Time synchronization adjusts the accuracy of distributed clocks through time-stamp comparison; it encompasses both absolute time synchronization (time alignment) and relative time synchronization, with the former focusing on the precision of time values and the latter emphasizing the consistency of time values. Frequency synchronization adjusts the accuracy of frequency sources through frequency comparison; it includes absolute frequency synchronization (frequency calibration) and relative frequency synchronization, targeting frequency source calibration and consistency adjustment, respectively.
Time synchronization requires signal frequency and phase to remain consistent (phase synchronization), whereas frequency synchronization only requires maintaining a specific relationship between frequencies or phases (clock synchronization). Satellite navigation systems (such as GPS and BeiDou) rely on time synchronization precision for positioning, while fiber-optic link time-frequency synchronization technology can achieve a frequency stability of 10⁻¹⁷/day and time synchronization precision at the 100-picosecond level. Bistatic SAR systems address the issue of independent clock source inconsistency by using GPS to discipline an OCXO system.
Since Tsinghua University achieved a frequency stability of 7×10⁻¹⁵/s and a synchronization precision of 50 ps in an 80 km experiment in 2011, fiber-optic synchronization technology has been gradually applied to projects such as Very Long Baseline Interferometry (VLBI) and the Square Kilometre Array (SKA) radio telescope. The SKA project requires time synchronization to achieve a short-term precision of 1 ps and a long-term stability of 10 ns over 10 years. The PTP protocol achieves sub-microsecond clock synchronization through a master-slave hierarchy and supports MAC or UDP encapsulation.