5.dme. Distance Measuring Equipment (DME)
Distance Measuring Equipment (DME) is a UHF radio-navigation system that provides the pilot with a continuous digital readout of slant-range distance in nautical miles from the aircraft to a selected ground station. DME is one of the original components of the VOR/DME and VORTAC systems and remains a primary range source for instrument flying, particularly for arc procedures, step-down fixes, holding fixes, and as a backup to GPS.
Frequencies and Pairing
DME operates in the UHF band between 962 and 1213 MHz, but the airborne unit is tuned indirectly. Each VOR or localizer frequency in the 108.0–117.95 MHz VHF band is paired with a discrete DME channel, so when you select the VOR/ILS frequency, the avionics automatically tune the associated DME. A VORTAC or VOR/DME provides azimuth (VOR) plus distance (DME) from the same site; an ILS/DME or LOC/DME uses DME for fix identification and missed approach guidance.
How DME Works
DME uses a transponder principle:
- The airborne interrogator transmits paired pulses on a discrete frequency.
- The ground transponder receives the pulses, waits a fixed 50-microsecond delay, and replies on a frequency offset by 63 MHz from the interrogation.
- The airborne unit measures the round-trip time of its own pulse pair (recognized by a unique random pulse spacing) and converts it to distance.
Because the calculation is based on time-of-flight, DME measures the straight-line distance from the antenna to the station — not horizontal distance.
Slant-Range Error
The distance displayed is slant range, the hypotenuse of a triangle whose legs are the aircraft's altitude above the station and the horizontal distance over the ground. The error is greatest when the aircraft is high and close to the station and is essentially zero at long ranges and low altitudes.
A practical rule: slant-range error in NM is approximately equal to the aircraft's altitude above the station in thousands of feet, divided by 6,000, when directly overhead. For example, at 6,000 ft AGL directly above a DME, the indicator reads about 1.0 NM even though horizontal distance is zero — the aircraft is in the station passage cone where the readout briefly becomes unreliable. Slant-range error becomes negligible (less than 1 percent) when the horizontal distance from the station exceeds the aircraft's altitude in NM by a factor of three or more.
Groundspeed and Time-to-Station
Many DME receivers compute and display groundspeed (GS) and time-to-station (TTS):
- These computations assume the aircraft is flying directly to or directly from the station (TO/FROM track aligned with the radial).
- When tracking a DME arc or flying a course offset from the station, the displayed GS will be less than actual groundspeed and TTS will be inaccurate. On an arc with the station abeam, GS reads near zero.
Identification and Monitoring
A DME station broadcasts a Morse code identifier every 30 seconds at a tone of 1350 Hz, which is approximately one octave higher than the paired VOR ident. When a single ident is heard, the VOR and DME share a common identifier; when both are received, both facilities are operating. You must positively identify the DME before using it for navigation. If only the VOR ident is heard, the DME may be out of service even though a distance still appears on the display.
Operational Use
DME is required or used for:
- DME arcs on instrument approach transitions, flown by maintaining a constant distance from the station while turning.
- DME fixes that define IAFs, step-downs, FAFs, MAPs, holding fixes, and missed approach holding patterns.
- DME-required approaches, annotated "DME REQUIRED" in the briefing strip; the pilot must have an operable DME (or an authorized GPS substitute) to fly the procedure.
- Class A airspace and certain Class B/RVSM operations where DME is part of the required equipment for IFR.
GPS in Lieu of DME
Under current FAA policy (AIM 1-2-3 and AC 90-108), a properly installed and operational IFR-approved GPS may be used as a substitute for DME on conventional approaches and en route procedures, provided the GPS is operating in a mode that provides equivalent integrity (RAIM or SBAS). Pilots must still confirm the substitution is authorized and load or identify the appropriate fix from the database.
Limitations
- Line of sight: Like VOR, DME range is limited by terrain and altitude. Reliable reception at 12,000 ft is roughly 130 NM.
- Station saturation: A ground transponder can serve only about 100 aircraft simultaneously. Beyond that, weaker signals (more distant aircraft) are dropped first.
- Search vs. Track: After tuning, the receiver enters a search mode for a few seconds before locking into track. A momentary loss of lock will cause the indicator to flag or revert to search.
- Cone of confusion directly over the station, where the readout fluctuates and any slant range is dominated by altitude.