On Saturday, December 19, 2015 at 8:12:21 PM UTC-8, Bill T wrote:
You no longer need to trigger ground stations to get TIS-B or FIS-B data on the UAT frequency.
I have a Raspberry Pi set up to receive UAT TIS-B and FIS-B data. Get great data with no "trigger".

BillT

You never had to do anything to get FIS-B data. It is just broadcast.

If you are seeing TIS-B messages (and are you discriminating between UAT direct, ADS-R and TIS-B?) then you are likely seeing transmissions made for client aircraft. And in some busy areas that might mean near continuous coverage. There are no ADS-R messages just transmitted blind.