Automatic Dependent Surveillance–Broadcast (ADS-B)

ADS-B has aircraft continuously broadcast their GPS-derived position, altitude and identity to ground stations and nearby traffic. 'ADS-B Out' is mandated in most controlled airspace; equipage is a checklist item on any purchase, since a non-compliant aircraft is restricted from that airspace until upgraded.

Why it matters: ADS-B is progressively mandatory equipment — a growing share of controlled airspace worldwide simply requires it. An aircraft without compliant equipment is locked out of that airspace, which is why compliance status directly moves aircraft value.

Note the two directions: ADS-B Out, broadcasting your position, is what mandates require; ADS-B In, receiving traffic and, in some regions, weather, is the optional capability that improves the pilot's picture. When buying, verify the installation actually meets the mandate applicable where you fly — a non-compliant transponder is a known, quantifiable retrofit cost.