of the GPS signal. This paper presents key findings, conclusions, and recommendations from a Timing Criticality analysis of the impact of loss of GPS time and frequency on important sectors of U.S. society. The focus in this paper is on civil and commercial sectors, including telecommunications and electric power. The goal of the analysis was to assess the consequences of disruption of GPS time on applications critical to the safety, security or economic well-being of the United States. The corollary goal was to determine the benefits and costs of backup systems - including modernized Loran - that mitigate the impact of the GPS disruption. The timing criticality analysis extends and updates prior work, in particular the DOT/Volpe Center and Homeland Security Institute (HSI) GPS vulnerability studies, and the HSI timing criticality study. The prior work facilitated identification of Subject Matter Experts (SME) in timing and in the critical applications (for example, electric power distribution) that depend on precise time and frequency. For each sector, assessments of timing requirements and of the performance impact under representative GPS outage scenarios were conducted. An early finding is that use of GPS backup systems needs to be clarified to some GPS timing users. Another finding is that GPS disruption has resulted in lost utility billing information. While this particular economic disruption is marginally tolerable for the time being, many electric power distribution SMEs predict a growing dependence on GPS time (hence, on the backup to GPS) for real time maintenance and grid stability.
Published in:	Proceedings of the 40th Annual Precise Time and Time Interval Systems and Applications Meeting December 1 - 4, 2008 Hyatt Regency Reston Town Center Reston, Virginia
Pages:	485 - 506
Cite this article:	Carroll, James, Montgomery, Kirk, "Global Positioning System Timing Criticality Assessment – Preliminary Performance Results," Proceedings of the 40th Annual Precise Time and Time Interval Systems and Applications Meeting, Reston, Virginia, December 2008, pp. 485-506.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In