GPS jamming or spoofing on a wide scale is definitely a viable attack! As another commenter noted well, making everyone appear at exactly the same location would be nigh-impossible, though.
GPS receivers will look for the 'strongest' PRN signal in the noise, so broadcasting louder than the (incredibly weak!) C/A signal is a valid way to jam or spoof GPS. It is, however, generally illegal for civilians.
GPS receivers operating with good practice do tend to try to mitigate this sort of attack, by (for example) ignoring signals with a too-high power level. It's a bit of a cat and mouse game, and there are academic papers exploring each side.
Lastly, GPS receivers also need to deal with interference from GPS itself! If GPS signals bounce off surfaces before reaching the receiver, the receiver might see two sets of GPS signals: one that arrived directly, and one that was scattered off a surface and arrives a bit later. This is called ‘multipath interference’, and part of what goes into making GPS receivers work well is mitigating multipath interference.
> making everyone appear at exactly the same location would be nigh-impossible, though.
I don't think this is actually the case. In a spoofing scenario, all of the rogue signals would typically be generated by a single terrestrial station. The time of flight of all of the generated signals will be the same, so all that matters is the position solution reflected in the transmitted signals, as the fundamental principle of GPS based on TOF is no longer in play. So I'd think that in a typical spoofing scenario, all receivers thinking they're in more or less an identical location is what you'd expect.
It might be possible in a borderline case for the receiver to receive some spoofed signals and some real signals simultaneously, in which case you'd expect weird results, but I think you'd definitely see a correlation around the position being broadcast by the spoofer.
Just to be a little clearer, jamming and spoofing are the not the same thing. Jamming obfuscates a signal, usually through noise. Spoofing impersonates the signal.
GPS receivers will look for the 'strongest' PRN signal in the noise, so broadcasting louder than the (incredibly weak!) C/A signal is a valid way to jam or spoof GPS. It is, however, generally illegal for civilians.
GPS receivers operating with good practice do tend to try to mitigate this sort of attack, by (for example) ignoring signals with a too-high power level. It's a bit of a cat and mouse game, and there are academic papers exploring each side.
Lastly, GPS receivers also need to deal with interference from GPS itself! If GPS signals bounce off surfaces before reaching the receiver, the receiver might see two sets of GPS signals: one that arrived directly, and one that was scattered off a surface and arrives a bit later. This is called ‘multipath interference’, and part of what goes into making GPS receivers work well is mitigating multipath interference.