What is required for the DME to calculate distances from the ground facility?

For the Distance Measuring Equipment (DME) to calculate distances from the ground facility, it requires an interrogation signal sent by the aircraft. The DME system operates by measuring the time it takes for the interrogation signal to travel from the aircraft to the ground station and back. When the aircraft sends out an interrogation signal, the DME ground station receives this signal and then responds with a reply signal. By timing how long it takes for this round trip to occur, the DME can calculate the distance to the ground facility based on the speed of radio waves.

The other options do not play a direct role in this distance calculation process. A constant signal does not provide the necessary timing information to determine distance. Active radar is used for tracking aircraft rather than measuring distance from a DME perspective. A transponder signal from the ground is not relevant in the case of DME, as it is the aircraft's interrogation that initiates the measurement process. Therefore, the interrogation frequency sent by the aircraft is essential for the DME to accurately determine the distance from its ground facility.