Originally Posted by
island_airphoto
Faking your ID:
In a Mode A/C transponder, you are whomever you want to be.
AFAIK with Mode S and ADS-B the identity of the airplane is hard coded into the transponder and not changeable in flight. Nothing stopping anyone from carrying around their own transponder, but setting it up in a 777 with access to an antenna would likely be quite the challenge
As far as popping up on a new code, this would work great in the middle of the USA at 4500 feet and 150 knots. Hi there! This is uncle Bill going to get a $100 burger and hit on the waitress. As far as flights crossing international borders and in the Class A - not so much.
AFAIK the aircraft identity is not coded into the transponder. ATC enters the aircraft ident into their system when they assign the transponder code. I would expect that if you select the same code it would cause problems with ATC.
Now if you were to coordinate with another known flight in the area and have them turn off their transponder as you turn yours on with the same code, that would be seamless with ATC.