In the context of aviation and aerospace, an "Apron Control Unit" (ACU) refers to a system or device used to manage and control aircraft movements on the apron, which is the area of an airport where aircraft are parked, fueled, and serviced.

The Apron Control Unit is typically responsible for coordinating the safe and efficient movement of aircraft, vehicles, and personnel on the apron. This includes tasks such as:

1. Guiding aircraft to and from parking stands
2. Controlling the movement of ground vehicles, such as fuel trucks and baggage carts
3. Coordinating the placement of ground support equipment, such as stairs and jetways
4. Monitoring and managing apron traffic to prevent congestion and minimize delays

The Apron Control Unit may be a physical location, such as a control tower or a designated area on the apron, or it may be a virtual system that uses cameras, sensors, and software to monitor and control apron operations.

The ACU plays a critical role in ensuring the safe and efficient operation of an airport, and its functions may be performed by air traffic control, airport operations staff, or a combination of both.