Doppler Sensitivity and Its Effect on Transatlantic TWSTFT Links
Shengkang Zhang, Tom Parker, Victor S. Zhang
Two Way Satellite Time and Frequency Transfer (TWSTFT or TW) is a commonly used technique to compare time and frequency precisely among the time and frequency laboratories. The precision of TWSTFT on time comparison is about 0.5 ns or even better, which makes it so important for the generation of the International Atomic Time (TAI). However, a daily variation pattern, or the diurnal, is observed in the time difference results in almost all of the TWSTFT links around the world. Usually, the diurnal has a peak-to-peak value of about 1 ns, which significantly degrades the performance of the present TWSTFT links. A lot of studies were done to understand where the diurnal comes from. V. Zhang and T. Parker calculated the daily variation of the ionosphere effect on TWSTFT and the daily variation of the Sagnac effect due to the satellite movement, and concluded that they are not the main reason for the diurnal. Recently, Yi-Jiun Huang et al. reported the diurnal in Asian TWSTFT links was eliminated by using a software defined radio (SDR) receiver. The SDR receiver is implemented with an open delay lock loop (DLL) which is insensitive to the Doppler effect on the TW signals. The SDR receiver also uses an algorithm that estimates and removes the interference from other TW signals transmitted at the same time. The Doppler response of the DLL in the SATRE* modem's receiver module. By simulating Doppler effects in the carrier and code of the time transfer signal, we find a type of Doppler sensitivity in the modem. That means it has about 0.5 ns offset in the delay measurement when the fractional Doppler is 1×10-9. We also find that the value of this Doppler sensitivity is almost the same among three different SATRE modems. By calculating the TWSTFT link geometry and the Doppler characteristic of the transatlantic link between NIST and PTB, we deduce that this Doppler sensitivity can only cause a very small amount of the diurnal and should not be the main reason for it.
TW, TWSTFT, diurnal variation, Doppler sensitivity, time transfer instability