Predicting Solar Outages

The satellites and Earth travel at the same rate of speed and thus stay in the same position in relation to one another. The sun, however, does not travel in this same pattern or at the same rate of speed. There are times when the sun passes behind a satellite that an earth station is receiving from. When this happens, the satellite's signal is obliterated by the tremendous energy of the sun. The home TVRO owner will lose partial or complete reception from his system for approximately 10 minutes.

More serious results occur when a satellite antenna has been painted with metallic or highly reflective paint. Satellite antennas are designed to collect radio frequency and concentrate it at a prime focus point, where the energy can be detected and amplified. During solar outage season, the sun's energy is in prime focus. When this energy is reflected in a satellite antenna, temperatures can reach thousands od degrees within a few minutes, even on cold days.

Antennas with highly reflective paint can suffer great damage. This damage can be averted by repainting the dish with a more suitable paint.

The actual part of the world undergoing solar outages at any time of the year is directly related to the latitude which is directly in line with the satellite and sun. Solar outages always occur within 3 1/2 weeks of the equinoxes (March 21 and September 22), when the sun crosses the equator during its annual journey north and south. Outages in February, March and April begin at the northernmost latitudes, moving southward until stations on the equator have outages at the time of the equinox. Then stations in the southern hemisphere begin having outages until the southernmost stations have experienced then 3 1/2 weeks after the equinox. In August, September and October, the entire pattern is reversed since the sun is moving in the opposite direction.

For 9.8 feet earth station, outage season lasts for a little over a week and during this time, there will be an outage on each satellite, once a day.

The outage appears first as a bit of video noise, rapidly becoming annoying interference and on the days at the center of the outage season, peaking out as total loss of incoming signal.

Brief disruptions of satellite reception occur every spring, and fall.

They are not:
1 - a problem within your system
2 - going to cause you more than a transient, and possibly avoidable, inconvenience in your watching habits. Nor
3 - likely to cause any physical/lasting damage, unless you have a solid/shiny dish. If you do have a solid/shiny dish, you should read the following paragraphs of this text, carefully.

"Brief" discribes these occurances, in that they occur over only a few days; and, even on the worst day, they last for only a few minutes on each satellite.

They occur every spring, and fall, as the sun (in transit from winter's low perspectives, to summer's high ones) lines up directly behind the satellites, from your point of view.

Our sun emits a wide range of radiation. Besides the physically familiar light, and heat, are strong microwaves in all the frequency bands of our satellites. The microwaves arrive, even when the sun is not "shining" !

The exact dates change slightly, from year to year, with the dates of each solar equinox; and, every year are earlier in the spring (and later in the fall), at greater latitudes, than at lesser latitudes.

The range of dates varies with the size, and shape/condition of your dish. Larger dishes, and those with better parabolic shape, enjoy better focus, and are "hit" for fewer days, and fewer minutes on their worst days. This table's/map's range of dates assumes a 10-foot dish, in reasonably good shape. The "occurance", on such a dish, will be momentary, at the beginning and ending dates; and last about six (6) minutes, on the worst day. The geometry is such that the dates are the same, for all practical/measurable purposes, across your entire visable arc of satellites.

Your exact "WORST DAY" times may vary, by a few minutes, depending on where you are (north/south, and east/west) within your time zone; and, your times of occurance on the days before, and after, your worst day will also vary by a few minutes.

The "few minute" variations are deemed tollerable, for the purpose of recognizing that "Sun Outage" is indeed what is happening to existing ystems. More precise data can be calculated, for your specific location, as are sometimes used in diagnosis as to the orientation, and condition, of commercial dishes. Such precise data may also be of value for exact, yet easiest, site surveys, and/or dish orientations for domestic dishes.

The times are earliest for the easterly satellites, and later for the westerly satellites. This offers some chance for avoidance of disruption to your viewing if you watch the feeds/programming from the westerly satellites in the early hours; and, shift your watching to the easterly satellites, for the later hours.

Because these phenomina are spaced half a year apart, even old/experienced "Bird Watchers" sometimes forget; and, worry/fret about their system's condition, when their picture "goes bad". Before calling your service man, or worse yet, diddling with your system's memory, check these dates/times to be sure that "The Problem" is not just this seasonal, and transient, pest.

Big dishes, and their Linearaly Polarized Analog Signals, are temporarly "wiped out" by the sun's being behind their Satellites. The small, Circular Polarized Digital, Systems show a dimunition of Signal Quality/Strength; but, often do not loose their picture (unless it is also raining). The satellites which serve these "Small Dish Systems" are included in the "Worst Day Time" tables, for those who may be curious as to where they have obstruction-free Sites, for a small dish (which exist wherever Sunshine falls, on their worst day, at that satellite's time); for those who want to watch their Signal Quality/Strength (just in case they wish to "see" the effect); or, in case it happens to be simultaneously raining.

Mesh dishes are usually not efficient reflectors of solar heat, but, solid/shiny dishes may be. The reflectively concentrated solar heat, at these times of solar transit, can damage the feed, and electronics (LNBs, Servo, etc). The build-up of steam, has even been known to cause the bursting/explosion of LNBs which have had scratches, or deterioriaton, in their paint (and thus have some moisture inside). As a preventative measure, anyone with a solid/shiny dish should move/point it (during these dates of occurance) in mornings toward the westerly satellites, until the local times for the easterly satellites have passed, and then move/point it to their easterly satellites.

Solar Outages Basics

Satellite solar outages occur because the sun which is a powerful broadband microwave noise source passes directly behind the satellite (when viewed from Earth) and the receiver with the beam directed towards the satellite picks up both the satellite signal and the noise from the Sun.

The degree of interference caused by a satellite solar outage varies from slight signal degradation to complete loss of signal as the downlink is swamped by the noise from the Sun.

