Time and frequency dissemination: advances in GPS transfer techniques

GPS World, Nov, 2004 by Thomas E. Parker, Demetrios Matsakis

The atomic clock is one of the greatest inventions of the 20th century. When the first commercial atomic clocks based on the resonance of the cesium atom were introduced in 1958, available clock accuracy jumped by orders of magnitude. The new clocks would not gain or lose one second in a thousand years. Scientists used laboratory versions of the cesium atomic clock to define the atomic second and to establish and maintain a time scale to which all clocks could be set. But how do you set a clock, particularly one which is accurate to a microsecond or better? Or once set, how do you keep track of its error? Clocks are monitored and synchronized using a time and frequency dissemination technique. In the past, radio signals and traveling clocks were used for this purpose. But GPS, itself dependent on atomic clocks for its operation, has become the best technique for intercomparing clocks and for helping to maintain the world's time systems. In this month's column, scientists from the 'United States' two national time-keeping laboratories, the National Institute of Standards and Technology and the U.S. Naval Observatory, discuss the past, present, and future use of GPS for accurately disseminating time and frequency.--R.B.L.

GPS is not only a high accuracy navigation system, it also delivers time with unprecedented accuracy. Free to the user, GPS is the world's most-accurate globally available one-way source of time and frequency. Declaration of full operational capability of GPS in 1995 revolutionized timing as well as navigation.

Dual use (by military and civilians) of GPS was announced as a formal goal in the wake of the 1983 downing of Korean Air-lines Flight 007, and civilian users now far outnumber military users. In addition to navigation, GPS also has had a huge impact on the use of precise time.

Each GPS satellite contains several atomic clocks and continually broadcasts the time and its position. A GPS receiver tracking at least four GPS satellites can solve for the receiver's unknown position and time at virtually any location on the globe, with a precision of a few meters and a time error of a few tens of nanoseconds (ns), excluding receiver calibration errors. A timing user operating from a known fixed location can derive time from GPS using just one satellite, and with averaging, a timing accuracy of a few nanoseconds is possible.

Time from GPS is now used for many civilian purposes, including synchronization of communications systems, cell phone networks, and power grids. It also is used for many commercial applications where accurate time tagging is becoming increasingly important.

The international timing community utilizes GPS to help produce the world's coordinated atomic time scale, Coordinated Universal Time (UTC). In some instances, GPS serves as the primary transfer tool, while in others it serves as a backup to two-way satellite time and frequency transfer (TWSTFT).

The International Bureau of Weights and Measures, or Bureau International des Poids et Mesures (BIPM), in France is charged with providing the time standard UTC. The BIPM collects data via GPS or TWSTFT from more than 200 atomic clocks and a few primary "absolute" frequency standards from more than 50 institutions around the world. Once a month, BIPM uses these data to produce the standard international references for frequency and time, International Atomic Time (TAI) and UTC, which is equal in rate to TAI, but adjusted by an integer number of seconds to account for variations in the rotation of the Earth.

Most of the contributing laboratories produce real-time realizations of UTC. The U.S. Naval Observatory (USNO) and the National Institute of Standards and Technology (NIST) produce official real-time realizations of UTC for the United States. These time scales are identified as UTC(USNO) and UTC(NIST). UTC(USNO) is the external time reference for GPS, and consequently the UTC time derived from GPS is considered fully traceable to international standards for civil and legal time, subject to user equipment errors.

Recent years have seen a number of improvements in techniques and equipment that have resulted in improved time transfer. This article reviews several advances.

Time Directly from GPS

The simplest and most-direct way to obtain time from GPS is to use the signals broadcast directly by the GPS satellites as illustrated in Figure 1.

Authorized GPS users, who have access to the encrypted P-code on both GPS frequencies (L1 at 1575.42 MHz and L2 at 1227.60 MHz) can directly estimate the ionospheric delay of the signals, which allows for a more-accurate position, velocity, and timing (PVT) solution. Most civilians can utilize only the C/A-code on L1, but in early 2005 GPS will introduce its first IIR-M satellite, which will broadcast a new unencrypted civil L2 signal, and in 2006 GPS will add a third civil signal, L5. There also are several commercial codeless and semicodeless GPS timing receivers available to all users that can derive timing information from L1 and L2.