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Wellll you could technically jam their uplink channels, but doing so may get the US in your doorstep quite quickly

This is a great plot for a B movie or a trashy military action book. “The bad guys are jamming GPS uplink and we only have two weeks until the almanacs are out of date and the whole system breaks down. Millions of innocent Americans will drive into rivers by accident.”

More to the point, to do that to this number of satellites on this big an area you'd need nuclear power plant levels of power, and it would only degrade GPS a bit (their clocks slowly desync when uplink is blocked)

My understanding was that each satellite broadcasts a coarse ephemeris for the whole network, and that that “almanac” isn’t accurate for very long (on the order of weeks). Without uploads to the satellites, those almanacs will go stale.

I don’t think the almanacs are necessary for the system to work, in theory. But I believe they’re commonly used by receivers to narrow down the range of possibilities when trying to find a PRN match for a signal they’re getting.

(I’ve dealt with GPS and similar navigation signals for work but am not an expert, this is just the impression I’ve gotten over a few years)

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