I'm trying to learn about co-located GEO satellites. When I first heard about them, coming from a planetary orbital mechanics background, the first thing I pictured was based on the epicyclic approximation: the satellites are located on an epicycle that is centered on a GEO orbit. However, as I've read up a bit more, it seems that the satellites are all in GEO orbits (no extra eccentricity as with my epicycle picture), just phased very slightly differently.

Which of these (or neither) is the way that GEO satellites are co-located?

  • $\begingroup$ Actually I think I'll delete my answer and let someone else have the honors. GEO Satellites are station-kept by thrusters. See Wikipedia's Orbital Stability of GEO and Station Keeping. See also tagged questions and answers for geostationary and station-keeping $\endgroup$ – uhoh Sep 26 '18 at 4:24
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    $\begingroup$ Currently I'm not sure what "satellites are located on an epicycle that is centered on a GEO orbit." means. Can you cite an example of a spacecraft that's been placed "on an epicycle"? So far all I've found is that the epicyclic approximation is a way to cope with axisymmetric terms without "doing the math" (numerical integration), useful when the potential is substantially axisymmetric or otherwise weird, and computing power is limited. What exactly would be an "epicycle" per se in this context, and how does one place an object on one of them? $\endgroup$ – uhoh Sep 26 '18 at 4:36
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    $\begingroup$ It sounds like the OP was visualizing that the cg of the co-located sat system (centroid of the line connecting them for 2 identical sats) was centered on the line of the orbit with the system rotating around the cg. $\endgroup$ – Organic Marble Sep 26 '18 at 10:20
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    $\begingroup$ @uhoh, what Organic Marble describes is indeed what I'm thinking. I can't find an online example of the epicyclic approximation being applied solely in a Keplerian case (which I realize is only an approximation of actual satellite orbits). The idea is, for small eccentricities, Keplerian orbits can be approximated as an object being on an epicycle (definition: "a small circle whose center moves around the circumference of a larger one") centered on the circular GEO. $\endgroup$ – NeutronStar Sep 26 '18 at 21:44

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