Have a look at 38:50 in the video below.
And also 47:30 is interesting, about how to send out fake TCAS signals.
"What happens when a hacker gets bored and starts looking at an aircraft tracking systems?
This talk will look at ADS-B (Automatic Dependent Surveillance-Broadcast), a common technology installed or being installed on a vast majority of commercial airliners that involves an unencrypted and unauthenticated radio broadcast.
This technology has some interesting features and weaknesses that are a useful lesson in failures when security is not built in from the beginning. This talk constitutes a work in progress and hopes to spur more research and investigation into this field."