What does DME measure to determine distance?

• A. The time between receiving an aircraft's signal and its response
• B. The time between aircraft transmission and ground pulse reply
• C. The angle of descent during landing
• D. The frequency of radio waves

Answer: (B)

Distance Measuring Equipment (DME) operates by measuring the time it takes for a radio signal to travel from an aircraft to a ground station and back. When the aircraft sends out a signal, the DME ground station receives it, processes it, and then replies with a pulse. By calculating the time elapsed between the transmission from the aircraft and the reception of the reply, the DME can calculate the distance. This is because radio signals travel at the speed of light, allowing for a precise measurement based on the time delay.

The correct answer reflects this process accurately, focusing specifically on the time between the aircraft's transmission to the ground station and the response from that ground station. This method is fundamental to how DME provides pilots with critical distance information for navigation and approach.