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Recall that geostationary satellites are placed into an orbit which is designed to stay above a fixed point on the Earth's surface. If the Earth and the satellite were both spherical cows in an otherwise-empty universe, the satellite, once placed in orbit at geostationary altitude over the equator, would presumably co-rotate with the Earth for the rest of time.

But, of course, there are no spherical cows. Real satellite orbits are presumably perturbed by various factors, like gravity from other large bodies (especially the moon?); collisions with "stuff" (dust, etc.); and the non-constant motion of the Earth itself (tidal deceleration, earthquakes altering the Earth's mass distribution, etc.).

This brings me to my two questions:

  1. How much does a typical geostationary satellite wobble away from its designated position over the Earth before being station-kept back into place?
  2. Which factors contribute the most to this wobble?
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  • $\begingroup$ I don't have data to answer the "how much" part of your question, but the main contributor to the need for North-South stationkeeping (out of plane motion) is lunar and solar gravitation. The main contributor to the need for East-West stationkeeping (drift along the orbit) is the non spherical Earth effects. North/South is a much bigger effect. $\endgroup$
    – Organic Marble
    Commented Sep 3, 2015 at 0:44

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Most geostationary satellites are kept within a very small margin, about 0.5 degrees. Wobbling happens due to a number of factors, including the Sun, Moon, and flattening of Earth at it's poles, causes some precession to occur (Wikipedia). It requires about 50 m/s of delta v to remain in a geostationary orbit per year, due to this effect. In addition, solar wind, sunlight and the unevenness of the Earth's equator all contribute to instabilities.

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    $\begingroup$ This box is ITU mandated to avoid interference with neighboring satellites. $\endgroup$
    – Deer Hunter
    Commented Sep 3, 2015 at 11:31
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    $\begingroup$ Additionally: The way they are kept there is by firing onboard thrusters at the appropriate times to correct deviations. Running out of stationkeeping fuel is one of the reasons they have to be replaced periodically. A fuel reserve is held back and used, at the end of the satellite's life, to boost it out of its "box" and into the geosync "graveyard". $\endgroup$
    – Jamie Hanrahan
    Commented Feb 3, 2016 at 21:24

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