For LEO missions, the source is now typically a GNSS receiver outputting a PPS line. Otherwise, you try to have a precise clock (TCXO, OCXO, and now CSAC are av

For LEO missions, the source is now typically a GNSS receiver outputting a PPS line. Otherwise, you try to have a precise clock (TCXO, OCXO, and now CSAC are available) as a reference distributing the time to all the subsystems, and you correct the satellite time offset from the ground (similar to NTP).