solar fade (sun interference)

Solar fade, also called sun interference, is a phenomenon that occurs in satellite communications on certain occasions when the downlink signal is aligned with the sun's position and it is overcome by signal noise from the sun. The term is used mainly in reference to geostationary (GEO) satellite systems.

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The sun is a powerful emitter of electromagnetic energy at all wavelengths, including those in the microwave portion of the radio-frequency (RF) spectrum, where most satellite communication is carried out. Normally, the sun does not affect the reception of microwave signals, because microwave-receiving antennas are rarely pointed right at the sun. But once in awhile, a signal source and the sun line up, and then they compete.

At the equinoxes, around March 21 and September 21 of every year, the sun is directly over the earth's equator. GEO satellites orbit over the equator. Thus, for about a week before and after the equinoxes, the sun lines up almost exactly with any given GEO satellite once a day for users living at the equator. For subscribers in the northern hemisphere, the same thing happens for a couple of weeks before March 21 and after September 21. In the southern hemisphere, the effect is observed just after March 21 and before September 21. Unless the satellite downlink satellite signal is exceptionally strong, RF noise from the sun overpowers it, and reception is degraded or interrupted. After a few minutes, the sun's course across the sky takes it past the satellite, and normal reception resumes.

Solar fade never occurs more than once a day for any GEO satellite, and presents a problem for only a few days out of the year. Nevertheless, it can be frustrating to satellite system users. It is important to realize that solar fade is not caused by a malfunction in system hardware or programming.

Compare with rain fade.