Influence of the ionospheric delays on the subnanosecond accuracy of Ku-band two-way satellite time and frequency transfer

Even if the subnanosecond accuracy of clock comparisons is not reached yet by the Two-Way Satellite Time and Frequency Transfer (TWSTFT), this technique appears to be the best available today for the frequency comparisons of the new generation of primary atomic frequency standards. It is generally assumed that the path non-reciprocity due to the influence of the ionosphere can be neglected for TWSTFT using Ku-Band carriers within an accuracy of 100 ps (1 σ). This paper presents some estimations of ionospheric delays for future frequency comparisons by TWSTFT between the primary standards of the BNM-LPTF (France), NIST (USA), and PTB (Germany). The computations are made for different approximate configurations of solar activities. The results show that specific schedules should be prepared for the comparisons of primary standards, where the frequency stability of the phase measurements should stay within 10^-15 or better