TWSTT: What It Is and How It Is Performed

The U.S. Naval Observatory (USNO) maintains precise time and the nation's Master Clock with the goal of providing the most precise and accurate time to remote users. USNO has been developing the two-way time transfer method (TWSTT) since the first communications satellites, Telstar and Relay-II, were put into operation in the very early 1960s. The recent development of low-cost, portable, very small aperture terminals (VSATs) and a new generation of time transfer modems have allowed the two-way time transfer method to be put into regular use at USNO.

Some of our base-station antennas (click on to enlarge)

The ultimate goals of the two-way project have been to improve the USNO Master Clock (USNO (MC)) and its operational utility to military and civilian remote users, and to improve time comparisons with other timing centers. The USNO (MC) is improved because two-way transfer allows inclusion into USNO's operational time scale remote clocks via the most precise and accurate timing links possible. The operational utility to military users is increased because high-precision military users are able to make direct comparisons of remote clocks with the USNO (MC) via independent means from other military operational timing systems, such as the Global Positioning System (GPS). The two-way timing links with national timing centers will improve the long-term stability of the International Atomic Time (TAI), which is the NATO timing reference.

Our mobile Earth station antenna; click on to display (USNO Image - JPG - 223Kb)

What is two-way time transfer? In its simplest form, two-way time transfer between two timing standards may be accomplished by having each of two time standards send a one-pulse-per-second (1pps) signal to the other time standard over a communications circuit. It is a point-to-point communications link. The communications circuit used is not important and may be made through any wide-band circuits, such as coaxial cable, fiber-optic cable, microwave transmission, television, laser light transmission, or communications satellites, to name a few. The transmission medium introduces delays, but this delay must be nearly reciprocal, i.e. the delay is the same in both directions, thus cancelling out. Each lab measures the time interval between the transmission of its local 1pps and the time it receives the remotely generated 1pps signal, typically using a time interval counter (see Figure 1 for a simple diagram of the procedure). The true time offsets of the two time standards can be measured very precisely (<0.2 nanosecond) and accurately (~1.0 nanosecond). By taking data over a period of time, the long-term behavior, i.e. frequency changes, rates, jumps, drifts, etc., that will affect the accuracy and stability may be characterized. The operational usefulness of the clocks are improved as well as the confidence in related decision making. The day-to-day stability of two-way time transfers can nearly reach the performance of the best reference clocks.

Figure 1. A simple diagram of the two-way timing method as applied between two timing centers. The communications link is a Ku-band link using a commercial satellite (courtesy of Bill Powell).

The two-way timing system employed at the USNO uses geostationary communications satellites for the communications circuit. Currently, commercial Ku-band satellites and X-band satellite links are possible. The time transfer modem being used is a very important piece of equipment for TWSTT to succeed. The modem codes/decodes the spread-spectrum signals of the 1-pps that is transmitted and received over the satellite communications circuit. The modem also is used to exchange the data collected during the run at each location, allowing near-real-time data processing to take place.

The U.S. Naval Observatory, in Washington, DC, has a wide range of steerable and fixed antennas, VSATs, and related equipment to fulfill any users requirement to transfer high precision time. The U.S. Naval Observatory's Alternate Master Clock (AMC), at Schriever AFB, Colorado has a 1.8-meter Ku-band VSAT and a 3.7-m Ku-band base-station and a AOA-TWT-100 time transfer modem, which are used to fulfill its backup timing source function.

Currently, USNO is conducting time transfer experiments with users the world over.