Distance measuring equipment (DME) is radio navigation technology that gives you precise distance from a station, similar to GPS. This system is composed of aircraft-installed and ground-based equipment. The ground-based equipment consists of a VHF omnidirectional radio range (VOR), or in some instances, an instrument landing system (ILS). The DME is an integral part of standard aircraft safety operations, allowing pilots to safely descend and ascend. With this in mind, this blog will cover the DME’s importance in the aviation industry.
The VOR provides the pilot with the slant-range distance to the DME transmitter. Slant-range distance is typically greater than the flight-planned distance to a DME station due to the fact that it accounts for the aircraft’s height above the station. Moreover, the DME is programmed to the VOR frequency that the pilot wants the distance from. When the pilot tunes the frequency of a VOR or ILS, with DME, the frequency of the co-located DME is automatically tuned.
The aircraft emits a pulsed signal to the DME station and the station replies. At the same time, aircraft equipment measures the time between transmission and reception to determine the groundspeed and time to the station. It is important to understand that DME is not always compatible with every VOR. The VOR necessitates the right equipment. Two types of VOR that have DME include VOR-DME and VORTAC stations.
DME radios operate over line-of-sight distances, similar to VHF communications and navigation radios. However, the transmission power of the aircraft’s DME unit can be a limiting factor. DME signals may be received up to 199 miles from a station, but this only happens in very rare instances. For smaller aircraft, the DME range is about 50 miles. If an aircraft comes in contact with mountainous terrain, the transmission signal’s DME range may be even less.
Some aircraft are equipped with more advanced systems that pair a small display with a NAV1 radio to show the distance on an independent display or the horizontal situation indicator (HSI). As technology became more advanced, aerospace engineers devised other navigation systems such as RNAV. These systems allow pilots to navigate the aircraft to any point as long as it references a VOR radial and DME distance. RNAV routes allow pilots to operate away from established airways and directly to their desired location.
Today, DME systems are not as common and have been replaced by the global positioning system (GPS). GPS allows pilots to travel from point A to point B without being limited by the location of VOR stations along the way. While GPS has replaced more area navigation systems, DME is still required for many instrument approach procedures.
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