A synchronization-free one-way ranging observable for detecting and characterizing coherent orbital-period systematics in GRACE-FO laser ranging data

We present a synchronization-free differential observable for one-way inter-satellite laser ranging, designed to suppress first-order Doppler effects without requiring clock synchronization between spacecraft. The observable is constructed from successive pulse-interval differences, which isolate time-varying signatures while eliminating static and slowly varying biases. Applied to GRACE-FO Laser Ranging Interferometer (LRI) Level-1B data over four seasonal epochs in 2019, the method reveals a stable, spectrally narrow modulation at the orbital frequency. The amplitude and phase of the detected signature remain consistent across all datasets, demonstrating a deterministic, mission-internal origin. The detection is independently confirmed through synthetic-signal injection, shuffle-based significance testing, and cross-comparison with K-band ranging data. These results show that the proposed observable provides a sensitive diagnostic for identifying coherent orbital-period systematics that may remain hidden in conventional range-rate analysis. The method offers a pathway toward improved characterization of instrument and dynamical effects in current and future satellite gravity missions.