ADS-B Out is an airspace requirement and already required in many parts of the world. A more stringent version of the ADS-B Out mandate in Europe (SESAR program) is due per December 2020 and in the USA (NextGen program) per January 2020. The existing ADS-B Out mandates require a version 0 (DO-260) ATC Mode S Transponder, while the coming mandates in Europe and USA require a version 2 (DO-260B) ATC Mode-S Transponder as well as a more capable GNSS Sensor.

To become compliant, the following will be needed:

An EASA / FAA / CAAC / other approved (STC) package (with installation instructions).
ATC Mode-S Transponders (compliant with DO260B).
ADS-B Out capable MMR’s or GNSS Receiver(s) with the latest software installed.
Wiring Interfaces (limited if the aircraft is already Elementary surveillance (ELS) and/or Enhanced Surveillance (EHS) compliant.
Flight Deck Indications.
Aircraft Documentation updates.

Regulatory documents:

USA (Mandate; January 01, 2020):
FAA: 14 CFR 91.225, 14 CFR 91.227 and AC 20-165B.
Europe (Mandate; December 07, 2020):
EU IR No. 1207/2011, No. 1028/2014, No. 386/2017, No. 587/2020 and CS-ACNS. China (Mandate; December 31, 2022).
South Africa (2022):
CAA AIC 023/2017.
UAE (Mandate: January 01, 2020)
GCAA UAE AIC 13/2019
Mexico; (Mandate; January 01, 2022):
CO AV-91.2/19.