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solar fade

(n.) Also referred to as sun fade, sun outage or solar outage, a signal degradation phenomenon that affects the transmission of radio signals in satellite communications.

GEO satellites orbit the earth above the equator and rely on microwave signals in the radio frequency spectrum for communications with an earth-bound antenna. The sun also emits large amounts of electromagnetic energy in the microwave portion of the spectrum. Through the majority of the year, the radio frequency energy from the sun does not affect satellite communications. However, during the spring and fall equinoxes the sun is positioned above the earth's equator (in the equatorial plane) and therefore will align itself behind the GEO satellites as they move with the earth's rotation. When the sun is directly behind a satellite the radio signals of the satellite communications are overpowered by the sun's radio frequency energy, creating noise, and the communications are therefore disrupted.

Signal interference only occurs once a day for any satellite and usually lasts less than eight minutes, though many factors, including antenna size and downlink frequency, effect the duration and intensity of the solar fade. The exact date and time of solar fade is different for each satellite. Many satellite operators have solar fade calculators accessible on the Internet.

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