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The BBC article SpaceX aims to launch internet from space discusses the SpaceX proposal for a dense array of 4,425 LEO satellites distributed among 83 orbital planes in five inclination groups as described in the linked document SpaceX non-Geostationary Satellite System Attachment A - Technical Information to Supplement Schedule S.

The altitudes of the five groups are all between 1110 and 1325 kilometers. Within each alt/inc group, collisions can be avoided by adjusting the phasing between planes. While there is nodal precession, it will be essentially identical for the entire group as nicely pointed out in this excellent, well thought-out answer. However, each inclination group will precess differently, so in order to avoid a nightmare of stationkeeping, the inclination groups are placed at slightly different altitudes.

I don't think the particular values of these five altitudes were selected randomly, but I don't see any compelling orbital mechanics arguments. So I'm thinking they were chosen to avoid stuff that's already up there in established orbits.

So I'm wondering for constellations of circular LEO satellites, are there allocations of available "slots" in altitudes? Assuming these were chosen for that reason, were they assigned, or allocated, or just dreamed up by SpaceX on-the-fly?

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above: screen captures from SpaceX non-Geostationary Satellite System Attachment A - Technical Information to Supplement Schedule S.

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There aren't really, because it's not practical. Any satellite in a given orbital altitude could potentially impact another satellite in the same orbital altitude. If there were very very tight constraints, one could in theory do such orbital planning, but it would be rather difficult.

I believe these orbits were created by SpaceX to optimally allocate their satellites, given all of the constraints. I suspect there is a desire to launch out of Vandenburg, which is constrained to 51-145 inclination, which would match very well with the 53 inclination for most of the satellites.

Basically, for any satellite launched, the launching country approves it. There has to be some justification for the orbits, but in general they are more worried about RF than orbital collisions, at least for now.

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  • $\begingroup$ RF rather than collisions is the big concern, at least for now. The radio astronomy community does not like this concept, at all. $\endgroup$ – David Hammen Nov 20 '16 at 15:29
  • $\begingroup$ @DavidHammen that's an interesting point re radio astronomy! About collisions, if indeed collisions are "not a big concern" it is because the different inclinations have different altitudes. If you put 4,425 satellites at a single altitude but five different inclinations, I think collisions would be quite a big concern. $\endgroup$ – uhoh Nov 20 '16 at 15:36
  • $\begingroup$ @uhoh There's already 32 different planes, which leads to a lot of potential collisions... $\endgroup$ – PearsonArtPhoto Nov 20 '16 at 15:52
  • $\begingroup$ But for a single inclination, all 1,600 satellitges (for example) have essentially exactly the same period, and so each plane can be staggered in phase so that close approaches are stable and identically repeating. - basically choreographed or interleaved. However, planes with disparate inclinations may not only have very slightly different periods but also precess differently, which means close approaches are constantly changing and unpredictable without continuous vigilance and careful modeling and monitoring. Of course in any case "station keeping" within a prescribed orbit is necessary $\endgroup$ – uhoh Nov 20 '16 at 16:37
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    $\begingroup$ Fair enough, given perfect orbital maneuvering. The spares and any imperfections will no doubt result in collision risk. $\endgroup$ – PearsonArtPhoto Nov 20 '16 at 16:40
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I suspect a major part of the reason for the various orbital heights is to allow the coverage from each satellite to nicely tessellate together to ensure there are no gaps (over time) in coverage with a fairly even RF field strength/ability to provide capacity to users. The following animation shows how it works for Iridium, a much simpler near polar constellation. https://en.wikipedia.org/wiki/Iridium_satellite_constellation#/media/File:Iridium_Coverage_Animation.gif

By using mostly low inclination orbits far more satellites will be over parts of the earth likely to require significant capacity at any time.

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  • $\begingroup$ The question here is "are there allocations of available 'slots' in altitudes?" rather than why do satellites have different altitudes. You may find the companion question SpaceX's 4,425 satellite constellation - what's the method to the madness of interest as well. $\endgroup$ – uhoh Jul 19 '18 at 23:46
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    $\begingroup$ I've read both questions; but may have answered the wrong one! Much of the governance I believe is through the ITU and FCC. Much of this is done on a first come - first served basis so we are probably seeing a land (or space) grab by these companies with these super constellations not fitting well into the current set of rules. $\endgroup$ – Lewis Davies Jul 20 '18 at 0:06

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