An Optical Frequency-Assisted Microwave Two-Way Time-Difference Measurement Link

High-precision satellite-ground microwave time-frequency comparison links are crucial for space-time synchronization and clock performance evaluation. However, traditional hardware devices suffer from temperature-induced transmission delay drifts in RF cables and severe phase noise degradation during electronic frequency multiplication. To address these device-level bottlenecks, this paper proposes an advanced hardware-level optical frequency-assisted scheme for picosecond-level time difference measurement. To compensate for thermal delay drifts, a reflection-interferometer-based fiber phase-stabilization technique is developed using fringe counting for high-speed phase demodulation, achieving a measurement resolution of 1.29 fs @ 1550 nm and a typical compensation precision within 12 fs over a 24 h period. To suppress phase noise accumulation, a microwave photonic frequency synthesis technique based on Mach–Zehnder Interferometer (MZI) repetition-rate multiplication is established; the generated 30 GHz local oscillator (LO) signal achieves absolute phase noise of −120 dBc/Hz @ 1 kHz and −136 dBc/Hz @ 1 MHz. Laboratory benchtop testing and on-orbit verification demonstrate a sub-picosecond equipment-level random measurement noise (1 σ) for the microwave two-way time-difference measurement system. This study provides critical hardware-level technical support for satellite-ground time-frequency comparison links and spatial spatiotemporal reference construction.