The Earth
The Solar System & Time
Charts
Using Aeronautical Charts
Basics of Navigation
Distance, Speed & Time
Vertical Navigation
Fuel Planning
Practical Navigation Techniques
Radio Navigation
Practice Exam

The DME

Distance Measuring Equipment (DME) tells the pilot the distance they are from a ground based DME station.

The pilot selects a DME frequency on the cockpit distance measuring equipment and the distance is displayed in nautical miles (nm). Many DME units also show the speed at which you are flying towards or away from the ground station and the time it will take you to reach it.

A typical cockpit DME display

How DME Works

The aircraft DME sends a signal out on the selected frequency which is received by the ground DME station. The ground station then sends this signal back to the aircraft and the time taken for this process is used to calculate the distance.

Slant Range

The distance given by a DME unit is the straight line distance from the aircraft to the ground station – known as slant range. This straight line distance is longer than the distance over the ground.

So when you are directly overhead a DME ground station, the cockpit equipment will not display zero – it will in fact be equal to your height in nautical miles.

The distance shown on cockpit DME is always the slant range

When an aircraft is directly overhead a DME station, the cockpit DME display will show the aircraft’s height above the station in nautical miles

DME Limitations

Line of Sight
Since the signals travel in straight lines, DME only works when the aircraft is in line of sight of the ground station.
No Failure Warning
The DME does not have any failure warning system, but a failure is still obvious to the pilot as the DME will stop displaying the distance.

Combined VOR and DME

VOR and DME ground stations are often combined to form a VOR/DME. This lets the pilot track a radial to a VOR while also knowing their distance from the station, so they can know their exact location (known as a position fix – more on this in the next lesson).