The remote control issue with strong encryption is no real large deal as the theory is that any encryption can be broken, __given_sufficent_time__ the point being that even if you used a public key encryption scheme and changed the key after every flight and if all flights last no longer than 22 or so hours, you could still use say PGP for example , you could generate a 1024 bit key wich is a pretty good length and to my knowledge there is no known computer that coulp crack this size Key in 22 hours and even distributed system like seti@home only procees data twice as fast as ASCII white does so we are still looking at something that is definatly not feasable to crack in the alotted amount of time

Secondly as far as overpowering a control signal goes, I would like to point out that just for the record the concorde can land in zero (0) visibility with NO pilot intervention, the auto pilot is good enough to handle landing at heathrow in thick fog

Lastly, GPS is at a point now or even 5 years ago that if you use more than 3 GPS recievers you can us triagulation to get a location resolution of something like 5 cm(2.5 Inches) which is not bad, landing a plane within 10 cm (5 Inches) of the centerline on a 150 foot wide runway is pretty damn good in my book.