GPS is such a weak signal that almost anyone transmitting ground based signals on the right frequency could easily drown out GPS - and do spoofing with a little extra research. So theoretically you could spoof near a data center and mess up a lot of data powering 1000s of applications.
On top of this civilian GPS is made to be jammed and innacurate - there's a better one the military has moved onto because of this (see the m-code section: https://en.m.wikipedia.org/wiki/GPS_Block_IIIA)
So yes, relying on an easily jammed atomic clock hurtling through space seems like a bad idea... But it's kind of the best we've got right now. At least until the cost of putting your own satellite into orbit goes down.
Although the US has always fantasised about its military having a much better GPS than everybody else, and it seems technically to be in a position to arrange this, the reality is always that a $5 civilian device is available in every store and the $8000 military device is back-ordered and won't be in stock until after the war is won, so you use the civilian device even though it's theoretically worse.
"Selective Availability" was the first incarnation of this pipe dream. Civilian GPS would be deliberately corrupted by up to 100 metres and the correct offset would be transmitted separately to authorised military devices. But, see above, in practice when the US military needed GPS it found civilian devices were available while accurate military ones were not, so it reduces the induced error to zero and bought civilian kit for the Gulf War.
The M-code won't be deployed until 2022 (probably later, these things always get delayed). That's when those Block IIIA birds "go live" even though some will be in orbit for years before that.
Jamming the M-code is not really harder than jamming other codes, this is a broadcast radio signal you can easily transmit a more powerful signal yourself. The Block IIIA birds have an extra transmitter so they can increase power... but they still can't drown out even a relatively disposable local jammer.
Spoof genuinely is harder with an extra code... and this is the supposed benefit with owning any of the GNSS systems, you can keep a secret shared key that makes your system spoof proof until bad guys learn the key. But again, in practice when war breaks out you turn out not to have thousands of expensive military-only devices with the shared secret, so you rely on the civilian stuff anyway.
GPS is such a weak signal that almost anyone transmitting ground based signals on the right frequency could easily drown out GPS - and do spoofing with a little extra research. So theoretically you could spoof near a data center and mess up a lot of data powering 1000s of applications.
On top of this civilian GPS is made to be jammed and innacurate - there's a better one the military has moved onto because of this (see the m-code section: https://en.m.wikipedia.org/wiki/GPS_Block_IIIA)
So yes, relying on an easily jammed atomic clock hurtling through space seems like a bad idea... But it's kind of the best we've got right now. At least until the cost of putting your own satellite into orbit goes down.