The FAA website says this:
Does this mean that a transponder is never needed if you have a UAT? Apologies if this is a dumb question!
Mode S transponder-based (1090 MHz) ADS-B equipment must meet the performance requirements of Technical Standard Order TSO-C166b. For aircraft operating at or above FL180 (18,000 feet), you must be equipped with a Mode S-transponder-based ADS-B transmitter. For aircraft operating below 18,000 feet and within U.S. airspace, you must be equipped with either a Mode S transponder with Extended Squitter
or
Universal Access Transceiver (UAT) equipment must meet the performance requirements of TSO-C154c. UAT equipment provides the ability to receive traffic and weather data provided by the FAA ADS-B network.
Does this mean that a transponder is never needed if you have a UAT? Apologies if this is a dumb question!