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

TL;DR

The paper tackles subtle coherent orbital-period systematics in high-precision intersatellite ranging data, which can bias gravity-field solutions. It introduces a synchronization-free one-way observable built from successive pulse-interval differences to suppress the dominant first-order Doppler without clock synchronization, preserving time-varying signals. Applied to GRACE-FO LRI Level-1B data across four seasons, it detects a stable, narrowband modulation at the orbital frequency with consistent amplitude and phase, and validates the finding via synthetic signal injection, shuffle-based significance tests, and cross-checks with K-band ranging. This diagnostic tool offers a path to better characterize instrument and dynamical effects, improving current data-processing pipelines and informing the design of future satellite gravity missions.

Abstract

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.

Figures (11)

• Figure 1: Conceptual illustration of expected (Exp.) versus observed (Obs.) one-way pulse intervals in the inter-satellite laser ranging configuration. In the idealized model (top), the emitted pulse intervals at the two satellites are identical, $\Delta\tau_{S1} = \Delta\tau_{S2}$, producing a symmetric timing pattern. In the actual measurement (bottom), the received intervals differ, $\Delta\tau_{S1} \neq \Delta\tau_{S2}$, due to dynamical and instrumental effects. This asymmetry is precisely what the synchronization-free pulse-differencing observable isolates, enabling detection of weak, coherent orbital-period variations in the GRACE-FO LRI data.
• Figure 2: Histogram of the difference between LRI and KBR range measurements. The sharp peak at zero confirms excellent consistency between the two independent ranging systems.
• Figure 3: Simulation of the observable $\delta_B(i)$ showing the successful recovery of an injected synthetic signal (red) from noisy data (black), validating the processing pipeline.
• Figure 4: Mean amplitude of the orbital-period component per epoch. Each point represents the mean amplitude of the $1.74\times10^{-4}$ Hz modulation in GRACE-FO range-acceleration residuals. Error bars denote one standard deviation over individual orbital periods.
• Figure 5: Mean phase of the orbital-frequency component for each epoch. Phase coherence across seasons indicates that the modulation is not stochastic noise.