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There is a current NOTAM:

!GPS 02/035 GPS Navigation PRN 28 unserviceable Feb 21, 2023 2020Z to May 31, 2023 2359Z

What exactly does it mean for a GPS PRN to be unserviceable, and what significance does it have for navigation?

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    $\begingroup$ gps.gov/technical/prn-codes $\endgroup$ May 25, 2023 at 20:10
  • $\begingroup$ @RodrigodeAzevedo Seems like good info, but I think it assumes I know a few too many things I don't know. $\endgroup$
    – Chris
    May 25, 2023 at 22:10
  • $\begingroup$ Do you know anything about digital circuits? $\endgroup$ May 25, 2023 at 22:13
  • $\begingroup$ @RodrigodeAzevedo Yes. $\endgroup$
    – Chris
    May 25, 2023 at 22:20

1 Answer 1

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GPS (Navstar) usually has more satellites in view than are strictly necessary for navigation. An IFR certified receiver has information to determine when the current computed position is within required specs based on which satellites are in view. A handheld GPS for hiking may just fall back to an imprecise solution with no warning when insufficient satellites or poor geometry is all that is available.

The protocol used by the system allows up to 32 regular satellites (numbered PRN 1 through PRN 32) to transmit timing information to the receivers used for the location (and another few satellites can transmit WAAS updates). But the minimum for global coverage is 24. This means that most of the time, the loss of 1 or 2 satellites doesn't cause a problem. Aviation users with a clear view of the entire hemisphere should have enough other satellites visible to give good solutions. Unless there are other problems, I would expect the unavailable satellites to have no impact.

PRN Unserviceable means that this satellite can't be used by your receiver. Your receiver already knows this. Most receivers have a page that can display the current vehicles in view, and will display their PRN on the hemisphere relative to your position.

If you have some sort of advanced mission planning that needs to guarantee a specific precision available during an upcoming flight, you could use this information to predict what satellites would be visible, in what geometry, and what the likely precision might be at any point in the flight. I'm not sure if anyone in civil aviation bothers.

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  • $\begingroup$ So a PRN number essentially just refers to a fixed GPS satellite? Or do physical satellites switch around PRN numbers? $\endgroup$
    – Chris
    May 25, 2023 at 21:20
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    $\begingroup$ They can switch. The PRN is a "logical" number that is reused as sats go into and out of service. There is also a "SVN" (Space Vehicle Number) that is not reused and isn't limited to 1-32. (Your receiver doesn't care about that number, just the PRN.) Status page: navcen.uscg.gov/gps-constellation $\endgroup$
    – BowlOfRed
    May 25, 2023 at 21:33
  • $\begingroup$ I see, thanks. :) $\endgroup$
    – Chris
    May 25, 2023 at 21:36
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    $\begingroup$ You answer without even explaining what PRN stands for $\endgroup$ May 25, 2023 at 21:38
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    $\begingroup$ For a good tutorial on GPS PRN codes, there's this series of articles on Github. $\endgroup$
    – Gerry
    May 25, 2023 at 22:23

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