What does the suffix 'U' represent regarding an aircraft's transponder?

The suffix 'U' in the context of an aircraft's transponder indicates that the aircraft is equipped with a transponder that has Mode C but does not have Distance Measuring Equipment (DME). Mode C allows the transponder to provide altitude information to air traffic control, which is essential for maintaining separation and ensuring safe airspace management.

In this case, while the aircraft has the capability to report its altitude via the Mode C transponder, it is not equipped with DME, which is a system used to determine the aircraft's distance from a VOR (VHF Omnidirectional Range) station. This distinction is important for air traffic controllers to understand the capabilities of each aircraft, especially in terms of navigation and altitude reporting.

Other answers focus on variations of transponder capabilities and designs, but the unique identifier 'U' specifically highlights the presence of Mode C without DME. Hence, this clarity in the designation helps ensure that pilots and controllers can accurately assess the equipment onboard and make informed decisions regarding air traffic management and safety.