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For example on the Iridium-4 launch from SpaceX on 22-12-2017, 10 satellites are deployed almost at the same time. How do the satellites go into their own separate orbits after they are deployed? When they are deployed like this, don't they just stay together in the same orbit?

While the question How were the 10 Iridium NEXT satellites deployed by SpaceX? and its answer address the mechanisms of deployment, I'm asking here about the orbits of the satellites. The deployment leaves all ten satellites in nearly identical orbits, but surely this would not be correct for a constellation providing global coverage.

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    $\begingroup$ Possible duplicate of How were the 10 Iridium NEXT satellites deployed by SpaceX? $\endgroup$ – Nathan Tuggy Dec 23 '17 at 16:32
  • $\begingroup$ Not really "almost at the same time". If you watch or listen to the video it takes about 15 minutes, they are deployed at regularly spaced intervals in time. youtube.com/watch?v=wtdjCwo6d3Q If you read the answer to the question linked in the comment above and have more questions, ask away! $\endgroup$ – uhoh Dec 23 '17 at 16:45
  • $\begingroup$ @NathanTuggy after a careful read-through of the question and the answer there, I don't believe there is yet a good answer to "How do the satellites go into their own separate orbits after they are deployed?" The discussion there is mostly focused on latching mechanisms and release. I don't think this question about orbits is a duplicate of that question, nor should it be closed. $\endgroup$ – uhoh Dec 23 '17 at 16:51
  • $\begingroup$ @uhoh Yes it does indeed take about 15 minutes but this still means that all the satellites are very close too each other. I don't think it would be usefull to have the same satelites so close to each other if they want to get full earth coverage as end-goal? So im geussing their must be a way they manoevre the satellites in different possitions or am I not right? $\endgroup$ – Matthias Nuttin Dec 23 '17 at 16:59
  • $\begingroup$ Yes you are very right! And that's one reason why I believe that this is a really good question, and not a duplicate of the one indicated above! $\endgroup$ – uhoh Dec 23 '17 at 17:06
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According to a reply to the the same question on youtube by "How does it really work":

"They will all remain in the same circular orbit, but they will spread out all around it to very accurately controlled positions. To do that, they have 8 tiny engines and enough fuel for 15 years. (141 kg of fuel each.)"

I found some additional information here

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  • $\begingroup$ The trick here is is to reword the statement to "nearly the same circular orbit" See my answer below. $\endgroup$ – Carlos N Jul 23 '18 at 3:33
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They do not achieve global coverage with one launch. Each launch targets a different orbital plane. The satellites are deployed - as you correctly observed - rather close to each other into an insertion orbit to do first tests. Then they are one by one raised to a parking orbit at a higher altitude to do further testing. In a last step, they are raised to mission orbit and inserted into the constellation at their proper slots. Since those three orbits all have different altitudes, the satellites travel at different speeds, which automatically takes them out of this "row of ducklings" that they start off with. Moving the satellites to their final positions requires some very sophisticated maneuvers with a lot of precision. This is done for each of the six planes (spaced app. 30 degrees apart) and requires eight launches.

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Standard procedure for spreading satellites within an orbital plane is to change their altitudes, and hence their periods (how long it takes for them to travel around an orbit). Once the altitude changes, the different periods will start spreading out the satellites around the orbital plane. Once their desired position is achieved the satellites are all brought to the same altitude. Their period is now the same, and the separation is roughly stable. Ongoing corrections to the orbit to account for perturbations will be needed, however, to keep the exact spacing.

Traditionally the change in altitude (and the ongoing orbit maintenance throughout the mission) is done with thrusters. There is a trade to be done, however. Changing the relative altitudes by a large amount will cause the relative drift to be faster, and therefore final positions will be achieved sooner. However the larger changes in altitude require significantly more propellant.

An interesting alternative is to vary the ballistic coefficient of the satellite to control changes in altitude. This can be done by feathering the solar panels, for instance. As more surface area is exposed to the "wind". drag increases, causing the satellite to drop in altitude. Each satellite within the plan can be made to drop different amounts by controlling the ballistic coefficient, and this in turn will cause them to spread out. This was first demonstrated by the ORBCOMM constellation. See this patent.

Note that the same problem in reverse is solved by vehicles attempting to rendezvous with each other. For instance the visiting vehicle to the ISS may start in a lower orbit, hence going faster - catching up and potentially lapping the ISS. Its orbit is slowly raised to match periods with the ISS. The timing is done so that when the period is perfectly matched, the two vehicles are very near each other.

Changing altitude/period only directly addresses spreading the satellites within the plane. Getting multiple planes in the constellation is normally done through different launches (one for each plane). However, it IS possible to spread out the planes in a similar fashion, if you are willing to wait long enough. The precession of the right ascension of the ascending node (RAAN) [or "swivel" of the orbit" is dependent on altitude. Different altitudes will result in different precession rates, slowly spreading out the planes around the earth. However this process is measured in months/years and usually not cost effective.

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