Robert M. Gary wrote:
Yes, there have been problems with authentication/security. There was
a
NOTAM (notice to airman) out a couple years ago in teh San Fransisco
area concerning some teenagers that were responding to radio calls to
airliners. Usually its a bit obvious when transmissions come from the
ground because only those within a *very* short distance can here
them
unless they are using a pretty serious antenna. Also, these teenagers
were using phrases that were not standard. Aircraft radios work well
because the aircraft are up in the air.

We had a NOTAM here in Phoenix about someone who was impersonating
both ATC and aircraft back in the mid 90s. He was usually on the
approach freqs. As I recall the guy kept it up for a couple of years.

I heard him one night while inbound to PHX. It was obvious that he
was on a handheld, and his mastery of even basic phraseology was
pathetic. Everyone just ignored him.

John Galban=====N4BQ (PA28-180)