> Presumably the spoofed GPS signals are all coming from the ground, or close to it.
I think that's a good presumption when operating in, say, Iraq or Afghanistan. Maybe less so when operating near Russia (which has its own GNSS system)?
> I would think it would work to use a semi-directional antenna that is on top of the aircraft, pointing upward. Even an antenna that has a 180-degree field of view should reject fake GPS signals from the ground, no?
GPS receivers already do this to reject ground reflections that could give false position readings (and the GPS signal is circularly-polarized so it can reject odd-ordered reflections off building and the ground). However, any kind of steerable (electronically or otherwise) directional GPS antenna is considered an ITAR no-no.
The other problem is that GPS signals are incredibly weak (actually below the thermal noise floor), and rely on processing gain from the CDMA chipping to raise the SNR to something useful. So even an antenna with very very good rejection from the lower hemisphere will easily pass a jamming signal from the ground which is both many times closer and probably of higher power.
> Only thing I can think of that would invalidate this is if the fake GPS signals bounce off the atmosphere above the planes and come back down. What else am I missing here?
Any country with a space program could transmit the jamming signal from above.