What is the relationship between slant range and distance in DME?

The relationship between slant range and distance in Distance Measuring Equipment (DME) is best described as 1 nautical mile (nm) per 1,000 feet of altitude. This means that for every 1,000 feet of vertical separation from the DME ground station, the distance measured will increase by approximately 1 nautical mile.

DME measures the distance to a ground station based on the time it takes for signals to travel to and from that station. However, because the aircraft is flying at altitude, the slant range—essentially the direct line distance between the aircraft and the DME station—will be longer than the true horizontal distance.

For example, if an aircraft is flying at 10,000 feet altitude, the slant range measured by the DME could be around 10 nautical miles. Therefore, this relationship is crucial for pilots to understand as they compute their distances for navigation and approach procedures, ensuring accurate positioning relative to the DME station.

This principle highlights the geometrical effects of altitude on distance measurements, making it essential knowledge for effective IFR navigation.