Re: How do Geostationary satellites remain in position?

	Leigh

The basic principle is that the satellite matches the Earth's angular velocity. 
This means it orbits the Earth in exactly one day and so stays in the same 
relative position above a point. If you can imagine a ball with a stick 
pointing out of it and rotate the ball through 1 degree, say, in one second. 
Both ends of the stick move one degree but the end furthest from the ball 
moves through a larger distance so it has a larger linear velocity. 

Technically, once in position there is no need for readjustment but minor 
perturbations do build up with time requiring very small corrections. I 
suspect that in most cases the extra load required to put in motors to make 
the corrections is not worth the expense and the sattellite is simply left to 
decay in its orbit. After all, the technology is usually obsolete after a few 
years anyway. 

Satellites get there by carefully choosing the launch vehicles' destination and 
then an onboard motor is fired for a precise period of time to position the 
sattellite where you want it. I suspect the NASA 
site will have details somewhere. 

Once in position most satellites use Solar Cell energy though some use small 
nuclear power plants. I remember there was a degree of panic about Skylab and 
a Russian Satellite crashing and irradiating inhabited areas with their 
onboard reactors.

As to cost, I don't know for sure but you probably will not get much change 
out of $50 Million for commercial satellites. Military hardware is probably a 
lot, lot more. 

Yours

Dave Barlow