[PeterisP]

Can you elaborate on why you think that "the spoofing signal needs to usually come from sky"? As far as I understand, it literally never comes from the sky, in every single case it involved ground-based transmitters.

The GPS system doesn't use the direction to the GPS satellite for localization but rather only the distance i.e. timing, so spoofing GPS is based on accurate control of the time of the transmitted (or replayed!) signals.

[nicce]

> The GPS system doesn't use the direction to the GPS satellite for localization but rather only the distance i.e. timing, so spoofing GPS is based on accurate control of the time of the transmitted (or replayed!) signals

GPS uses Signal-to-Noise ratio for determinating the signal quality and integrity. Horizontal signal will suffer pretty fast. Especially if your receiver is sophisticated and could actually detect the signal strength (power) outliers. If you want to spoof GPS signal very well, it should be also weak. But weak signal will quickly disappear with ground-based transmitters.

I used ”strength” incorrectly on the previous comment.

[PeterisP]

GPS spoofing is generally done at limited range and line of sight so the fact that "horizontal signal will suffer pretty fast" and having the range limited by terrain and curvature of the earth is not a problem but a feature that the spoofers generally want - affecting the target, but not affecting people 500 miles away; and sometimes even explicitly doing that from a pit so that spoofing or jamming affects airborne targets but not those on the ground.

And regarding "If you want to spoof GPS signal very well, it should be also weak" the scenarios I've seen (e.g. targeting drones in current conflicts) often explicitly target non-sophisticated commercial off-shelf GPS modules that don't attempt to detect spoofing and will gladly accept a signal that's 100 times louder than the actual satellites, so I think the spoofers often have no desire to do it "well" according to your criteria.