What does DME compare and decode?

The correct answer focuses on the function of Distance Measuring Equipment (DME), which is designed to provide information about a pilot's distance from a ground station. Specifically, DME compares the slant distance, which is the direct line distance from the aircraft to the DME ground station, with the relative closure rate, which indicates how quickly the aircraft is approaching or moving away from that station.

By measuring the time it takes for a signal to travel to the ground station and back, DME can provide accurate distance information essential for navigation and approach procedures. Understanding slant distance helps pilots maintain situational awareness regarding their position relative to the ground station, aiding in safe and precise navigation.

In contrast, the other options do not accurately reflect the primary function of DME. Vertical speed and altitude pertain more to vertical navigation but do not encompass the main role of DME. Flight path and navigation updates are broader aspects of navigation systems and do not specifically pertain to what DME measures. Wind speed and direction are factors that impact flight performance but are not related to the primary function of DME.