How can that be? Aren't geostationary satellites stationary relative to one another?

 

How can that be? Aren't geostationary satellites stationary relative to one another?

In an ideal world you could park a satellite in a geostationary orbit and it would stay there forever. Unfortunately the world, literally, isn't perfect. The very fact that the Earth isn't a nice, perfect sphere results in perturbing forces on all satellites. Then you have other effects including tidal forces from the moon and the sun plus solar wind, magnetic interactions, etc. Thus, without active correction (stationkeeping burns), satellites will drift both in longitude and also inclination.

 

In an ideal world you could park a satellite in a geostationary orbit and it would stay there forever. Unfortunately the world, literally, isn't perfect. The very fact that the Earth isn't a nice, perfect sphere results in perturbing forces on all satellites. Then you have other effects including tidal forces from the moon and the sun plus solar wind, magnetic interactions, etc. Thus, without active correction (stationkeeping burns), satellites will drift both in longitude and also inclination.

So you're saying that a geostationary satellite can deviate from its intended position many kilometers away before a corrective burn is performed to put it back into place?

 

So you're saying that a geostationary satellite can deviate from its intended position many kilometers away before a corrective burn is performed to put it back into place?

I have no idea how far from it's intended location it would be allowed to drift before a stationkeeping burn is made. I imagine that would depend on a number of factors, both technical (how far off axis can it be before the signal between it and the ground stations drop too much) and legal (how far can it be from it's allocated orbit before it violates the limits on its interference with other satellites). But without the burns it will precess in its orbit and oscillate around the two stable equilibrium points with a period of around half a century. It will also progressively increase its inclination at a good fraction of a degree per year up to a max of around 15 degrees.

 

So you're saying that a geostationary satellite can deviate from its intended position many kilometers away before a corrective burn is performed to put it back into place?

They normally wobble a bit in orbit. The degree of precision to the exact orbital required is a factor of the link budget with older birds being milked for extra life allowed to deviate more before a correction. The MH370 tracking by satellite transponder was helped (eliminate the north track) because of predictable satellite movements from geostationary orbit.

 

It will also progressively increase its inclination at a good fraction of a degree per year up to a max of around 15 degrees.

Would that be because of the Earth's magnetic field? Because it's tilted relative to its rotational axis?

 

Would that be because of the Earth's magnetic field? Because it's tilted relative to its rotational axis?

As I understand it, the north-south deviation is primarily due to the difference between the equatorial plane and the ecliptic and lunar planes. Thus it's the sun and moon that are trying to pull the satellite out of the ecliptic. The equilibrium point for this effect is at about fifteen degrees.

 

I have no idea how far from it's intended location it would be allowed to drift before a stationkeeping burn is made. I imagine that would depend on a number of factors, both technical (how far off axis can it be before the signal between it and the ground stations drop too much) and legal (how far can it be from it's allocated orbit before it violates the limits on its interference with other satellites). But without the burns it will precess in its orbit and oscillate around the two stable equilibrium points with a period of around half a century. It will also progressively increase its inclination at a good fraction of a degree per year up to a max of around 15 degrees.

As best I can tell, the station slots at geosynch are roughly 100 miles wide and since the precession rate is known, when the satellite nears the boundary it is given enough delta-v toward the other side so that the precession will halt and reverse its motion within the slot just before it gets to the other end. With newer thruster designs the satellite is kept closer to the center of its slot through more frequent but smaller thrust bursts.

 

How are you going to keep it stable in space? It will get pulled by the sun and earths gravity or pull satelites towards itself. Also it needs to be very big, not to mention a simple net will do better I think.