Posted: 7/22/2006 11:42:56 AM EDT
| ...does a sattelite make orbit adjustments by making the burn perpendicular to orbit to increase altitude or in the direction of orbit to increase speed? Or is it something inbetween these? |
Sigh....That was intended to be funny. A geosynchronous orbit is a geocentric orbit that has the same orbital period as the sidereal rotation period of the Earth. It has a semi-major axis of 42,164 km (26,200 miles). In the special case of the geostationary orbit, an observer on the ground would not perceive the satellite as moving and would see it as a fixed point in the sky. Such orbits are useful for telecommunications relays. In the more general case, when the orbit has some inclination and/or eccentricity, the satellite would appear to describe a more or less distorted figure-eight in the sky, and would rest above the same spots of the Earth's surface once per sidereal day. Synchronous orbits exist around all moons, planets, stars and black holes —unless they rotate so slowly that the orbit would be outside their Hill sphere. Most inner moons of planets have synchronous rotation, so their synchronous orbits are, in practice, limited to their leading and trailing Lagrange points. Objects with chaotic rotations (such as Hyperion) are also problematic, as their synchronous orbits keep changing unpredictably. If a geosynchronous orbit is circular and equatorial then it is also a geostationary orbit, and will maintain the same position relative to the Earth's surface. If one could see a satellite in geostationary orbit, it would appear to hover at the same point in the sky, i.e., not exhibit diurnal motion, while one would see the Sun, Moon, and stars traverse the heavens behind it. A circular geosynchronous orbit in the plane of the Earth's equator has a radius of approximately 42,164 km (from the centre of the Earth) or approximately 35,786 km (22,236 statute miles) above mean sea level. |
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If you burn in the direction of the orbit (speed up) you move into a higher orbit. If you turn around (facing against your orbital path) and hit the gas you slow down moving into a lower orbit. A combination of these two brings the desired orbit. Think of it this way: The object is ALWAYS falling toward the Earth, if its going fast enough it will never hit the Earth. Its a balance between speed, gravity and distance. |
I understand this part, it's always in freefall and there is no such thing as "zero gravity". I know that orbits decay as well and I was wondering how exactly they maintain it. I was just thinking about GPS the other day. It must be a bit of a ballet keeping all those up in the right places. |
| As the satellites age they start to run lower on fuel so do less station maintenance. They start to trace an "8" pattern in the sky as they drift a couple hundred miles this way and that. Eventually stationary dishes will lose signal from them and only active tracking dishes will follow them as they wander around. Typically by that time the company sells the now near empty satellite off to some third world country where they use it for a few years. They save enough fuel to push it out of the prime orbit location and drift it slowly off to the new location. |
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At geosynchronous orbit, which is 22,300 miles above earth's mean surface level, frictional effects are virtually zero. Typically there's no need for any orbital adjustments to a satellite in geosynchronous orbit due to it coming out of its designated position. It really isn't going to do that. Earth stations may be phased out and replaced by others, which may cause the satellite's orbit to be shifted slightly for better transmit/receive performance, or a satellite may be shifted to improve its ability to link its communications with other satellites in a lateral link system. But in general, once a geosynchronous satellite has been placed in its orbit and its proper positioning has been verified, it's just going to hang there in the same apparent position in our sky for its entire service life and never have to make any significant orbital adjustments. CJ |