Two-way satellite time and frequency transfer (TWSTFT)

Img.: The TWSTFT station on the roof of the Laue Building of PTB serves to compare the time scale UTC(PTB) with the respective time scales of institutes in Europe and USA.

The so-called two-way time comparisons (TWSTFT, from the English term “two-way satellite time and frequency transfer”) represent the most exact procedures to compare time scales of time institutes which are far away from one another. The participating institutes agree to simultaneously exchange signals in pairs via a geostationary telecommunication satellite. For this purpose, transmitting and receiving facilities are needed on both sides, the photo shows the respective station on the Laue Building of PTB. Although TWSTFT has already been proposed and performed in the early eighties, it has been recognized as indispensable technique and globally implemented only in the past few years, in connection with the further development of the atomic clocks. BIPM uses a series of TWSTFT connections for calculation of the international atomic time TAI

Stations (1) and (2) simultaneously transmit signals at agreed moments via a geostationary satellite and determine the time difference between emission of the signal and reception of the signal coming from the opposite side. If the signals really are on route at the same time, the difference of the measurement values furnishes the time difference searched practically independent of the position of the stations and of the satellite. Averaging over a measurement time of one to two minutes, the technique currently used allows a statistical uncertainty of clearly less than one nanosecond to be achieved. If the internal signal delays of both earth stations have been calibrated, the time difference T₁ - T₂ can be determined with an uncertainty of less than one nanosecond. Details of the data processing and evaluation can be found in a recently published BIPM report.

Advantages:

• Small measurement uncertainty
• Availability of the measurement result quasi in real time
• Exact knowledge of the station coordinates and of the satellite movement not required
  

Disadvantages:

• Sophisticated equipment required in each station (costs: approx. 100 000 €)
• Transmission authority and approval of the station necessary
• Transponder must be made available by the satellite operator (sometimes at great expense)