NIST Method Improves Reliability Of GPS Clocks

Widelyused by the military, first responders, surveyors and even consumers,GPS is a navigation and positioning system consisting of ground-basedmonitors and a constellation of satellites that rely on atomic clocks.A statistical method, developed by the National Institute of Standardsand Technology (NIST) and tested and implemented with the help ofseveral collaborators, has made the job of analyzing the accuracy andreliability of these satellite-borne time signals significantly fasterand easier. The method will help ensure that GPS clocks produceaccurate location and distance measurements and remain closelysynchronized with official world time.

The NIST method, describedin a recent paper,* has been incorporated over the past few years intothe GPS clock analysis software system managed by the Naval ResearchLaboratory (NRL). The satellite clocks—commercial devices based in parton research originally done at NIST—use the natural oscillations ofrubidium atoms as "ticks," or frequency standards. The algorithm helpsdetect and correct GPS time and frequency anomalies. The algorithm alsocan be used to improve the control of other types of atomic clocks andhas been incorporated into commercial software and instruments forvarious timing applications, according to NIST electronics engineerDavid Howe, lead author of the paper.

A GPS receiver pinpointsits location based on the distance to three or more GPS satellites atknown locations in space. The distance is calculated from the time ittakes for satellite radio signals to travel to the receiver. Thus,timing accuracy affects distance measurements. The NIST method makes aseries of mathematical calculations to account for numerous measures ofrandom "noise" fluctuations in clock operation simultaneously. Thismakes it easier to estimate many sources of error and identify theonset of instabilities in the clocks in minutes or hours rather thandays. Adjustments then can be made promptly. The technique also couldaccelerate the evaluation of clocks during the process of building GPSsatellites, where test time is at a premium. "Ultimately, it shouldimprove reliability, stability and accuracy for many people who use GPSfor time and navigation,"said Howe.

Co-authors of the paperinclude scientists from NRL, the Jet Propulsion Laboratory at theCalifornia Institute of Technology, the Observatoire de Besancon inFrance, and Hamilton Technical Services in South Carolina.

###

* D.A. Howe, R.L. Beard, C.A. Greenhall, F. Vernotte, W.J. Riley,T.K. Peppler. Enhancements to GPS operations and clock evaluationsusing a "total" hadamard deviation. IEEE Transactions on Ultrasonics,Ferroelectrics, and Frequency Control. August 2005.