Astronomy

Sun Outage and its Effects



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What is a Sun Outage?

A sun outage, also known as a solar or sun transit outage, is an electromagnetic phenomenon that interrupts or distorts a geostationary satellite signal caused by interference from solar radiation making a station downlink temporarily unable to receive a satellite signal.  As the sun radiates strongly at the microwave frequencies used to communicate with satellites, the sun swamps the signal from the satellite.  Even at times of low solar activity, the effect is very noticeable and can result in noise levels of between 10 and 20 dB.

What are the effects?

The effects of a sun outage include partial degradation.  Solar outages affect all satellite systems.  This has had an effect on stock market trading where trading terminals communicate through satellite signal.  In most situations when a sun outage is about to occur, trading at the affected stock exchanges is halted until the outage is over.  During a solar outage it may not be possible to receive the signal from the satellite.  Interference gradually increases through the time period, then peaks, and then fades as the period ends.

Why?

The effect is caused by the sun's radiation overwhelming the satellite signal.  They come from the basic physics behind the operation of a satellite system, and any radio communications link.  The effect is due to the fact that the receiver picks up both the satellite signal and the noise from the Sun.

When?

Sun outages occur around the the time of equinoxes.  In the Northern Hemisphere, solar outages usually occur in early March and October.  In the Southern Hemisphere, solar outages usually occur in early September and April.  Stations along the equator will experience solar transit right at the equinoxes, because the satellites in geostationary orbit are located directly over them.  At these times, the apparent path of the sun across the sky takes it directly behind the line of sight between an earth station and a satellite.  The time of day varies mainly with the longitude of the satellite and receiving station.  Outages will occur at roughly the same time each day and may repeat on a daily basis for a week or more during the season.

How long does it last?

Sun outages are usually short in duration.  Reception interruptions span a period of about ten days as the sun aligns directly above the satellite and the downlink antenna.  Solar outages are normally experienced as a result of the widespread use of satellite direct broadcast TV reception.

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