ADS-B Out

ADS-B Out broadcasts your aircraft's GPS-derived position, pressure altitude, speed, and tail number once per second to ATC ground stations and to other aircraft within range. Unlike a standard Mode C transponder, it uses a precise WAAS GPS position source, making the targets on controllers' and pilots' screens far more accurate.

The 2020 FAA mandate requires ADS-B Out in Class A, B, and C airspace, inside the Mode C veil around Class B airports, and above 10,000 ft MSL in Class E. Essentially anywhere you previously needed a Mode C transponder, you now need ADS-B Out. If a training aircraft predates 2020 and hasn't been upgraded, it is legally limited in where it can fly.