Basic jamming:

Transmit a strong signal on the main GPS frequency (1575.42 MHz). Receivers get saturated and can't hear the real signal.

Medium Jamming:

Transmit the signal that a receiver would receive at a specific location loudly. GPS receivers will lock onto that signal, and report they are at the location you choose, rather than the real location they're at.

Advanced Jamming:

Pick a target, such as a VIP plane. Calculate the aggregate GPS signal that that plane is receiving right now from all the GPS satellites. Now start transmitting that same signal towards your target (you'll have to transmit ahead of time due to the speed of light delay, but that isn't an issue because GPS signals are fully predictable). Now gradually modify the signal to make the target think they're moving off their desired course, and to make them make corrections. Watch them in realtime, and adjust the signal so they correct in the direction you choose.

This is how Iran stole a drone[1].

Military GPS is encrypted (ie. XORed with a crypto-stream), which makes the signal not predictable ahead of time, which makes the advanced attack impossible. The basic and medium attacks are still possible though.

[1]: https://en.wikipedia.org/wiki/Iran%E2%80%93U.S._RQ-170_incid...

The medium and advanced method would become ineffective if civilian GPS was to be authenticated. (cryptographically signed, not encrypted)

It seems that progress is being made: https://insidegnss.com/what-is-navigation-message-authentica...

Was the drone not using the encrypted variant of GPS?

It is alleged that the RQ-170 was expressly designed for risky operations (where loosing it eventually is likely, and retrieval/destruction may not be possible) and so was created without sensitive technology.

The crypto for US military GPS uses the same keys worldwide, and rotated ~ weekly. That means if you capture any device which has the crypto keys, then you can spoof military GPS for the rest of the week, allowing you to do lots more damage.

Therefore, the military often doesn't even use the keys in their own devices.

How do you know this?

It's in the published specs of the satellites. They have a whole key schedule bit.

IIRC there was speculation Russia helped to decrypt it?

Synchronous and Asynchronous GPS Spoofing are the correct terms to use

The format of GPS civilian signal is public and you could just spoof it with any transmitter like a HackRF SDR box. There’s no cryptographic signature to verify or anything. The military code has had cryptographic anti-spoofing from the earliest days.

Or you could just send any super strong signal on the frequency to cover the satellite signal up. That can also work.

Also - what's the point nowadays when you got at least 3 more constellations + jam-proof (?) GPS2?