Satellite Signal Rain Fade - Causes & Explanations
Even the most reliable satellite communications technology can sometimes be out-matched by the forces of nature. It’s a phenomenon known as “rain fade” or “rain attenuation” – a weakening of the satellite signal as it passes through raindrops.
Rain fade is one of the most common, and often most misunderstood, phenomena to affect satellite signals. But the more you learn about the causes of rain fade, the better your chances are to lessen its impact on your satellite system. Rain-fade is not service provider dependant, DIRECTV and Dish Network equipment are equally susceptable to the effects of signal loss due to rain-fade.
The Causes of Satellite Rain Fade
Any satellite communications system network operator using a Ku-Band system (12/14 GHz or higher frequencies) will face the effects of rain fade at some time. But to understand why this weakening occurs with Ku-Band transmissions, you must first understand the causes of satellite rain fade. Two of the most common causes are listed below.
Absorption – Part or all of the energy generated when a radio wave strikes a rain droplet. The droplet is converted to heat energy and absorbed by the droplet.
Scattering – A non-uniform transmission medium (the raindrops in the atmosphere) causes energy to be dispersed from its initial travel direction.
Scattering can be caused by either refraction or diffraction:
Refraction – The refractive index of the water droplets encountered by the radio wave.
Diffraction – the travel direction of the radio wave also changes as it propagates around the obstacle in its path (a water droplet).
These different reactions ultimately have the same effect – they cause any satellite system to lose some of its normal signal level. Don’t expect to lose your satellite signal every time it rains, though. Rain outage will only occur during the heaviest rains (convective and stratiform are the most predominant types) with only a small portion of the transmission path experiencing attenuation. In fact, of a typical satellite transmission path measuring 22,300 miles, less than .02% will be affected by rain fade.
The Impact of Satellite Rain Fade
Rain rate is the most common factor used to determine rain fade. Rain fade seems to correlate very closely with the volume of raindrops (expressed in cubic wavelengths) along the path of propagation. This is opposed to the common misconception that the degree of attenuation is proportional to the quantity or individual size of the raindrops falling near the receive site.
Pinpointing the specific factor that lead to attenuation is essential to accurately predicting the problem. Models can be developed from this data to chart the effects of rain fade on a regional or individual site basis.