What is two way satellite 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 two-way timing system employed at the USNO uses geostationary communications satellites for the communications circuit.  The USNO utilizes time transfer modems that operate over commercial Ku-Band and X band (Defense Satellite Communication System or DSCS) satellites.  The timing modems code and decode the 1-pps signals in to spread-spectrum signals which can be used over a satellite communication link.  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 antennas.

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 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.