Perhaps I just don't understand how this works all that well, but shouldn't it be easy to defeat GPS spoofing when you're 30,000+ feet in the air? Presumably the spoofed GPS signals are all coming from the ground, or close to it. 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?
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?
Radio goes through and reflects off things, including aircraft. GPS signals are incredibly weak, and even if an aircraft attenuates or reflects 99.9% of the signal, a ground based system will nearly always be able to overpower it.
It can be as simple as reflecting off the tail section of the aircraft.
Incredibly weak is correct and even bordering on understatement. They are below the noise floor by about 50dB (which is a logarithmic scale, so is a factor of 100,000x).
It still blows my mind that gps even works given how far below the noise floor it is. I am very far from being any sort of expert in radio theory but my layman understanding is that the receiver "knows" exactly what a gps signal should look like and somehow magically uses that to extract the data out of the noise. It is one of those absolutely genius aspects of modern life, and we tend to just take for granted
Same. I am pretty comfortable as a programmer and that translates to feeling moderately comfortable at gluing together blocks of digital electronics. Analog electronics are much more of a struggle and RF is entirely black magic. The idea that GPS works at all defies my understanding of what ought to be possible.
> 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.
You're right, a cheap GPS isn't going to do that.. however, it is well within then realm of possibility to design a GPS receiver system that uses beamforming to lock onto satellites and ignore everything else for orders of magnitude better jamming resistance.
Its called controlled reception pattern antenna (CRPAs), which unfortunately is an ITAR restricted thing because of insane paperwork required, so no one wants to build it for civilian receivers. Which is stupid because beamforming (or null-steering) algorithms are not that difficult to implement, and so we already have Chinese and Iranian receivers with CRPAs.
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?