Distance Measurement Equipment (DME) is a transponder-based radio navigation system that measures the slant-range distance in nautical miles between an aircraft and a ground station. It determines this distance by measuring the time delay between interrogation and reply, applying the time-of-arrival principle to compute the separation. The computed slant-range distance is then presented to pilots on a digital readout, providing continuous distance guidance. Functionally, the DME system is identical to the distance-measuring unit of TACAN, and like all radio systems it can be affected by atmospheric conditions, terrain, and interference, leading to potential errors in the indicated range.
How does a DME work in aircraft?
DME works by using paired radio signals between the aircraft and ground stations. The DME avionics in the aircraft send a pulse signal to the ground based DME, which responds with an answer pulse signal. When the pilot of a DME equipped aircraft tunes the frequency of a VOR with DME, the frequency of the co-located DME is automatically tuned. 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 after a fixed delay of 50 seconds. The time taken for this process is used to calculate the distance.
The DME receiver in the aircraft searches for reply pulse pairs (X mode = 12 s spacing) that match its original interrogation pattern. It operates on the line of sight principle. Slant range measurements will always exceed planned distances, because slant range includes aircraft altitude. Slant range error is minimized at lower altitudes. Accuracy is better than 1/2 mile (0.8 km) or 3% of the distance, whichever is greater.
How to know if your plane has DME?
To know if the plane has a DME, look at the navigation display: a letter D in the top left of the navaid box tells you the ground station has DME capability. If your cockpit can show range in nautical miles when you pass overhead, the aircraft is equipped. DME antennas are normally mounted on the underside of the fuselage so the thin, white blade or spike has an unobstructed view of the ground transmitters.
What is the difference between VOR and DME in aviation?
VHF omnidirectional range and distance-measuring equipment solve different navigation problems. VOR navigation technology uses a ground-based antenna at a station to send a directional signal that rotates clockwise thirty times a second. The receiver displays the bearing from the station on a gauge called a VOR indicator, letting pilots select desired course or radial. DME replies to an airborne interrogator with a slant-range distance, independent of bearing. When the two aids are paired, the result is a radio beacon that combines VOR with DME: VOR-DME for civil-only sites, VORTAC when the co-located VOR and TACAN also share the same DME system. All collocated DME, VOR and TACAN facilities share the same service volume and same DME frequency. The DME-coded identification is transmitted one time for each three or four times that the VOR-coded identification is transmitted, maintaining synchronized timing.
A VOR approach does not have vertical guidance, so it is a non-precision approach. With a VOR-DME one can determine the exact point at which to start CDFA, converting bearing plus slant range into continuous position. ILS systems transmit navigation signals over VHF and provide vertical guidance, making ILS the more precise option, yet coverage remains limited by line-of-sight while GPS now offers global coverage. As VORs are becoming decommissioned, the VOR MON Program will implement new service volumes advertised above five-thousand feet AGL, eventually replaced by GNSS technology.
What are the DME limitations in aviation?
DME is limited to line-of-sight and terrain and distance beyond the horizon prevent it from working. Small physical obstacles block or reduce the signal path. Over rough terrain the reliable range shrinks to 30 NM or 40 NM, and at low altitude the horizon cut-off further shortens coverage even though, at line-of-sight altitude, the U.S. Aeronautical Information Manual states reliable signals are received up to 199 NM.
Each ground station can only respond to a certain number of interrogations in a given period of time: a typical beacon saturates at about 2,700 pulse pairs per second, which translates to a practical capacity of roughly 100 aircraft. If too many aircraft interrogate the ground station, it automatically desensitizes its receiver and ignores the farthest or weakest signals, so overloaded equipment denies service to those aircraft. These constraints - line-of-sight blockage, terrain masking, slant-range error and finite interrogation capacity - define the operational limits of DME in aviation.
What are DME requirements for use in aviation?
DME must meet the performance requirements set forth in the International Standards and Recommended Practices Aeronautical Telecommunications. The frequency range 960 MHz to 1215 MHz is assigned in accordance with ICAO Annex 10. Aircraft equipment must be FAA-certified before installation. Non-Federal ground stations must meet the requirements of 171.157 before the Federal Aviation Administration will approve an IFR procedure. A suitable frequency channel must be available and line-of-sight between the aircraft and the ground station is required.
FAR 91.205(e) requires DME above FL 240 when VOR navigation is required under IFR. Aircraft certified to fly IFR above that level must therefore be equipped with approved DME or a suitable RNAV system. Some countries require any aircraft operating under IFR to carry a DME interrogator. For RNAV routes, RNAV Standard Instrument Departures and RNAV Standard Terminal Arrivals, at least two DME facilities are used. The NEXTGEN DME program sustains High-Power DMEs required for RNAV en-route procedures as a backup for GPS navigation.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
