Review and preview of two-way time transfer for UTC generation — From TWSTFT to TWOTFT

Bi-directional methods are widely used in scientific and productive measurements. By the symmetric principle, systematic errors are largely cancelled. The technique of Two-Way Satellite Time and Frequency Transfer (TWSTFT) is a typical example. Using the reciprocated radio signals emitted at two Earth laboratories and exchanged through a geostationary telecommunication satellite, the atmosphere delay effects are greatly reduced in the combination of the up and down signals. Applications to time transfer of the optical fibre technique, here under the acronym TWOTFT (Two-Way Optical fiber Time and Frequency Transfer) developed rapidly. The reciprocity of the signal in both the directions allows balancing the propagation path delays in the fibre since they average out almost entirely by employing the two-way method. Based on historical documents and the latest developments and in view of metrology, we briefly review and preview the two-way techniques for application in accurate time transfer for UTC.