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When comparing GPS, Beidou, Galileo, GLONASS, and potentially other GNSS, we can see that the eccentricities of GPS spacecraft are increasing with time (i.e. apogees increasing and perigees decreasing).

Is this an orbital perturbation that GPS has decided to leave uncorrected in the space segment (preferring a software solution, which should work) while other GNSS maneuver to minimize?

The below plot was generated using TLE eccentricities from Space Track for all GNSS spacecraft launched after ~October 2011.

Comparison of GNSS eccentricities over time

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    $\begingroup$ @uhoh Specifically GPS, as I can show in a few minutes on a plot, it appears that only GPS spacecraft feature large changes in ecc over time, while the other constellations seem more constant. $\endgroup$ – costrom Sep 17 '18 at 20:22
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    $\begingroup$ @uhoh just added the plot. These are only spacecraft launched after ~October 2011. $\endgroup$ – costrom Sep 18 '18 at 14:10
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    $\begingroup$ @costrom Beautiful plot, that's excellent!! I'm curious if you have plotted only the satellites with low eccentricity in MEO? It looks like you did that but I can't quite tell yet. For example, BeiDou has 14 satellites in GEO (!42,170 km versus !28,900 km) and should probably be treated separately, and Galileo has two satellites (GSAT0201 and 0202) with huge eccentricity and inclination that are probably doing something interesting. $\endgroup$ – uhoh Sep 18 '18 at 21:04
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    $\begingroup$ I think this will turn out to be something related to orbital resonance. GPS period is 1/2 sidereal day, but the others are nearby fractions of 17 (see Does the “17” really mean anything with respect to GNSS orbits being rational factions of a sidereal day? which reminds me that @Uwe gave me some “ homework” to do there that I haven't finished!). $\endgroup$ – uhoh Sep 18 '18 at 21:35
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    $\begingroup$ You can have a look at this paper, I don't have time to do this for another day or two hindawi.com/journals/mpe/2009/347835 and also researchgate.net/publication/253322326/download $\endgroup$ – uhoh Sep 18 '18 at 21:35
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I wouldn't have suspected GPS satellites to be corrected less often than other GNSS. But I would guess they don't need to be as close to a circular orbit so long as the orbital parameters are reasonably accurate in the ephemeris the satellites send to GPS receivers. So long as the user can determine the satellite's position, it shouldn't matter too much if the orbit is more eccentric than other satellites.

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    $\begingroup$ "...it shouldn't matter too much if the orbit is more eccentric than other satellites..." is probably true, I think the question is asked in order to better understand orbital mechanics itself, not GPS operation. But orbital maneuvers are the last thing you would want a GPS satellite to do! The whole system is based on knowing where the satellite is. Do a propulsive maneuver and the satellite's signal becomes instantly useless, at least until enough data can be built up to generate a new ephemeris and then to feed it back to the satellite. $\endgroup$ – uhoh Sep 18 '18 at 12:03
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    $\begingroup$ hmm... now I'm rethinking that, so I've just asked Do operating GPS satellites ever make orbital maneuvers for station-keeping? $\endgroup$ – uhoh Sep 18 '18 at 12:11
  • $\begingroup$ suspected, guess, shouldn't - can you add some references to make this a solid answer? As it is now it reads more like a comment. $\endgroup$ – Jan Doggen Sep 18 '18 at 15:41
  • $\begingroup$ aha I was wrong! There's now a very nice answer about GPS satellite station keeping $\endgroup$ – uhoh Sep 25 '18 at 14:02

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