Overview

As the forthcoming “Time and Navigation” opening next March at the Smithsonian National Air and Space Museum reminds us, modern navigation depends profoundly upon time — the more precise, accurate, and stable a navigation system’s timing is, the more precise, accurate, and stable its positioning capability.

Not only that, precise time — as so affordably provided by GNSS satellites — lies at the heart of global security. A Department of Homeland Security (DHS) analysis found that 15 of 18 critical infrastructure/key resource sectors in the United States depend upon GNSS-provided time — such things as communications, banking and finance, emergency services, electrical power grids. Those dependencies ripple throughout the increasingly complex networks that bind the developed world together.

We asked Francine Vannicola to bring us up to date on the status of time/frequency capabilities of GNSS systems. Vannicola is a mathematician at the U.S. Naval Research Laboratory, Advanced Space PNT Branch in Washington, D.C. Her current work involves GPS on-orbit and Block IIF life test atomic clock analysis. Vannicola currently serves as chairman of the PTTI (Precise Time and Time Interval) Executive Committee.

The NRL has been involved with the GPS program since its beginning and has provided expertise for space qualified atomic frequency standards (AFS) for all phases of the program. On a quarterly basis, NRL produces a performance analysis report for all operational GPS AFSs.

Since 2004 the NRL has been conducting life tests of the GPS Block IIF cesium AFS (CAFS) and rubidium AFS (RAFS) in the NRL Precision Clock Evaluation Facility (PCEF), one of the major facilities within the Naval Center for Space Technology.

Life testing of the Block IIR RAFS was conducted there from 1997 to 2004.

The PCEF was developed at NRL to support development of high-precision clocks for GPS spacecraft and ground applications, primarily atomic standards. Investigations of on-orbit anomalies are performed within the PCEF to attempt to duplicate similar effects in space-qualified hardware under controlled conditions.

To date GPS has launched 51 operational satellites and activated 125 AFSs. The current constellation consists of 31 operational satellites that include three generations of satellites (Block IIA, IIR/M, and IIF) and seven types of CAFS and RAFS.

The Block II/IIA satellites were launched from 1989 through 1997, and each satellite contains two CAFS and two RAFS. The Block IIR/M satellites were launched from 1997 through 2009 containing three RAFS each. The Block IIF satellites began to launch in 2010 with one CAFS and two RAFS on board each.