DME — Distance Measuring Equipment
DME (Distance Measuring Equipment) is a secondary radio-nav system that indicates the slant range from aircraft to ground station. Often co-located with VOR (VOR/DME, VORTAC) or ILS (ILS/DME) — together they give a two-dimensional position fix.
Source: ICAO Annex 10 Vol I §3.5.
Operating principle — time measurement
The DME determines distance based on the principle of time measurement:
- Onboard DME interrogator transmits a UHF pulse to the ground station.
- Ground transponder receives the pulse and replies on a frequency shifted by 63 MHz (avoids self-interference).
- Onboard DME receiver measures the time difference between transmit and receive.
- Distance = (time difference − antenna delay) × c / 2 (speed of light, halved for round trip).
Result shown in the cockpit as distance in NM.
Frequency band
- DME transmitter (onboard): 1025-1150 MHz (UHF).
- DME receiver (ground): 962-1213 MHz (UHF).
- Automatically set via VOR frequency pairing — selecting a VOR frequency activates the paired DME channel.
Slant range — definition
The DME reading is a slant range — the geometric line from aircraft to ground station, not the horizontal (chart) distance:
Slant range = √(horizontal² + altitude_above_station²)
Consequence: in level flight at 10 000 ft over a DME station the slant range is ~1.6 NM (= 10 000 ft / 6076 NM/ft), while horizontal distance is 0 NM.
Vertical distance overhead
On direct overflight the DME gives the vertical distance in NM above the station:
- Directly overhead the horizontal distance = 0 NM, so the slant range equals the vertical distance (altitude above station).
- Example: overflight at 6000 ft AGL over a DME station shows ~1 NM on the DME (= 6076 ft / 6076 ft/NM ≈ 1 NM).
Slant range vs horizontal distance — error
The difference between indicated DME slant range and horizontal distance from the DME station increases when approaching the DME station:
| Horizontal | Altitude above station | Slant range | Difference |
|---|---|---|---|
| 30 NM | 5000 ft (0.82 NM) | 30.01 NM | 0.01 NM (negligible) |
| 10 NM | 5000 ft (0.82 NM) | 10.03 NM | 0.03 NM |
| 5 NM | 5000 ft (0.82 NM) | 5.07 NM | 0.07 NM |
| 2 NM | 5000 ft (0.82 NM) | 2.16 NM | 0.16 NM (~8 %) |
| 1 NM | 5000 ft (0.82 NM) | 1.29 NM | 0.29 NM (~29 %) |
| 0 NM (overhead) | 5000 ft (0.82 NM) | 0.82 NM | = 100 % |
→ At range > 10 NM and low altitude the error is negligible — DME ≈ horizontal distance. Near overhead the DME reading is increasingly dominated by altitude alone.
Use
- En-route: distance to next waypoint (e.g. "10 NM to VOR Frankfurt").
- Approach: distance to threshold using step-down fixes or approach lines (e.g. "5 NM to threshold").
- Position fix with co-VOR: together with VOR bearing → unambiguous position (distance + bearing).
- Ground speed: on direct approach DME changes with ground speed — pilot can estimate GS.
Identifier
DME stations transmit the identifier (Morse, same 3-letter code as the paired VOR) every 30 seconds as a short "beep". Verify the identifier before use.
Accuracy
ICAO Annex 10: DME accuracy better than 0.2 NM or 3 % of distance (95 % confidence), whichever is greater. Often better in practice: ±0.1 NM.
Range
Line-of-sight (UHF, quasi-optical) — same range formula as VOR:
- 1000 ft: ~39 NM
- 10 000 ft: ~123 NM
- FL400: ~246 NM
TACAN
TACAN (TACtical Air Navigation) is the military counterpart — transmits both bearing and distance. Civilian aircraft can use only the DME component of a TACAN (distance, no bearing). VORTAC = civilian VOR + military TACAN co-located.