Two-way time and frequency comparison

Using the method of two-way time and frequency compare (usually with TWSTFT referred, the abbreviation of English two-way satellite time and frequency transfer ) that are time scales of two time institutes compared. The aim is to have the most accurate international atomic time possible .

With the help of TWSTFT, for example, the atomic time TA (PTB) determined by an atomic clock of the Physikalisch-Technische Bundesanstalt in Braunschweig with the corresponding atomic time TA (NPL) of the British National Physical Laboratory in Teddington or with the atomic time TA (NIST) of the US American National Institute of Standards and Technology in Gaithersburg .

The two-way time and frequency comparisons are currently (2017) ^{[out of date] the} most accurate method for comparing the time scales of distant institutes and are increasingly replacing GPS time comparisons.

The results of the TWSTFT and the GPS time comparisons of numerous time institutes are transmitted to the Bureau International des Poids et Mesures (BIPM) in Sèvres near Paris , which uses them to form a weighted average, which is the basis of the International Atomic Time (TAI) determined by the BIPM is published. Details on the processing and evaluation of the TWSTFT procedure are set out in more detail in a BIPM directive.

Schematic representation of the process

In a very simplified way, institutes A and B simultaneously send a signal via a geostationary telecommunications satellite to the other institute at an agreed time , whereby the transmitting and receiving systems should be close to the atomic clock of the respective institute. Each signal is provided with a time stamp of the exact transmission time according to the atomic time TA of the transmitter. When sending a in which respective transmitter interval counter (time interval counter - TIC) activated. When the signal arrives at the receiver, its interval counter is stopped (which was triggered by the sending of the signal there). The accuracy of the two atomic clocks can be determined by comparing the transmission times according to the atomic clock of one institute with the transit times according to the interval counter of the other institute, regardless of the position of the two institutes and the satellite.

The delays affecting the signals usually have the same effect on both signals, so that they neutralize each other in the computational comparison. Differences due to different transmitting and receiving systems can be reduced considerably by calibrating the devices. The effect of the ionosphere is only completely neutralized if the same frequencies are always used; However, often for the signals to the satellite ( english uplink ) and from the satellite to ground stations ( english downlink ) used for technical reasons different frequencies such as 14 GHz uplink and 11 GHz downlink. Delays within the satellite are neutralized if the same transponder is used for both signal paths. The Sagnac effect must be taken into account for both stations .