DME System stands for Distance Measuring Equipment. Used extensively for the purpose of navigation, a DME system can be considered as a Radar, providing with the information of distance between an aircraft and a ground station. 

A DME system primarily consists of two components responsible for all the communication purposes; an aircraft interrogator (installed on the aircraft) and a ground transponder (located at a geographical location).  A DME system usually operates in an Ultra-High Frequency (UHF) spectrum in a range of 960 MHz to 1215 MHz. 

Working Principle of a DME System

 The operation of a distance measuring equipment requires a complete exchange of information loop between the aircraft interrogator and ground transponder. The aircraft interrogator broadcasts a signal in all directions towards the ground transponder. The ground transponder receives the signal, adds a time delay of 50 microseconds and retransmits the signal in all directions towards the sky.  

The aircraft interrogator acknowledges the signal, and performs computation to calculate the distance based on a round trip time measurement. The velocity of the signal is approximately equal to the speed of light.

DME Operation
Cockpit view

This should be noticed that a DME system only provides the slant distance between the aircraft and the ground station. Ideally, it should have provided the ground distance. In terms of radar terminology, slant distance is defined as the line-of-sight distance along a slant direction between two points that are relatively not at the same level.

A Distance Measuring Equipment is more accurate in terms of obtaining the distance of an aircraft from the ground station than a VOR navigation system. However, it is lesser in terms of accuracy when compared to the current state of the art systems such as a Global Positioning System or GPS