How do emergency squawk alerts work?

A transponder squawk of 7500, 7600 or 7700 signals unlawful interference, lost communications, or a general emergency. TallyAir watches for the change itself — the first report carrying one of those codes — on anything on your follow list and on anything inside your alert radius, which is 50 NM by default and adjustable from 10 to 250 or switched off entirely.

When one fires you get a push naming the code, the aircraft and its distance from you, and the strip moves to the top of your board. It is one notification per aircraft per event: the squawk has to sit at a routine code for fifteen minutes before that aircraft can alert again. When it returns to routine the strip drops back to AIR and the event stays in your log with its timestamps.

TallyAir reports what an aircraft is broadcasting and nothing more. It is not an alerting service, it carries no guarantee of coverage or timeliness, a code can be transmitted in error, and it must not be used for flight operations.