"The whole spectrum" for GPS FHSS being a relatively tiny frequency range. So still trivial to jam.
There are physical restrictions on transmitter aerials which make really wide FHSS impossibly difficult. Especially from a satellite. So GPS still transmits on the same standard 5 frequencies, with some special codings.
The antijamming features work a little differently. If you know a random code sequence you can use various digital noise reduction techniques to improve the SNR and help punch a signal through noise.
But if you blanket blast the transmitter frequencies and their associated components with noise, you're still going to have an effective jammer. It will just have a smaller effective range.
In the military context, violence is an option. Powerful radio transmitters are vulnerable to anti-radiation missiles: https://en.wikipedia.org/wiki/Anti-radiation_missile In a free fire zone a GPS jammer would have a lifespan measured in minutes. A hypothetical conflict with a near-peer would involve a lot of jam and counter-jam drones destroying each other, until both sides run out of equipment.
Not as trivial as jamming detection and countermeasures, so simple that it has found its way into base model commercial components. If you are in the air: use directional antennas - why are you listening to the ground? If you are on the ground: you get an easy 30deg arc pointing to the hostile transmitter... put a building or terrain between it and you.
I thought so for a long time, but then learned about spread-spectrum and frequency-hopping (with cryptographically randomized pattern) radio communication techniques which are employed in military-grade communications, and suddenly reliably jamming GPS without jamming literally everything becomes very hard.
I'm literally experiencing GPS jamming when I'm driving my car near president palace. Like one second I'm moving on road and next second I'm jumping to random places few hundred meters away until I passed long enough. Navigator sometimes goes crazy because of that. So at least civilian GPS is possible to jam.
The US doesnt have a presidential palace, so this must be in a foreign country. There isnt really anything the US can do for localized jamming in a foreign country.
