Impact of Mountainous Terrain on Satellite RF Propagation

This paper characterizes how steep Alpine terrain modifies low-Earth-orbit (LEO) satellite links at L‑band, using the Iridium short‑burst data (SBD) service as a representative system. A portable Iridium 9603N terminal was deployed at three sites near Saas‑Fee, Switzerland: a high ridge station (~2887 m), a mid‑valley ledge (~2300 m), and a valley‑floor site (~1800 m). These were chosen as to span open, partially shielded, and deeply shadowed geometries. At each site, tagged SBD messages were transmitted at discrete azimuth–elevation points during multiple satellite passes, and +SBDIX session outcomes were logged via serial AT commands. High‑resolution SwissALTI3D digital elevation models were imported into QGIS to derive terrain profiles along each measured azimuth. From these profiles we computed the ridge height above the line of sight, the terminal‑to‑ridge and ridge‑to‑satellite distances, and the corresponding knife‑edge diffraction parameter v. Diffraction‑loss predictions from ITU‑R P.526 were then compared with the observed Iridium link behavior, using successful versus degraded +SBDIX codes as a coarse proxy for received signal quality. A second, shorter field campaign repeated the measurements later in the season, and controlled 10 m spatial “dither” offsets of the antenna at selected geometries were used to probe small‑scale fading. Results show a clear ordering of performance with terrain exposure. The ridge‑top site maintains reliable links over a wide azimuth sector, consistent with negative or small v values and negligible knife‑edge loss. At the mid‑valley site, partial intrusion of nearby ridges into the first Fresnel zone produces moderate v (~0.3–1.0) and a higher incidence of degraded sessions. At the valley base, large positive v (>1) and deep geometric shadowing coincide with repeated degradation until the satellite elevation is high enough to clear the controlling ridge. A diffraction‑loss‑versus‑v diagram with superposed measurement points shows that knife‑edge theory captures the main trend of increasing attenuation with increasing v, while the spread of outcomes at fixed v and the spatial‑dither ensembles are consistent with additional multipath fading. The approach which combines low‑cost field hardware, open‑source GIS, and a standard diffraction model provides a practical framework for quantifying satellite RF performance in complex terrain and for siting L‑band terminals for small‑satellite ground segments, alpine research stations, and safety applications.