Two-way Satellite Time Transfer

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

The ultimate goals of the two-way project have been to improve the USNO Master Clock (USNO(MC)), its operational utility to military and civilian remote users, and to improve time comparisons with other timing laboratories. 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 laboratories will improve the short-term stability of the International Atomic Time (TAI), and with it Coordinated Universal Time (UTC).

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 1 pulse per second (1-pps) 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, e.g. the delay is the same in both directions and thus cancels out to 0. Each lab measures the time interval between the transmission of its local 1-pps and the time it receives the remote labs' 1-pps signal, typically using a time interval counter (see Figure 1 for a simple diagram of the process). The true time offsets of the two time standards can be measured very precisely ( 0.2 nanoseconds) and accurately ( 1.0 nanoseconds). 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, and thus the operational usefulness of a clock may be characterized. Improved confidence in decision making is a key benefit. The day-to-day stability of two-way time transfers can nearly reach the performance of the best reference clocks.

A simple diagram of the two-way timing method as applied between two timing laboratories. The communications link operates at the Ku-band frequency (12 GHz) 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 Ku-band, Satellite Business Systems (SBS-6 and SBS-5) and X band (Defense Satellite Communication System or DSCS) links are possible. The USNO Washington, DC site includes two 4.57-meter Ku-band satellite earth station terminals, one 3.7-meter X band terminal, and several smaller X-band and Ku-band stationary and fly-away antennas, a van-based antenna, and associated hardware including modems. The modem is a very important piece of equipment. It codes/decodes the spread-spectrum signals of the 1-pps that is transmitted and received over the satellite communications circuit.

In addition testing is conducted continually to improve the process.