GPS is quite easy to jam with very low power. Something as low as 500mW could easily disrupt a fairly wide area. The reason being that GPS signals are generally already below background noise as measured from the ground.
Indeed. When I was experimenting with GPS spoofing, an output of 20dBm (100mW) was enough to make devices think they were in the middle of the ocean in a 200m radius (line of sight). With a quickly improvised yagi antenna I was able to push it to 500m, even after losing 3dB from switching from circular to linear polarization(!). IIRC, GPS satellites have 25W transmitters, that's insanely powerful, but they're 20000Km away, so it's rather easy to overpower their signal from close due to the inverse-square law.
Even with the first tests with 10mW in my room, iPhones at my house picked up GPS signal to know the time and timezone, which caused my flatmates to miss appointments and many websites stopped working due to certificates being expired by "2030".
> IIRC, GPS satellites have 25W transmitters, that's insanely powerful, but they're 20000Km away
Both numbers are correct, but 25W isn’t “insanely powerful”. The spec for GPS satellites lists the antenna gain at 13dBi. 25W fed through a 13 dBi antenna is just shy of 500W EIRP. Sitting near me is a 2W 2.4ghz amp and a 24 dBi parabolic antenna. That combo is just over 500W EIRP. The reason you stated (distance) is the big factor, over 12,500mi, the free space path loss is huge (182dB assuming flat gains). 500W is just shy of 27 dBW, so 27-182 is -155, which is a very very very weak signal.
The old AM border blasters were legitimately "insanely powerful". John R. Brinkley's XERA was transmitting at 1 megawatt EIRP back in the 1930s. It covered almost the whole of the US and parts of Canada, even on a crystal set.
Today, the Solt transmitter in Hungary operates at 2 megawatts.
Yeah, my wording wasn't the best. It's insanely powerful, (the transmitter leaving the antenna aside) in this sense:
Low earth orbit satellites are much closer to the Earth, so transmitters need less power to cover the range with high quality signal. For example, NOAA satellites are able to send images of the earth at 5W that you can reliably receive with a dipole and a $5 amplifier.
Geostationary satellites are significantly higher, ~35000 Km, but stationary (duh), so they benefit from high gain, very directional antennas both on the satellite and the ground. They send high data and symbol rate signals for HD video of hundreds of channels, and each transponder sits at 15 to 150W (depending on many factors: band, area to cover, etc). They tend to have multiple transponders, though.
GPS satellites sit in the middle, pretty high up, so there's a ton of free space loss, in a non geostationary orbit, so high gain antennas are useless, plus receivers are supposed to be portable, hence there's no room for helix/axial antennas or arrays on the ground. So for this reason they have to ramp up the power to geostationary, TV broadcast levels, only to push a very, very slow data rate (50 bits/s).
GPS is quite easy to jam with very low power. Something as low as 500mW could easily disrupt a fairly wide area. The reason being that GPS signals are generally already below background noise as measured from the ground.