TWSTFT

TWSTFT stands for Two-Way Satellite Time and Frequency Transfer. It is a method used to compare clocks at distant locations with high precision by exchanging timing signals through a geostationary satellite. The technique is favored for its ability to mitigate many path-related errors that affect one-way time transfer.

In operation, each station timestamps its transmitted signal and its received signal with its local clock, then shares these timestamps with the other station. By combining the timestamps from both directions, the clock offset between the two sites can be determined along with the round-trip satellite link delays. The two-way geometry largely cancels common path delays, improving accuracy over simple one-way links. The method requires well-calibrated transmit and receive paths, knowledge of the satellite transponder delay, and stable, well-characterized equipment.

TWSTFT is widely used by national metrology laboratories and within international timekeeping networks to synchronize clocks and contribute to coordinated time scales such as UTC and its related primary standards. It is often deployed as part of a broader time-transfer infrastructure that may include other methods for cross-validation.

Accuracy and limitations: TWSTFT typically achieves nanosecond-level precision, with performance dependent on the quality of equipment, calibration of uplink and downlink paths, and knowledge of satellite delays. Potential sources of error include asymmetries in the ground stations, transmitter/receiver biases, and environmental conditions affecting hardware. Despite these factors, TWSTFT remains a robust and established method for high-precision clock comparisons, especially in regions where fiber-based links are unavailable or impractical.