During the last few lessons, the great advantages of satellites in a geostationary orbit (GEO) were espoused about ad nauseum. The characteristics of persistence in communications and observations are the direct benefits of using a satellite in GEO. Include the huge field of regard and simplified ground system requirements, and it’s really a no-brainer to use GEO satellites for communications, remote sensing, observations, etc.
But, there are also a few disadvantages affecting satellite operations. Problems with names such as “solar interference,” “eclipses,” “latitudinal limitations,” and “space weather.” And while being at such a high altitude (35,786 km (22,236 mi)) from the Earth’s surface is good for seeing the whole world, a GEO satellite won’t “see” the details (technology is getting better, however). What exactly, then, do these problems pose to satellite operations? And how do some satellite operators deal with them?
Solar influence (or sun fade/solar transit) sounds somewhat benign. It is also fairly easy to explain, and we’ll explain it with music. Specifically, loud music. If you’ve ever been to a rock concert, rave, etc., you understand the meaning of “loud music.” You and your friends stand in front of the stage while your ears are sonically overpowered. One of your friends tries to talk with you during this sonic assault. You can’t hear the conversation. So your friend tries yelling. You still can’t hear the conversation. Your friend’s voice simply can’t compete with the band’s amplified speakers. Attempts at conversation stop until a lull.
This is essentially what occurs during solar interference, except it’s happening with radio waves. There are times when the sun is directly behind the GEO satellite and above the satellite’s ground station. The sun emits a lot of noisy radio wave energy. It’s the equivalent of the amplified speaker overpowering your friend, the satellite. During this alignment of the sun, GEO satellite and ground station, the ground station is unable to sort out the satellite’s communications signal from the sun’s noisy radio wave assault. The video below shows the alignment occurring. During that time of alignment, no signal could be received from the satellite.
This communication “outage” (no satellite communications), is predictable and typically starts gradually—a few minutes at first. Slightly longer when it’s in direct alignment. It’s the reason why certain agencies issue notifications like the one below.
As the Earth and satellite rotate, the sun will slowly come into alignment, with the time of the communication outage peaking at the exact time of the alignment of the sun, satellite, and ground station. Then the communication outage shortens as the sun slowly moves away from the alignment, until it’s not even close to the alignment anymore. Contact between the ground station and GEO satellite means satellite operators will be able to conduct 24/7 operations once again. For more detailed information about this, please go to this Intelsat page and this Celestrak page.
Now you satellite television subscribers know why your signal goes out about twice a year. You’re under the influence–solar influence, that is. It’s only for a few minutes and hopefully you’re watching nothing terribly important.
More about the other GEO satellite disadvantages next week.