DME

The DME (Distance Measuring Equipment) provides the distance between the aircraft and a ground station. It is often paired with a VOR to give both azimuth and distance.

Operating principle

The airborne equipment sends pulses (interrogations) to the ground DME station. The station responds with a calibrated delay. The receiver measures the round-trip time and derives the slant range.

Slant range vs ground distance

The DME measures the straight-line (slant) distance, not the horizontal ground distance:

Slant range = sqrt(ground_distance² + altitude²)

In practice: The difference is negligible except at high altitude and in the immediate vicinity of the station.

Altitude	DME distance	Ground distance	Error
FL100 (10,000 ft)	10 NM	9.95 NM	< 1%
FL100	3 NM	1.7 NM	43%
FL350	20 NM	19.5 NM	2.5%

Rule of thumb: The error is significant only when the DME distance is less than the altitude (in NM).

Position identification

By combining a VOR radial and a DME distance, a unique position (fix) is obtained:

Radial = azimuth relative to the station
DME = distance from the station

Groundspeed calculation

The DME allows calculation of groundspeed (GS) by timing the change in distance:

GS (kt) = delta_distance (NM) x 60 / delta_time (min)

Example: If the DME goes from 30 NM to 25 NM in 2 minutes:

GS = 5 x 60 / 2 = 150 kt

DME arc

A DME arc is a procedure where the aircraft maintains a constant distance from the station. It is used in certain approaches.

Technical characteristics

Parameter	Value
Frequency band	UHF (paired with VOR)
Range	Same as the associated VOR
Accuracy	+/- 0.5 NM or +/- 3% (whichever is greater)
Information	Slant range in NM