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This answer to Where are MEO satellites put at the end of their operational life? says (currently in full):

This is an active area of research. As you noted, the main satellites that are in MEO are navigation satellites. The short is they have their own disposal orbits, a bit further beyond the current constellations. It seems that GPS satellites are disposed by raising their apogee by about 1800 km (The perigee seems to remain the same)

The GPS constellation consists of six planes of intersecting circular orbits all with an altitude of 20,180 km and periods of exactly one half sidereal day. They are (likely) choreographed to not hit each other at those intersections.

If the only thing that is done to a decommissioned or otherwise unused satellite is to raise its apogee, leaving its perigee and inclination unchanged, that could potentially be a recipe for a collision, especially if control or station-keeping capability of the spacecraft is lost. So I'd like to ask:

Question: Why would end-of-life GPS satellites be given orbits that seemingly still intersect active satellite orbits but with a different period? Isn't this a recipe for disaster?

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“As of Feb 2019, there are 31 satellites in the GPS constellation, 27 of which are in use with the rest allocated as stand-bys… More decommissioned satellites are in orbit and available as spares.” Wikipedia

When the GPS system decommissions a satellite, and the system wants to use that orbit location for a replacement, they can either dispose of the decommissioned satellite by raising it to a higher circular orbit, or raise the apogee so the decommissioned satellite is in a quasi-synchronous orbit. The period of the new orbit is an integer fraction longer than the period of the remaining satellites in that orbit, such that its perigee marches retrograde. This means the decommissioned satellite always remains thousands of kilometers away from the operational satellites.

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If there are n GPS satellites in the orbit with a period of P, the disposal orbit has a period of P(1+1/n). As example, all GPS satellites have a P=12hrs. If the decommissioned satellite is sharing the orbit with 4 functioning satellites, its apogee is raised so its orbital period is 12hr(1+1/4)=15hr. The perigee is located half way between two functioning satellites (say, #1 and #2). On the next orbit, the perigee will be between #4 and #1. The perigee intersects the orbit of the active satellites. But there is never an active satellite in that location at perigee.

High circular orbits and quasi-synchronous orbits each have advantages, depending on plans for re-commissioning. If the old satellite is damaged beyond any use and it has enough fuel for the maneuver, a circular graveyard orbit is the most secure way to dispose of it. If the old satellite may be re-commissioned, re-establishing its orbit will use less delta-v from a quasi-synchronous orbit than a circular orbit.

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  • $\begingroup$ Can you add a supporting source to your answer? Where does this information come from? And what is an "integer fraction"? What is a "quasi-synchronous orbit?" Can you add a numerical example to make this clear? Does the perigee still intersect the orbit of the active satellites but at a different period? If so, why isn't there a collision? Right now I not only don't see how this answers the question, I can't yet even understand what it says. Numbers and citing sources will help. Thanks! $\endgroup$
    – uhoh
    Dec 12, 2021 at 19:37
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    $\begingroup$ @Ohoh. If there are n GPS satellites in the orbit with a period of P, the disposal orbit has a period of P(1+1/n). As example, all GPS satellites have a P=12hrs. If the decommissioned satellite is sharing the orbit with 4 functioning satellites, its apogee is raised so its orbital period is 12hr(1+1/4)=15hr. The perigee is located half way between two functioning satellites (say, #1 and #2). On the next orbit, the perigee will be between #2 and #3. Yes, the perigee intersects the orbit of the active satellites. But there is never an active satellite in that location at perigee. $\endgroup$
    – Woody
    Dec 12, 2021 at 20:07
  • $\begingroup$ Okay when you have the time please add that back into your answer. Comments are considered temporary and can disappear without warning, and that explanation is very helpful to both me and likely many future readers. Thanks! $\endgroup$
    – uhoh
    Dec 12, 2021 at 20:12
  • $\begingroup$ @Ohoh. Sorry, no sources except Kepler. gps.gov/systems/gps/control/LADO doesn’t include orbital data on disposal strategies. unoosa.org/pdf/icg/2015/icg10/02pf.pdf deals with disposal orbits and the problems of long term eccentricity growth. This paper hindawi.com/journals/mpe/2015/382340 says that, for “American GPS, the disposal strategy consists in changing the altitude of the nonoperational satellites to 500 km above or below their nominal orbits.” $\endgroup$
    – Woody
    Dec 12, 2021 at 20:44
  • $\begingroup$ But it does not mention parking orbits for potentially re-commissionable satellites. gpsworld.com/… mentions maintaining outdated, but functional, satellites “in a residual status” which likely means recommissionable. However, at the end of operational life, “the vehicle is put into a safe configuration by depleting the leftover fuel and battery life and shutting off the satellite vehicle transmitters so no one else can access the satellite in the future.” $\endgroup$
    – Woody
    Dec 12, 2021 at 20:44

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