Figure: Mechanics behind a Solar Outages

For geostationary satellites, the solar outage can typically cause disruption to the received signal for a few minutes each day for a few days.

The duration of the sun outage depends on several things: the beam width of the field of view of the receiving ground antenna, the apparent radius of the Sun as seen from the Earth (about 0.25), the RF energy given off by the Sun, the transmitter power of the satellite, the gain and S/N performance of the ground station receive equipment, and other factors. All of this can be used to determine the outage angle of the receive antenna. The outage angle is defined as the separation angle (measured from the ground station antenna) between the satellite and the Sun at the time when sun outage or signal degradation begins or ends (see diagram).

Parameters such as the antenna directivity can make large differences to the amount of time of the solar outage. Antennas with a very wide beamwidth could be affected for as much as half an hour, whereas antennas with higher gain and directivity levels as are more commonly used for satellite reception will be affected for much shorter periods of time. Typically only a few minutes.

The effect of the solar noise causing the outage is very marked. Even at times of low solar activity, the effect is very noticeable and can result in noise levels of between 10 and 20 dB above the signals from transponders, dependent upon a host of factors.

It is possible to defined what is termed and outage angle of the receiving antenna. The solar outage angle of the antenna angle is defined as the separation angle (measured from the ground station antenna) between the satellite and the Sun at the time when sun outage or signal degradation begins or ends.

It is not always possible to exactly predict the duration of a solar outage. The exact point at which the solar outage begins and ends is a gradual transition. In addition to this there are many differences between different installations and systems. Accordingly some stations may experience a complete loss of signal while others may only experience a tolerable degradation of signal. In view of this it is not possible to exactly determine the exact solar outage angles without complete information about the ground station equipment and the satellite parameters. However an approximation can be gained from the following equation:

Outage angle   =   11
(Frequency) x (Diameter)
+ 0.25

Frequency: Downlink frequency in GHz (3.95 for C and 11.95 for Ku band).
Diameter: Downlink antenna diameter in meters.

When do solar outages occur ?

For geostationary satellites, solar outages occur around the equinoxes, i.e. March/April and then again in September/October. At these times of year the Sun crosses the equator and it traces an arc that places it directly behind geostationary satellites.

It is found that the in the Northern Hemisphere, solar outages usually occur in early March and October as a result of the geometry and relative positions of satellites and the solar transit of the Sun. In the southern hemisphere, outages normally occur in early September and April. The time of day varies mainly with the longitude of the satellite and receiving station, while the exact days vary mainly with the station's latitude. Stations along the equator will experience solar transit right at the equinoxes, because the satellites in geostationary orbit are located directly over them.

The exact dates and times of the solar outages are easy to predict and many calculators are available. All that is needed is a knowledge of the satellite position (sometimes just the satellite is needed as its coordinates may be held within the calculator), the position of the receiver in latitude and longitude. The beamwidth of the antenna is often needed as it will enable the time of the satellite solar outage to be determined.

Summary and the final considerations

Solar Transit on Satellite Dish Video

This is a three hour time-lapse video on March 4, 2009 taken at the satellite earth station of Full Channel TV, Inc. in Warren, R.I. of the solar transit of a satellite TV in earth orbit, the sun and a satellite dish antenna on the ground. The video was taken by Jesse Dufault.

The shadow of the dish's feed horn can be seen passing perfectly under the center of the dish. This biannual occurrence happens around the equinoxes and causes what is known as sun-fade or sun-outage. During this brief (10-20 minutes) period the TV signal may be degraded or lost.

Twice a year, in the spring and the fall the sun passes across the equator directly behind each geo-stationary satellite in the Clark belt. When the main beam of the satellite dish is in direct line of sight with the sun, a sun outage occurs. The LNB is bombarded by the sun's RF radiation focussed by the dish. This causes a drastic deterioration of the receive C/N (carrier to noise ratio). Basically, the satellite signal is overwhelmed by the unwanted signal from the sun; the signal from the sun is what we call noise. This phenomenon is also known as Sun Transit or Solar Interference. Solar outages only affect downlinks (receivers), not uplinks (transmitters). They also occur in cloudy weather, as the RF radiated by the sun is only dampened slightly by clouds.

A solar outage will effectively cause about 10 minutes loss of signal from that satellite. On the days before and after when the sun is directly aligned with that satellite there will be less outage. The actual days and times depend on your latitude and longitude, longitudinal position of the satellite, diameter of the dish and the received frequency. The larger the dish diameter and higher the frequency, the shorter time and the fewer days sun outages last.

While satellite solar outages, or satellite sun outages can be annoying, they are normally short lived and they are totally predictable. They affect all satellites, and in particular these solar outages are commonly experienced as a result of the widespread use of satellite direct broadcast TV reception. However all satellites are affected, whether geostationary or in other orbits, and the effects can cause short lived disruption to the radio communications.

Can it damage my system ?

Modern LNBs can handle the high levels of RF noise (using moderate sized dishes). But during a solar outage all visible sunrays will be focused on your LNB. Normally a LNB can cope with this, but in exceptional cases (a wet, shiny dish surface and bright, sunny weather), the plastic cap covering your LNB can burst!

Can I prevent this ?

You cannot block the RF noise from the sun without blocking the signal from satellite, so everyone will experience loss of signal during a solar outage. But you can block the visible sunrays by putting a non-transparent plastic bag over the LNB or dish without blocking satellite signal. Alternatively you could swing the dish out of position during a sun outage to prevent damage to the LNB.

When will I be affected ?

To calculate the date and time of solar outages you need to know your latitude and longitude, the longitudinal position of the satellite, the receive frequency and the diameter of your dish.

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