Tri-frequency transfer in the non-rotating frame for ACES mission. The ground station emitted a signal, f₁, and the ISS antenna received f′₁, immediately two down-link signals with frequencies of f₂ and f₃ emitted from the ISS antenna and received at the ground station with frequencies of f′₂ and f′₃. The emitted time is t_i and the received time is t′_i, i = 1, 2,3 (modified after Shen et al. 2016 and Sun et al. 2021).