Mode C veil refers to a kind of airspace which currently surrounds all primary Class B airports within the United States.
An additional requirement for the transponder to have ADS-B Out became effective January 1, 2020.
[2] As of August 2017[update], all 37 Class B airports in the United States[3] have Mode C veils centered on them.
[4] Prior to November 2014, two Class B airports did not have a Mode C veil (at least de jure): William P. Hobby Airport in Houston[5] and Marine Corps Air Station Miramar in San Diego.
[6] Mode C veils were implemented after the collision of Aeroméxico Flight 498 and a Piper Archer on August 31, 1986, within the terminal control area of Los Angeles airport.