Rain fade and link budgets

A satellite link has a margin: the difference between the signal quality you receive and the minimum the demodulator needs. Weather eats into that margin, and understanding it turns a mysterious outage into a predictable one.

Why rain matters

Rain absorbs and scatters microwave energy, and the effect worsens sharply with frequency. Ku-band is noticeably affected. Ka-band more so. C-band much less, which is why it remains preferred in regions with heavy tropical rainfall despite needing larger antennas.

It is not only rain at your site. It is rain anywhere in the path, including at the teleport, which may be hundreds of kilometres away and enjoying quite different weather from the one outside your window.

The number to watch

Eb/N0 or Es/N0 — energy per bit, or per symbol, relative to noise. Your modem reports it. Learn what it reads on a clear day, because that baseline is the only thing that makes a reading meaningful.

The behaviour is not linear. The link is fine, fine, fine, and then a couple of decibels below threshold it stops entirely. Forward error correction masks degradation right up to the point where it cannot, which is why outages feel sudden even though the underlying degradation was gradual.

Adaptive coding

Modern systems change modulation and coding as conditions vary, trading throughput for robustness. The link stays up at reduced capacity rather than failing. Users experience this as “slow in bad weather”, and the modem statistics will show the modulation having stepped down — which is the system working correctly, not a fault.

Operational practice

Trend the signal quality alongside throughput and errors. A link whose clear-sky margin has quietly fallen over months has a physical cause: a shifted antenna, water in a connector, a degraded feed, or corrosion. Finding that during routine review is far better than finding it during the first heavy rain of the season, when everyone assumes it is just weather.