As correct as that documentation is, I'm going to suggest we stop all discussion of ADS-B in this thread. The topic was ACARS and to unintentionally confuse a "Squit" from a transponder on an aircraft with a message type "SQ" from an ACARS or VDL ground station is only going to lead to deeper and deeper cross-polarization of the conversation.
At this point, and correct me if I'm wrong, outside of SSR interrogation (or 'beacon mode' in FAA circles) radar, there is no implementation of the ground network polling the transponder equipment in aircraft or sending data to aircraft either as a broadcast or selectively based on the 24 bit ICAO transponder ident., which will be part of TIS-B in the future.
So, back to ACARS SQ messages. If I remember correctly, these are sent from the ACARS ground stations on a 2 minute interval, and 32 seconds on HFDL. In the case of VHF ACARS (incliding VDL2), this is your best indication of being within range of a ground station. They are also exactly how an aircraft knows it is in range of a ground station when no other traffic is being sent.
