A Sign-Stable Earth-Fixed Modulation in Earth Flyby Anomalies

The Earth flyby anomaly—a small, unexplained residual in spacecraft velocity—remains a persistent challenge in astrodynamics. While the empirical Anderson relation captures the general trend of reported anomalies, it does not account for the suppressed or near-null results observed in several later flybys. Here we construct a geometry-aware, Earth-fixed coupling proxy based on distance-weighted surface visibility over a land-sea distribution mask. We identify a robust sign structure across the primary flyby set that remains stable under variations in weighting choice and integration window. The results indicate that Earth-fixed asymmetry acts as a modulation of the Anderson prediction rather than as an independent force, offering a potential pathway for reconciling discrepancies in anomaly magnitude.