Satellite Solar Outage

A solar outage (or sun outage) happens twice a year around the times of the Spring and Autumn. In the northern hemisphere, sun outages occur before the March equinox (February, March) and after the September equinox (September and October).

The solar outages occur when the sun’s path in the sky takes it directly behind the satellite and swamps the signal with interference. Since the sun is a giant source of radiation, the antenna noise temperature increases, decreasing the overall ink performance. Depending on the receive antenna size, its efficiency and the frequency band used, this interference can cause degradation of the satellite signal and a service outage.

During this time there can be an interruption or loss of satellite signals caused by interference from solar radiation, which can last about 10-15 minutes per day. The actual timing, the duration of the interference and the level of service degradation varies depending the location and size of the receiving satellite dish.

When the sun passes directly behind the UK TV satellites there can be a loss of some of the weaker satellite signals. This loss of channels is due to the Sun’s radiation swamping the satellite signal.

Why do the satellite solar outages happen twice a year? Sun outages for geostationary earth orbit (GEO) satellites happen during the equinox seasons (February-March and September-October) for a period of around two weeks, when the sun passes through the equatorial plane, that is used by the GEO satellites. During these periods, the sun crosses the equatorial plane and in specific time intervals is directly behind the satellite as seen as from the receiving earth station.

 

The solar outage in the mornings can actually be useful. When the outage happens, the dish is aligned to the satellites and the sun. You can use this time to check to see if there are any shadows from obstructions on the dish. Overhanging trees, branches etc can reduce the signals, and so using the shadows at that time you can identify what could cause a drop in your satellite dishes performance.

On the flip side at night, satellites pass through the umbra, or area of total eclipse, forcing the satellite operators to switch to built-in batteries, compensating for the absence of sunlight. For some weeks either side of total eclipse, satellite reception in fringe areas is disrupted owing to the reduced amount of sun the satellite’s solar panels receive as they pass through the penumbra, or outer shadow.

This loss of solar power results in the satellites swapping to their on board batteries, again resulting in a small drop in power to the satellite and slightly weaker signals for people in fringe reception areas.

 

A more detailed explanation

Geostationary satellites are stationed at approximately 22,300 miles (36,000 kilometers) from Earth and located directly over the equator. Given the equator is offset by 22.5 degrees, the sun aligns directly with satellites and receiving earth stations twice a year—once in the spring and once in the autumn. This event is called a sun outage, and is also known as sun fade or sun transit.

The heat emitted by the sun is an intense source of noise radiated at all frequencies, including the frequency range communication satellites use. That noise is called thermal noise. When the sun—and its thermal noise—aligns perfectly with a satellite and the receive antenna on the ground (line-of-sight), the noise floor, as seen by the receive earth station, is significant enough that it rises above the satellite’s carrier signal and causes a temporary loss of reception.

A sun outage occurs because the earth station cannot distinguish between the energy from the sun and its intended communication signal.

The duration of the solar interference depends on the receive antenna’s location on the Earth, the satellite’s orbital location above the equator, the size of the receive antenna and the reception frequency. These sun outages start with a signal loss of only a few minutes. The outages start small—when the sun is very near alignment with the satellite and the earth station.

The sun’s thermal energy is strong enough to temporarily interfere with the satellite signal and cause an outage as it approaches direct alignment. Each day as the sun moves further north, the sun’s alignment with the satellite and earth station move ever so slightly. As the sun becomes more aligned with the satellite and the earth station on the ground, the outage duration increases. Peak outage time occurs when the sun, satellite and the earth station are exactly aligned with each other. The interference declines gradually as the sun starts moving away from the satellite and earth station alignment, until it is no longer a factor—until the next interference season when the sun starts heading south (northern hemispheric in autumn).

Given that all geostationary satellites are over the same geographic plane—the equator—and orbiting at the same distance, 22,300 miles or 36,000 kilometers, the sun outage will apply to every antenna at a given location. For instance, if there are four antennas looking at four different satellites (assuming the dishes are of the same size and same frequency reception), the solar outage will travel through all four satellites and antennas at that location in the same day. The four events will happen at different times of day for each satellite, as Earth’s rotation creates alignment.

The duration of the outage is inversely related to the size and frequency of the satellite receive dish. The larger the antenna, the shorter the duration and intensity of the outage. Similarly, the smaller the dish, the great the duration and intensity.

When do Sun outages occur?

A sun outage typically occurs around the time of the equinoxes. This happens twice a year when a satellite and a receiving earth station come directly in line with the sun. In the northern hemisphere, sun outages occur before the March equinox (February, March) and after the September equinox (September and October), and in the southern hemisphere the outages occur after the March equinox and before the September equinox. The sun radiates strongly across the entire spectrum, including the microwave frequencies used to communicate with satellites (C-band, Ku band, and Ka band). So the sun swamps the signal from the satellite.

How long do they last?

For geostationary satellites, the solar outage can typically cause disruption to the received signal for a few minutes each day for a few days. It will last longer the smaller the antenna involved.
A smaller dish will have a longer period of interference than a larger dish. This is because a small dish has a greater signal acceptance angle than a big dish.

How does affect watching TV?

The effects of a sun outage range from partial degradation (increase in the error rate) to total destruction of the signal. During this time interference may occur with picture quality and sound when watching television. Sun outages do not affect internet or phone service.

( some of this information provided by Intelsat from intelsat.com/tools-resources/library/satellite-101/satellite-sun-interference/ )