What does a "Controlled" airport primarily rely on for its operation?

A "Controlled" airport primarily relies on Air Traffic Control services for its operation. This means that the movement of aircraft in and around the airport is managed carefully by ATC to ensure safety and efficiency. Air Traffic Control services provide critical oversight, helping to manage takeoffs, landings, and the flow of aircraft both in the airspace surrounding the airport and on the ground. This systematic approach is essential, especially in busy airports like O'Hare, where a high volume of air traffic occurs.

In contrast, airports that operate under Visual Flight Rules (VFR) depend more on the pilots' ability to navigate and manage their own flights without ATC assistance. Ground control communications are important for managing movements on the airport surface, but they are just one aspect of the broader ATC services. Private ownership doesn't play a primary role in the operational definition of a controlled airport, as it is the ATC that establishes and manages the control of airspace in a structured manner.