The daily variation (diurnal) in two-way satellite time and frequency transfer (TWSTFT) difference is one of the most significant instabilities of the TWSTFT technique. The diurnal comes from the daily delay change of the TWSTFT signal paths. In recent years, there is a persistent diurnal, at the 1 ns to 2 ns level, in the transatlantic and the Europe-to-Europe TWSTFT links. In this paper, we study the diurnal in the transatlantic TWSTFT differences. We will show that a small part of the diurnal is due to the path nonreciprocity between two earth stations and a satellite; the diurnal can come from multiple TWSTFT signals transmitted at the same time; TWSTFT using different satellites and frequencies results in different diurnal levels; and the environmental effects on the earth stations and the satellite transponders also contribute to the daily delay variation of the TWSFTF difference.
