The U.S. has provided the highly accurate and dependable GPS service to users worldwide for nearly two decades. In addition to this service, the U.S. has made corresponding commitments as to the types and quality of current and future GPS service through various documents, such as the GPS Standard Positioning Service Performance Standard, the Federal Radionavigation Plan, and numerous interface specifications. The question is, how is the U.S. doing in meeting these commitments?

The service provided by GPS has historically been excellent with each succeeding year demonstrating improvements in reduced range, position, and timing errors and better availability of signals.  Despite these improvements, however, signal and service aberrations have occurred, resulting in brief periods of poor performance for users.  Examples include large range errors resulting from clock runoffs by SVN22/PRN22 and SVN23/PRN23 experienced in July 2001 and January 2004, respectively, and large range errors in the SVN54/PRN18 signal due to ephemeris errors experienced in April 2007.  These anomalies highlight the issue.  How can the magnitude and duration of these errors be minimized?  One way is to improve the monitoring of the GPS signals and service, thereby providing early detection and resolution of these anomalies. 

On January 1, 2004, the user range error from the signal of PRN 23 grew to more than 200,000 meters in magnitude.

Figure credit: University of Texas Applied Research Laboratories

There are other objectives that can be met in providing civil GPS signal and service monitoring.  Monitoring of the signal improves the situational awareness to the satellite operators, providing an up-to-date assessment of GPS performance worldwide.  It provides the U.S. Government the hard evidence that it is meeting its commitments for GPS service to its users.  And when monitoring data is stored in a database, it can be used for predicting potential future failures and analyzing past events.

In designing the original GPS ground control system, called the operational control system (OCS), the developers provided a means to monitor the military signals, but chose not to include capability to continuously monitor the civil signals and service.  Besides this limitation, the original tracking network had only five monitor stations, leaving area gaps where some signals could not be tracked.  This has since been mitigated somewhat by the addition of real-time data from ten monitor stations of the National Geospatial-Intelligence Agency (NGA), eliminating the gaps and providing redundant observations for each signal[1].  However this only addresses P(Y)-code signals, and the OCS remains unable to continuously process the civil signal (C/A code).

As GPS is modernized, and new civil signals (L2C, L5, and L1C) are added, the problem compounds.  The need to continuously monitor the new civil GPS signals as well as the old grows with the expanding capability. To remove the ambiguity as to what monitoring services are required for the civil GPS service, Tom Nagle, currently Program Manager, Civil Applications at the GPS Wing, initiated the development of a GPS Civil Monitoring Performance Specification (CMPS) published by the U.S. Department of Transportation, that would provide a clear statement as to what is meant by monitoring the civil GPS signals and service.  The current version of the CMPS was released in April 2009, and is available at the Space-Based Positioning, Navigation & Timing web site: http://pnt.gov/public/docs/2009/CMPS2009.pdf.

 The CMPS provides a comprehensive compilation of requirements for monitoring the GPS civil service and signals.  It does not define new requirements for GPS, but instead interprets existing high-level requirements that call for monitoring all signals all the time.  It accomplishes this by translating the commitments contained in guidance documents such as the GPS Standard Positioning Service Performance Standard and requirements contained in the signal interface specifications into a set of monitoring requirements.  These requirements are generally in the form of specific metrics that are be evaluated and thresholds that are to be met.

 In addition to requirements regarding direct monitoring of the service and signals, the CMPS considers requirements associated with non-broadcast data, reliability of monitoring, reporting dissemination, and archiving of results. It provides background material on the requirements (notes, algorithms, and explanations).  It even provides a partitioning of requirements indicating which are civil only, and which requirements overlap military needs.  

The CMPS was driven by the need to improve service to civil users of GPS.  The document is intended to be used to guide the U.S. Government in implementing ways to monitor the GPS civil signal and service.  A number of benefits result from having improved monitoring capability.

  • The reduced probability of failures adversely affecting users.  When such failures do occur, the magnitude and duration of range errors will be mitigated. 
  • Satellite operators will have full situational awareness into the service being provided to civil users.  Anomalies affecting civil users will be unambiguously identified for rapid isolation and resolution. 
  • With access to an up-to-date database of performance data, the operators will be able to identify potential failures earlier and head them off before they occur.
  • With archived data, operators will be able to perform historical analysis, such as investigations into service performance at the time of a transportation incident involving GPS.
  • The U.S. Government will have the ability to quantitatively confirm that GPS is meeting its performance objectives.

The 2009 version of the CMPS provides comprehensive and unambiguous guidance on what is needed to support civil monitoring of the GPS signals and service.  The CMPS provides traceability back to source documents ensuring all essential signals and services are monitored. It provides much helpful background information, including use cases, algorithms, and detailed discussions that go beyond the statement of the requirement to explain how to effectively implement the requirement.  The CMPS will be a valuable resource for those involved in the modernization of GPS, to ensure user needs will continue to be met as new signals and services are implemented.

The material in this blog was derived from the paper “Proving the Walk Matches the Talk – Verification of GPS Performance Assertions”, presented by John Lavrakas and Brent Renfro at the Institute of Navigation International Technical Meeting, January 2010, San Diego, CA, USA. For a copy of the paper, go to www.ion.org.

This blog will also be available on the GPS World website in their TechTalk section at http://techtalk.sidt.gpsworld.com/

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