How does DME determine distance from the station?

The distance measuring equipment (DME) determines distance from the station by timing the signal pulse sent from the aircraft to the ground facility and back. This process involves the aircraft transmitting an interrogation signal to the DME ground station, which then responds with a reply signal. The DME onboard the aircraft measures the time it takes for the signal to travel to the ground station and back. Since radio signals travel at the speed of light, the DME calculates the distance by dividing the total time by two (to account for the round trip) and multiplying by the speed of light.

This method provides a direct and accurate measurement of distance, which is essential for navigation and approach procedures. Other methods mentioned, such as measuring altitude loss or calculating ground speed, do not provide a direct measure of distance. Additionally, atmospheric pressure differences are not used in DME calculations, as DME relies solely on the timing of radio signals. Thus, the response involving timing the signal pulse is the correct and fundamental principle behind how DME calculates distance.