NavList:

Richard,
Thank you very much for passing that along to Prof. Humphreys for comment, and please send thanks back to him for the reply. He writes, "We were onboard but could have been miles away."

From how far off would it really work? If the potential "pirates" have to be within, let's say, three miles to pull this off, then that significantly reduces the threat. On the other hand, if the pirates (assumed to be very wealthy) put the spoofing package in a high-altitude UAV, then they could be invisible to surface ships and even aircraft at normal cruising altitudes. Generally, though, I would guess that this is line-of-sight limited, right? For a UAV at 40,000 feet, you could see to a range of 200 nautical miles or so. On the other hand, this would make the spoofing detectable to a large number of GPS users in most cases, right? And any one of them could put out the alarm. An aim-able spoofer, targeting a single aircraft or surface vessel from high altitude, would be much more expensive, I would think.

Just off the top of my head here, it seems to me that the principal detectable feature of a spoofed GPS signal would be its apparent directionality. A spoofed signal would appear to emanate from a single source. GPS satellite signals come from multiple directions around the observer at various altitudes and azimuths. That directionality is not detected and not used for anything in common GPS hardware, as far as I know. This would seem to imply that we could build a "spoofer detector". Rather than retrofitting every GPS receiver to be spoof-proof, we combine a basic no-frills GPS receiver with directional scanning. I believe that's possible in this frequency range, but I could be way off. If a "spoofer detector" finds multiple signals coming from altitudes and azimuths that don't make sense given the satellite orbits --for example, if it detects six signals coming from a single five-square-degree spot in the sky (or, for aircraft, from a direction below the horizon!), then it sets off an alarm, and we go to backup procedures and call the local authorities.

And:
"Also, the open Glonass signals are just as spoofable as the open GPS ones."

That makes sense, of course. I was wondering more about the reaction of receivers to a mixture of spoofed and clear signals. I think it's more likely that "pure" GPS spoofers would be available on the black market first --not because that's a logical move, but because that's easy to sell. Now suppose I have a vessel that receives both GPS and Glonass, and I start spoofing the GPS signals only, would commercially available navigation systems issue any kind of warning? Or would they just ignore the Glonass signals (that would be my guess)? Or would they attempt to process both sets of signals and perhaps display a steadily growing error ellipse?

-FER

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