A potential issue for satellite systems is the so-called "satellite sun outage" or "sun outage".
During a sun outage, the signal from a satellite may be unavailable or may be degraded.
Satellite Sun Outage Basics
A sun outage occurs because the Sun is a powerful, broadband microwave noise source. When the Sun is positioned directly behind a satellite (as seen from the ground), the antenna beam pointing at the satellite receives both the satellite signal and noise from the Sun at the receiver.
The level of interference from a satellite sun outage varies, ranging from slight signal attenuation to a complete loss of signal when the downlink is overwhelmed by solar noise.
Mechanism Behind Sun Outages
For geostationary satellites, a sun outage typically causes interruptions in the received signal lasting minutes, and it can persist for several days around the peak. The exact dates, times, and durations depend on multiple factors, including:
- receiver location
- the satellite's specific position
- the size or, more precisely, the antenna beamwidth
- the apparent angular radius of the Sun as seen from Earth (about 0.25°)
- the alignment accuracy of the antenna pointing toward the satellite
Antenna directivity and related parameters strongly influence the duration of a sun outage. Antennas with very wide beamwidths may be affected for up to half an hour, while higher-gain, more directional antennas (commonly used for satellite reception) are typically affected for only a few minutes.
The impact of solar noise that causes outages can be significant. Even during periods of low solar activity, solar noise can raise the noise level by 10 to 20 dB above the transponder signal, depending on many factors.
Sun Outage Angle
The outage angle can be defined for the receiving antenna. The sun outage angle is the separation angle between the satellite and the Sun, measured from the ground station antenna, at the moment when the outage or signal degradation begins or ends.
It is not always possible to predict the exact duration of a sun outage. The start and end times are gradual transitions. In addition, there are many differences among installations and systems. Some stations may experience a complete signal loss while others only see a tolerable signal reduction.
Given the above, it is not possible to determine the exact sun outage angle without complete information about the ground station equipment and satellite parameters. However, an approximate value can be obtained from the following relation:
where frequency = downlink frequency (GHz) and diameter = receiver parabolic reflector or dish antenna diameter (m).
When Satellite Sun Outages Occur
For geostationary satellites, sun outages occur around the equinoxes, typically in March/April and again in September/October. At these times of year the Sun crosses the equator and passes along an arc that places it directly behind a geostationary satellite as seen from Earth.
Studies indicate that in the Northern Hemisphere, sun outages most commonly occur in early March and early October due to the relative geometry of the satellites and the solar transit. In the Southern Hemisphere, outages typically occur in early September and early April. The time of day depends mainly on the longitudes of the satellite and the receiving station, while the exact dates depend primarily on the station latitude. Stations near the equator will experience solar transit at the equinoxes since geostationary satellites pass directly overhead.
The precise dates and times of sun outages are straightforward to predict, and many calculators are available. The required inputs are the satellite position (sometimes only the satellite name if its coordinates are stored in the calculator) and the receiver latitude and longitude. The antenna beamwidth is also usually needed because it determines the duration of the satellite sun outage.
Although satellite sun outages can be inconvenient, they are typically short in duration and are predictable. They affect all satellites, and as direct satellite broadcasting is widely used, these sun outages are common. All satellites, whether geostationary or in other orbits, can be affected, and the effects may cause brief interruptions to radio communications.
