What is the primary function of Distance Measuring Equipment (DME)?

The primary function of Distance Measuring Equipment (DME) is to measure the slant range distance between the aircraft and a ground-based transponder. It does this through the use of radio signals; the DME system broadcasts a signal from the aircraft to the ground station, which then responds with its own signal. The equipment then calculates the time it takes for the radio signals to return, converting that data into a distance measurement.

This understanding is crucial for pilots, as DME provides valuable navigation information, allowing them to determine how far they are from a certain waypoint or navigational aid, enabling more accurate flight planning and situational awareness. The use of terms like sonar may be misleading, as DME relies on radio waves rather than sound waves to ascertain distance.