A few aircraft (for example here: flightaware.com/live/flight/SWA3 … S/tracklog )
seem to alternate between MLAT and ADS-B. If you look at that track log, initially, the aircraft is MLAT, but later changes to ADS-B. Obviously, I’m pretty sure they did not upgrade the transponder in-flight, so does anyone know what may have happened here, or why this occurs?
Someone with a broken setup may be backfeeding us MLAT as ADS-B.
You will also see the same scenario if the aircraft is UAT equipped and there is a local UAT receiver feeding to FA. So, unless there is additional data hidden, I wouldn’t assume it’s bad data. But being that it’s SWA, probably not UAT.
I don’t think SWA would be UAT equipped though, right?
Not to make things more complicated, but what if it was UAT equipped, then rebroadcast over 1090 by an FAA tower as TIS-B traffic? Would that be reported by piAware if it sees the 1090 transmission from the tower?
Yes, but piaware does identify these as TIS-B.
I’ve seen aircraft above 18 kft on UAT. The civilian owned T-38 being one of them. He had no 1090 squitters, so just a wierd configuration. Mode-S only on 1090.
Lately though, I haven’t seen much on UAT. Seems like a big waste of money.
I’m thinking about buying one of these for my car just to have some traffic. 
I have found also one now (ICAO AE1167) that shows both MLAT and ADS-B Version: v0 (DO-260).
Ident is: BOXER23
That can happen if you have an ADS-B transponder that’s transmitting some ADS-B info (e.g. ident) but not transmitting position messages.