Two-Way Satellite Time and Frequency Transfer (TWSTFT) is a technique that utilizes commercial geostationary satellites for time and frequency transfer. The technique is active in contrast to GPS, that is, time signals are transferred between laboratories via satellite transponders.
The International Bureau of Weights and Measures (BIPM) outside Paris make use of the technique in cooperation with national laboratories to compare atomic clocks as well as for the calculation of the international atomic time scales TAI and UTC. TWSTFT is in this consideration a more accurate complement to the more common time transfer methods utilizing GPS.
About 15-20 time laboratories use the TWSTFT method presently, and SP is one of them. In late autumn 2001 it was decided that SP should receive funds from The national Post and Telecom Agency for the investment of a TWSTFT-system. The design of a TWSTFT-system to be installed at SP began during 2002 with financial support from the Foundation for Knowledge and Competence Development and a complete station was installed late 2003.
Presently, the satellite INTELSAT 707 is used for time links between Europe and North America. SP transmits to 9 stations in Europe and 2 in North America. A more detailed information about TWSTFT can be find for example at The National Physical Laboratory (NPL) in England.