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With more and more satellites in orbit and increasing risk of collisions, should satellite owners not select satellite altitudes more diligently or even at "random"? Is there a specific advantage to selecting a specific altitude close to others (apart from mission requirements which may require a certain range) - could be carrier capabilities, governmental restrictions, ease of tracking? Are the spikes in the picture below simply driven by large constellations operating at a specific altitude - or is this driven by some kind of round number bias of mission developers who just pick a typical round number around the required range of altitude?

I got the question by reading Newmam & Mashiku (2022) where I also found the picture below. They write "An orbit that results in a high number of close approaches with other satellites can lead to the need to perform many conjunction risk mitigation maneuvers, [...] For instance, if a spacecraft is being designed to fly “near” 500 km, a plot of orbit density (see Figure 1 below) shows that small changes in mission altitude to target 480 km or 520 km instead would drastically reduce the number of close approaches."

Newman, L. K., & Mashiku, A. K. (2022, September). Conjunction Assessment: NASA Best Practices and Lessons Learned. In 23rd Advanced Maui Optical and Space Surveillance Technologies Conference.

My question is, how much room do satellite owners/operators have here to choose different altitudes and what are the reasons for this bunching at certain altitudes?

Thank you

operating altitudes

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    $\begingroup$ I don't know about bias as such. Airliners at cruise altitude, which have a lot of the same limiting factors as satellites, all fly at or very near even thousands of feet because it makes collision avoidance easier than if everyone was just at random altitudes. Eastbound flights use odd-thousands and westbound use even-thousands, so while there's still a possibility of a crossing path, it's much less likely to come up as everyone on your flight level is going more or less the same direction, and if it does come up, the closing speeds are much lower so there's more time to manage it. $\endgroup$ Commented Oct 18, 2024 at 14:05
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    $\begingroup$ Three words describe the proclivity for this bunching: Repeat ground track. $\endgroup$ Commented Oct 18, 2024 at 14:22
  • $\begingroup$ @DavidHammen Talking about planes or satellites? Can you elaborate? $\endgroup$ Commented Oct 18, 2024 at 18:23
  • $\begingroup$ @Adrian Satellites. Airplanes can't fly at the altitudes in your plot. The maximum altitude an airplane can fly at is about 100 km. $\endgroup$ Commented Oct 18, 2024 at 23:21
  • $\begingroup$ DYAC. I don't know how the site got "copelling" as the reason for my edit. I meant to type "corrected spelling and added a link to the cited material." $\endgroup$ Commented Oct 19, 2024 at 12:48

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Are the spikes in the picture below simply driven by large constellations operating at a specific altitude. Or is this driven by some kind of round number bias of mission developers who just pick a typical round number around the required range of altitude?

It's both. For example, SpaceX intends to operate its huge Starlink satellites in constellations at a small number of altitudes. This alone makes for spikes (huge spikes; the image predates Starlink) in a diagram of satellite count versus altitude.

Regarding the latter point (and regarding Starlink as well), a number of low Earth orbit satellite operators very much want a repeat ground track orbit. In other words, in $j$ orbits, the satellite's ground track will repeat after $k$ days, where $j$ and $k$ are both positive integers (and $k$ is preferably small). These repeat ground track orbits are highly desirable for a number of reasons. The choices of $j$ and $k$ and the need to also be sun synchronous are what cause the bunching that you have observed.

One small example of such a constellation was NASA's A-train of satellites, a few of which remain operational and remain in the original orbit. This constellation was controlled to be in a 233:16 ratio (233 nodal orbits in 16 nodal days). The Starlink satellites also appear to use repeat ground tracks, although I'm having a harder time finding a specific reference for that.

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    $\begingroup$ I don't think this is "round number bias", since the "good" altitudes in this approach are not necessarily round numbers. But the orbit density peaks in the OP aren't actually round numbers either, so the "repeat ground track" explanation makes more sense with the data than "round number bias". $\endgroup$ Commented Oct 18, 2024 at 20:21
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    $\begingroup$ An example of selection of j and k explaining one of the spikes would be very illustrative. $\endgroup$ Commented Oct 18, 2024 at 22:18
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    $\begingroup$ @RussellBorogove Figure 8a (top of page 18) of The Use of Resonant Orbits in Satellite Geodesy: A Review (also archived) I don't see a lot of matches that jump out, except ~550 km in the upper left corner of the plot being 15:1. $\endgroup$ Commented Oct 19, 2024 at 0:06
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    $\begingroup$ @ufoh That paper is mostly about CHAMP, GRACE, and GOCE, where those repeat ground tracks were considered to be something to avoid as they resulted in greater errors in what those satellites were intended to measure. Other satellite operators perceive those repeat ground tracks as a big plus rather than as a big minus. $\endgroup$ Commented Oct 19, 2024 at 13:04
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    $\begingroup$ The above comment was intended to be addressed to @uhoh. Uhoh, I mistyped. $\endgroup$ Commented Oct 19, 2024 at 15:30

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