Fluid Antennas for LEO Channel Stationarization: An Ephemeris-Driven Approach

Abstract

The orbital motion of low-Earth-orbit (LEO) satellites changes the line-of-sight direction seen by a ground array, requiring continuous spatial-channel tracking. This letter counteracts this variation physically with ephemeris-driven fluid antennas (FAs). From a Keplerian orbital--electromagnetic model, we derive a necessary and sufficient condition for an exactly time-invariant array response, closed-form FA counter-motion laws, aperture, spacing, and speed requirements, and an error bound under discrete ports, finite speed, and pointing and positioning errors. At 20~GHz, few-mm/s FA motion preserves the full coherent gain with a phase-shifter-free combiner, whereas a fixed array averages $0.21$, and a $\lambda/16$ port pitch matches $4$-bit electronic beam steering.