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From what I've read, GPS satellites have been always put in orbit one at a time. In contrast, Galileo (aboard Ariane or Soyuz) and GLONASS (aboard Proton) sometimes are launched at the same time. Is there a clear reason GPS doesn't follow the same approach? It seems a lot cheaper. I think it's important most of the time consecutive GPS launches don't aim for the same orbital plane, but this was not the case for SVN-31 and SVN-37. They both ended on plane C within a two month difference.

Some guesses: Delta or Atlas rockets don't allow multiple payloads? They don't think it's worth risking several satellites on a single launch? Really tight schedules?

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  • $\begingroup$ JIT? $\endgroup$
    – uhoh
    Nov 23, 2019 at 14:37
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    $\begingroup$ GPS satellites in the same orbital plane are placed in a large distance. $\endgroup$
    – Uwe
    Nov 23, 2019 at 22:07
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    $\begingroup$ Could be mass. Block III GPS is four to five times more massive than Galileo or GLONASS. $\endgroup$
    – Schwern
    Nov 24, 2019 at 4:51
  • $\begingroup$ Apart from launch mass and delta-V considerations for distributing multiple satellites in a single launch, satellites have a finite service life, so I imagine that the units launched together would likely expire together and therefore (1) need to be replaced together and (2) leave a bigger hole (or multiple holes) as they expire; individual launches could help distribute in time the replacement schedule and minimize the impact of any gaps. $\endgroup$
    – Anthony X
    Nov 24, 2019 at 23:01
  • $\begingroup$ @Uwe building separation is not really an issue, especially during insertion $\endgroup$
    – Antzi
    Nov 25, 2019 at 4:06

2 Answers 2

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Originally, it was because the GPS satellites are heavy. An Atlas E/F with SGS-1 upper stage could just barely put a GPS Block I satellite into a 12-hour orbit, while the Delta II was designed around placing a single Block II satellite into orbit at a time.

Now, it's because the constellation is complete. GPS Block III satellites are being launched to replace failed or aging Block II satellites, which doesn't require launching more than one at a time. A Falcon Heavy could fill up an orbital plane with six satellites in a single launch, but there's no need for it.

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    $\begingroup$ I've just asked: Could a Falcon Heavy really put six GPS Block III satellites in orbit? $\endgroup$
    – uhoh
    Nov 26, 2019 at 4:15
  • $\begingroup$ That seems reasonable. Just to confirm, putting two sats at different orbital planes at the same time is not possible? Or at least the satellite needs to be redesigned? $\endgroup$
    – jinawee
    Nov 26, 2019 at 8:24
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    $\begingroup$ @jinawee putting two sats at different orbital planes at the same time requires a lot of additional fuel for plane change. A heavier launch rocket may be needed to transport that fuel. $\endgroup$
    – Uwe
    Nov 26, 2019 at 10:05
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    $\begingroup$ This answer is partially incorrect: FH can put the tonnage of 6 Block III sats into orbit, but only 2 will fit in the fairing space.stackexchange.com/a/40111/6449 $\endgroup$
    – Ian Kemp
    Nov 27, 2019 at 12:59
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    $\begingroup$ @Mark the size increase would make the aerodynamics of the rocket completely different, requiring a rewrite of the control software as well, possibly affecting performance and certainly stability. It'd need to be recertified. Given that FH is a stopgap until Starship/Superheavy becomes available for orbital launches, which could probably launch an entire constellation in a few launches, that's not something SpaceX is going to do as it's very expensive and time consuming. $\endgroup$
    – jwenting
    Nov 28, 2019 at 4:22
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According to Silverbird, it may be that neither Atlas V 552 nor Falcon 9 block 5 can put 8.8 tons into the necessary 1200 km x 20000 km 55º transfer orbit.

Silverbird's data is estimated, and has a fairly wide band of uncertainty, so it might be possible on the Atlas, or the satellite might be able to expend more of its own delta-V to complete circularization from a slightly lower-energy transfer orbit (thus shortening its operational lifetime), but launching two at once seems like it's right at the edge of what's possible.

Another factor is risk: these satellites take a long time to build and are extremely expensive; launching them separately may be seen as an insurance policy reducing the chance of losing two of them at once.

The production rate of GPS sats seems to be about 2 per year and I've seen a lot of references to schedule slips, so launching two at once might also entail one sitting on the ground unused for six months when it could be put into service on a separate launch instead.

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