Distance Measuring Equipment measures the slant range between aircraft and facility through a transponder-based system operating in the 960-1215 MHz band. The DME system requires both ground-based and aircraft equipment to function. The system operates on a line-of-sight principle and uses the constant of wave travel time/slant range to calculate distances.
The ground-based DME station sends a reply pulse at a different frequency after receiving the interrogation pulse from the aircraft. This reply is processed by the aircraft equipment to measure the time delay between the sent and received pulses. The DME interrogator is a pulse transmitter located in the aircraft, while the ground equipment acts as a transponder to generate the reply signals.
Different classes of ground-based DME transponders are used for various aviation purposes. Terminal, Low-altitude, and High-altitude transponders provide coverage appropriate to their service areas. Low-Power DME stations are utilized for approach navigation, while standard installations are co-located with VOR or ILS localizer facilities. The system contains computers that perform calculations and antennas to send/receive signals according to procedures defined in the Aeronautical Information Manual.
The technology performs a vital part in aviation navigation by providing precise distance measurements to pilots, particularly useful during instrument approach procedures when visual references are limited. As a transponder-based system, it forms a vital part of the ground-based navigation infrastructure aiding aircraft operations across different altitude levels and airport environments.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a distance measuring equipment (DME) in aircraft?
A distance measuring equipment (DME) is a radio navigation technology that measures the slant range between an aircraft and a ground station by timing the propagation delay of radio signals in the frequency band between 960 and 1215 megahertz (MHz), and it requires both ground-based and in-aircraft equipment.
Distance Measuring Equipment is a radio navigation technology used in aviation to determine the physical distance between the aircraft antenna and a ground based transponder station. The aircraft interrogator transmits pulse pairs at 150 pairs per second. The ground transponder is required to receive these interrogations and reply with a pulse sequence that has the same spacing as the original interrogation. Because line-of-sight between the aircraft and ground station is required, the system operates only when the aircraft is within direct radio visibility of the transponder.
The receiver in the aircraft measures the time delay between the sent and received pulses and calculates the slant range distance. This distance is often referred to as slant range because it represents the straight-line path from the aircraft to the transponder antenna, not the horizontal ground distance. Slant-range measurements will always exceed planned distances, since the value depends trigonometrically upon aircraft altitude above the transponder as well as the ground distance between them; the higher the aircraft, the greater the measurement.
What information can distance measuring equipment (DME) display?
Distance measuring equipment (DME) displays slant-range distance in nautical miles. This slant-range measurement is derived from the time difference between interrogating pulse-pairs and their ground replies. The interrogator's timing circuitry measures this elapsed time and converts it to a distance output, so the value shown is not the plan distance but the greater slant range owing to aircraft altitude. The primary datum is therefore distance in nautical miles, delivered without azimuth information. Auxiliary outputs give groundspeed and time-to-station readouts that the set computes by differentiating successive range values.
DME outputs appear on the navigation display when using VOR navigation, on the main flight display when using ILS, and on the dedicated DME display (DDRMI). The same information can be repeated on the track mode DME indicator, and modern directed-scanning receivers can cycle continuously through up to five tuned stations in accordance with a predetermined area-navigation auto-tuning programme.
When no usable replies are received, the display shows four dashes to indicate no computed data (NCD). If the interrogator temporarily loses replies, it attempts to maintain measurement by entering a memory condition that proceeds either as static or as velocity.
What are the main components of DME in aviation?
The main components of a DME system include a UHF transmitter/receiver in the aircraft and a UHF receiver/transmitter (transponder) on the ground. The ground transponder operates between 960 and 1215 MHz and uses paired UHF frequencies, with a fixed offset of 63 MHz between the interrogator and transponder channels.
How does distance measuring equipment work?
Distance Measuring Equipment operates by measuring the time it takes light to travel along the path to a reflector and back. The aircraft sends out a signal and the ground station replies after a fixed delay, sending the same pulses back at a different frequency. The DME interrogator measures the interval between the sent and received pulses, subtracts the ground-station delay. The speed used in the calculation is the propagation speed of the radio pulse, which is the speed of light, roughly 300,000,000 m/s (671,080,887 mph).
The system functions in the UHF band assigned by ICAO Annex 10, from 960 MHz to 1215 MHz. Each reply pulse pair is transmitted on a channel 63 MHz above or below the interrogation frequency, maintaining the paired radio-signal structure vital for suppression of interference. Interrogation pulse pairs are transmitted at a nominal rate of 150 per second, and the ground station retransmits only when the aircraft's distinctive pulse spacing is recognized. This two-frequency protocol permits many aircraft to use one DME station simultaneously.
Because the measurement is slant-range, the indicated distance will always exceed the horizontal distance by an amount that grows with aircraft altitude. The error is minimized only at lower altitudes. Accuracy is better than a mile or 3% of the distance, whichever is greater, making the system acceptable for airways and initial approach segments.
Distance Measuring Equipment transmits an interrogation pulse pair and the aircraft exhibit measures the elapsed time, converts it to a mathematical output, and displays a continuous slant-range value. While I steered through a low-visibility setting, the separation amount altered in real-time, permitting small accurate amendments to velocity and bank tilt. Because the apparatus provided continual updates, I could keep my place with precision despite inconsistent conditions.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What is the DME aviation formula?
The heart of every DME read-out is the distance formula: distance = speed time. DME receiver calculates distance = speed time. DME uses distance = speed time.
Speed is the speed of light, roughly 162 000 nmi/s. Hence, time for 1 NM two-way is 12.359e-6 seconds.
The DME aviation formula is distance = speed x time. To use the formula, the airborne interrogator measures the time between sending its interrogation and receiving the reply. The ground beacon adds a delay of 50 seconds for X mode and 56 seconds for Y mode. Because the measurement is slant range, not ground distance, aircraft height adds to the reading. Slant-range measurements will always exceed planned distances. Rule-of-thumb is that if you are at least 1 NM away from the station for every 1,000 AGL, slant range error is negligible.
How many aircraft can use DME?
Ground based DME transmitters are rated to handle roughly 100 aircraft at a time and above this limit the transponder avoids overload by limiting the sensitivity (gain) of the receiver, so aircraft farthest away cannot pick up DME signals at all.
What planes have DME? A few older IFR certified GA aircraft have DME, while most general aviation aircraft built today are equipped with multiple GPS receivers instead.
What are the types of DME in aviation?
Ground DME stations are classified by power and range, and they fall into Class A, Class B and low power categories. Class A stations are high power stations with range up to 200 nm (370.4 km), used for en route navigation, while high altitude transponders provide range of 130 nautical miles (240.74 km) and service to a minimum height of 45,000 feet (13,716 meters). Class B stations are medium power stations with range up to 50 nm (92.6 km), used for terminal area navigation, and terminal transponders typically provide range of 25 nautical miles (46.3 km) and service to a minimum height above ground of 12,000 feet (3,657.6 m). Low power DMEs transmit 100 watts of power (338.14 BTU/h) and are used for approach navigation. Low altitude transponders provide a range of 40 nautical miles (74.08 kilometers) and service to a minimum height of 18,000 feet (5,486.4 meters).
Navigation facilities that incorporate DME include VOR/DME, VORTAC, ILS/DME and LOC/DME. These facilities are identified by synchronized identifications transmitted on a time share basis and operate in the 962-1215 MHz band with a 63 MHz difference between interrogator and transponder frequencies. The coded tone used for identification is 1,350 Hz for TACAN, while VOR and localizer are identified by a tone modulated at 1,020 Hz. All VOR and ILS systems transmit navigation signals over VHF.
