What is rain fade?
Rain fade (or rain attenuation) is the additional signal loss experienced by satellite radio links when the signal path passes through rainfall. Rain drops absorb and scatter microwave energy — the effect scales dramatically with frequency: at C-band (4/6 GHz) the effect is modest (1–2 dB in extreme rain), at Ku-band (12/14 GHz) it becomes significant (5–10 dB), and at Ka-band (20/30 GHz) it is severe — a tropical cloudburst of 100+ mm/hour can cause 40+ dB of excess attenuation, effectively blocking the link entirely.
Statistical characterisation
Rain fade is characterised statistically in terms of the percentage of time a given attenuation level is exceeded at a specific location. ITU-R P.618 is the standard recommendation for predicting rain attenuation statistics. A typical Ka-band earth station in northern Europe might experience fade exceeding 3 dB for 0.1% of the year and exceeding 10 dB for 0.01% of the year. Tropical locations — Brazil, India, Southeast Asia — have much worse statistics: exceeding 3 dB for 1% of the year and 20+ dB for 0.1%.
Mitigation techniques
Link margin: Design the link with extra power budget (typically 3–10 dB for Ka-band in temperate climates) to absorb rain fade without link outage. Adaptive Coding and Modulation (ACM): DVB-S2/S2X ACM dynamically lowers the modulation order and increases FEC redundancy during rain events, trading throughput for availability. Uplink Power Control (ULPC): User terminals increase transmit power during rain fade (up to regulatory limits) to compensate for uplink attenuation. Site diversity: For gateway stations, deploying two sites 50–100 km apart and switching traffic to the clear-sky site during rain events can improve availability from 99.5% to 99.99%.