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Distance measuring equipment (DME) is a type of navigation technology used by aircraft pilots for the purpose of determining distance from navigational fixes. Found attached to many very high frequency omnidirectional range (VOR) or localizer stations, this technology establishes a connection between a ground-based transmitter and an airborne receiver box. Once the connection is established, pilots are able to determine their distance from the ground station. A pilot can combine the information gathered from distance measuring equipment with the relative position from one or more VOR stations to determine the aircraft’s exact location.
To use distance measuring equipment, the pilot tunes his receiver to the specific frequency assigned to the ground-based equipment. Only 200 possible frequencies exist, so some stations may share the same frequency. Using distance measuring equipment requires the space between the aircraft and the ground station to be unobstructed. This requirement allows stations far apart from one another to operate on the same frequency without interference.
Distance measuring equipment operates on the concept of propagation delay. After the pilot tunes the receiver, an interrogation signal is sent to the ground-based station. Precisely 50 milliseconds after the station receives the signal, it will send a return signal to the aircraft. The airborne receiver measures the time necessary for the signal to be returned and subtracts the 50 milliseconds. It then takes that number and multiplies it by the speed at which the radio wave travels, resulting in the distance between the aircraft and station.
One major consideration while using distance measuring equipment is that it measures the straight line distance between aircraft and station, not the distance over the surface of the earth. In reality, the distance displayed to the pilot will be slightly farther. The higher the aircraft’s altitude, the larger this margin of error is. Another limitation of distance measuring equipment is that ground-based stations are only capable of supporting a predetermined number of aircraft. Transmitters will evaluate each interrogation signal based on its strength, granting priority to the closest aircraft.
As long as its limitations are understood, distance measuring equipment is an effective tool for enhancing pilot navigation and increasing situation awareness. This technology enables pilots to determine exact locations while en route as well as identify descent points on an instrument approach. This aging technology is gradually being replaced by the more sophisticated global positioning system (GPS), yet it still plays a fundamental role in modern aviation.