I just bought a SIRIUS satellite radio receiver. It’s a beautiful thing. One of the things that supposedly makes SIRIUS better than XM is their satellite arrangement:
Now, I always thought geosynchronous meant the satellite stayed in the same position relative to the earth, but apparently not. Some of their brochures even show a figure eight pattern, with three satellites moving around it, which seems pretty strange to me.
How can a geosynchronous satellite move back and forth relative to the earth? Why would this be a good thing?
There’s a difference between geosynchronous and geostationary orbits. A geostationary orbit is a circular orbit with zero inclination (parallel to the equator) and 24-hour orbital period. A satellite in geostationary orbit appears to stay fixed in the sky. A geosynchronous orbit is any orbit that has a 24-hour period. It can be elliptical, in which case it’ll appear to drift back and forth in the sky in the east-west direction. If it’s tilted, it’ll drift north and south. If it’s both, it’ll draw a circle or a figure 8 in the sky. It’ll come back to the same position in the sky every 24 hours.
The main advantage of a geostationary satellite is that you can use a fixed directional antenna to receive signals, which is why all TV broadcast satellites are in geostationary orbit. But satellite radio is intended for mobile users who use automatic antennas. They want a satellite as high up in the sky as possible and don’t care if it stays fixed in the sky. A geostationary satellite is directly above the equator so even though it’s above the horizon 24 hours a day but it’s very low in the sky if you are at high latitude. A geosynchronous satellite in a high-inclination orbit will move north-south, so it’s higher up for half the day and lower the rest of the day. Put three satellites in a high-inclination orbit, and you have at least one satellite high up in the sky at any hour.
Small correction to my above post: a geosynchronous satellite doesn’t need to spend exactly 12 hours a day on each side of the equator. If the orbit is elliptical, the satellite moves more slowly near the apogee (highest point of the orbit). So if the apogee is above the northern hemisphere (say, above Canada) the satellite spends more than 12 hours a day north of the equator. I believe the Sirius satellites are in such orbits.
Thanks, scr4, you put that better than I could have. I’m glad the SIRIUS people are providing a little information in their brochures. I’ve met people who had satellite tv, thought “satellite” meant the receiving dish, and were totally unaware that there were artificial objects in Earth orbit that sent them the pretty pictures.
One more thing to add: Even a circular orbit will produce a figure-8, if it’s inclined. The satellite always has the same speed, but when it’s crossing the equator, it’s moving at an angle to the latitude lines, so the east component of its speed is less than when it’s at maximum latitude. So sometimes it’s a little ahead of the rotation speed of the Earth, and sometimes a little behind, producing a figure 8.
But Sirius probably has a lot more customers in the northern hemisphere than in the southern, so they probably do use more elliptical orbits as described by scr4.