GPS Civil Signal Monitoring – Advancing Toward Implementation

Andrew Hansen, Karen Van Dyke, Calvin Miles, John W. Lavrakas

Abstract:	The Global Positioning System provides user positioning, navigation, and timing service. Fulfillment of this service entails not only performance commitments on the specification and broadcasting of the GPS signals but also monitoring of those broadcast signals so that both operators and users are aware that the service is meeting its stated commitments. The GPS Civil Monitoring Performance Specification (CMPS) has been adopted by civil federal agencies to identify the requirements for civil signal monitoring. The CMPS is a mature document and defines a set of metrics for assessing GPS performance against standards and commitments defined in official U.S. Government documents such as the Standard Positioning Service Performance Standard, the Navstar GPS Space Segment/Navigation User Interfaces (IS-GPS-200), Navstar GPS Space Segment/User Segment L5 Interfaces (IS-GPS-705), and Navstar GPS Space Segment/User Segment L1C Interfaces (IS-GPS-800). To the extent practicable, each metric defined is traceable to one or more specifications or commitments of performance. The CMPS also identifies the scope and range of monitoring needs not directly traceable to the key GPS reference documents but expected by civil users. These needs include the ability of the system to detect defects in signal and data, the rapid report of anomalous service behavior to satellite operations for resolution, and notification to users of the causes and effects of such anomalies for their various service types (e.g., positioning, timing, and navigation). Two key objectives needed to successfully fulfill Civil Signal Monitoring (CSM) are: 1) Direct notification to GPS operators when critical, time sensitive specifications on signals used by the civil community are violated, and 2) Publication of the observed performance level of signals from the GPS service available for civil use. In some ways—mostly relating to Information Assurance (IA)—these are competing objectives. However, the following six groups of monitor functions are important to both objectives. a. Metrics verification – verification that signal and service performance meets commitments made in SPS Performance Standard, interface specifications, and other government specifications b. GPS Operator notification – timely notification to satellite operators of real-time GPS anomalies and situational awareness of civil signal performance c. Civil user notification – notification to civil authorities and agencies or GPS anomalies and situational awareness of civil signal performance d. Signal quality monitoring – assessment of carrier waveform and code performance to ensure within designated limits e. Archive and retrieval – archival of CSM reports and data for retrieval by the GPS operators, analysts, and civil authorities f. Signal monitoring – monitoring of the four broadcast signals, L1 C/A, L2C, L5, and L1C, accessible to civil users There are various ways a CSM system may be implemented for the future GPS service. That variety is spanned, roughly, by the allocation of monitoring functions to either the Next Generation Operational Control System (OCX) which is currently under development or a Non-OCX system using other US Government reference networks that observe the GPS signals. The central theme of the paper is to present the recommended CSM functional architecture with an implementation that leverages both OCX and Non-OCX elements in a complementary fashion to satisfy the two key CSM objectives. Description of architectural allocations of the monitoring functions, operational aspects of implementation, and a cost-effectiveness trade space for implementation alternatives is described. The paper closes with thoughts on the future of CSM, including expected capabilities and milestones.
Published in:	Proceedings of the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2014) September 8 - 12, 2014 Tampa Convention Center Tampa, Florida
Pages:	3363 - 3372
Cite this article:	Hansen, Andrew, Van Dyke, Karen, Miles, Calvin, Lavrakas, John W., "GPS Civil Signal Monitoring – Advancing Toward Implementation," Proceedings of the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2014), Tampa, Florida, September 2014, pp. 3363-3372.
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