Are 'they' talking 'jamming' or 'reflections due to spurious signal operation' or some such? Sending air craft off course for this or that reason. I believe air craft have other means (manual) to check their course but not the computer operated craft on 'auto-pilot.
I don't know for sure but it seems that jamming could be done quite easily... 'child's play' as it were.
I don't know squat about air craft GPS but I remember that the auto GPS up in the State of Maine could be quite hazardous to your health.
I digress to Land GPS in Maine:
There was a story that was floating around up in Maine about making sure when those little boxes told you to take the next left or right, you should pay extra attention because you could very well end up in the middle of a lake or pond very easily. No joke. I have rode in autos that used the directions from those boxes and you could get very lost very quickly. For some reason some of those units (not all) didn't work all that well when driving in the boonies. Like where I lived... 14 miles from the Capital City of Augusta.
Oh BTW folks, if you plan to head up to Maine for the summer or whatever... avoid the traffic circles unless you are well versed on how they work. In Maine the circles, in and out, work ass backwards than they do in the other 49 States... Beware! Many folks from out-of-State get into crack-ups because of this and the police love writing tickets because of this little oddity.
Enjoy and play nice <heh>