JoesPiper wrote: "Keep in mind with the Mode S transponders you will also need a pressure switch installed (Design Flex PSF100) coupled to your pitot system that tells ADS-B that you are either on the ground or in the air (required). With a UAT box such as the King KGX 150 you don't need that as it has an internal GPS for altitude reporting. They don't tell you that either. Correct me if I am wrong please."
Some solutions don't require a pressure switch. The Avidyne IFD540 GPS uses speed/altitude data to determine when the aircraft is airborne, and can provide that status to a compatible Avidyne ADSB-out transponder (AXP340).