Error Budget for Two-Way Satellite Time and Frequency Transfer at the Nanosecond and Sub-Nanosecond Level

Two-way satellite transfer remains the workhorse of operational time comparison because it cancels most path delay by symmetry. Its floor is set not by the satellite but by the terms that do not cancel: station delay calibration, equipment asymmetry and residual atmospheric terms. This budget assembles seven such terms in picoseconds, combines them, and expresses the result as a range equivalent and as a one-day frequency-transfer floor, allowing a direct comparison with fibre and optical methods. The computed results, taken from the deposited script, are as follows in summary: The computed components_ps modem is 300. The computed components_ps transponder is 100. The computed components_ps iono_residual is 80. The computed components_ps tropo is 50. The computed components_ps station_cal is 200. The computed components_ps sagnac_model is 20. The computed components_ps orbit is 70. The computed rss_ps is 393. The computed rss_ns is 0.393. The computed range_equiv_m is 0.118. The computed frac_freq_1day is 4.54e-15. The study is a computation from stated assumptions, not a measurement, and its figures should be read at the precision those assumptions support.