Behind the Forecast: How rain can ruin your TV time

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Science Behind the Forecast 4/5/19

LOUISVILLE, KY (WAVE) - Rain fade describes the absorption of microwave radio frequency by precipitation ranging from rain to snow. Satellite television and internet can both be affected by the phenomenon.

The issue arises because the size of the space between the precipitation is similar to signal wavelengths. A raindrop is defined by the American Meteorological Society as a drop of water with a diameter greater than 0.5 mm. Drops of drizzle have diameters between 0.2 and 0.5 mm. Direct broadcast satellite customers, think DirecTV and DISH Network, receive their signals in the Ku frequency range (10.7GHz to 14.5 GHz). Early satellite tv was transmitted in C-band radio (radio in the 3.7 GHz to 6.4 GHz range). The Ku frequency range translates to a range of 28.0 to 20.7 millimeters. (Around an inch)

When signals are close to this wavelength they are often impacted when traveling through areas with high water droplet and ice crystal concentrations. When the distance between water droplets and ice crystals are similar to the wavelength of the signals then the signal may be degraded leading to static or a fuzzy image. Once the precipitation is gone then the rain fade ends.

Rain fade isn’t just caused by precipitation at the location where the signal is being received. It can also be caused by precipitation in areas between where the signal is being sent to the satellite, also known as the uplink location. Usually to compensate for rain fade companies will increase their transmission power.

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The sun can also cause issues for satellite communications. Many satellites used for communication, whether it be television, radio or phone, are geostationary. This means that they sit around 36,000 km, 22,300 miles, away from the Earth’s surface and their orbit matches the Earth’s rotation, according to NASA. Many weather satellites are geosynchronous because they offer a consistent view over a particular location. Twice a year, the sun aligns directly with satellites and their receiving stations on Earth causing a sun fade (also known as a sun outage or sun transit). The sun’s heat causes noise that radiates at all frequencies; this is called thermal noise. When the sun lines up with a satellite and it’s reciever, the thermal noise overwhelms the satellite’s signal causing a loss of reception. A sun fade’s length is related to the size of the receiving antenna, the frequency being used, the antenna’s location and the satellite’s location. According to Intelsat, outages begin as the sun is quite close to alignment, usually lasting a few minutes. Durations increase as things become more aligned with peak outages when everything is perfectly aligned. Every geostationay satellite’s antenna is affected by a sun outage since they are all on the same geographic plane, the equator, Intelsat explained. The bigger the antenna, the shorter the duration and intensity of the outage.

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