Field Guide · technology

Also known as: DME

DME (Distance Measuring Equipment) is a radio navigation aid that tells an aircraft its slant-range distance to a ground beacon by timing a radio round trip. The airborne set transmits pairs of pulses; the ground station waits a fixed delay and replies with its own pulse pairs; the aircraft measures the total elapsed time, subtracts the known delay, and converts to distance.1 Operating in the L-band (960–1215 MHz), DME is almost always paired with a VOR or ILS so a crew gets both bearing (or course) and distance from one tuned facility.

aircraft ground beacon interrogation pulse pair reply after fixed 50 µs delay round-trip time − delay → slant range
DME times the round trip of pulse pairs — interrogation out, reply back after a fixed delay — to compute slant-range distance.

How it works

DME is a form of secondary radar. The aircraft’s interrogator sends closely spaced pulse pairs (the pairing and spacing distinguish DME from other L-band traffic and help reject noise). The ground transponder receives them, waits a standardised 50 µs fixed delay, and re-transmits pulse pairs on a paired reply frequency. The airborne receiver correlates its own replies out of the stream — every beacon serves many aircraft at once, so each interrogator jitters its pulse timing and looks only for replies that track its own pattern. Elapsed round-trip time, less the 50 µs delay and processing allowances, converts to distance at the speed of light.

The result is slant range — the straight-line distance to the antenna, which exceeds ground distance when the aircraft is high and close. Because the encoding lives in pulse position and spacing, DME is conceptually related to pulse-position modulation. The military TACAN system reuses the DME distance function and adds bearing, which is why civil “VOR/DME” and military “VORTAC” facilities interoperate.

Relevance to SDR

DME is harder to receive casually than the AM navaids because it is pulsed L-band traffic shared with many users, but its principle — round-trip timing of coded pulse pairs — is the same idea behind Mode S and other secondary-radar signals a software-defined radio hobbyist may sample. GopherTrunk does not decode DME; it is a land-mobile trunking scanner, and DME is documented here to complete the aviation-navaid picture alongside VOR, ILS, and TACAN.

Sources

  1. Distance measuring equipment — Wikipedia, for the DME pulse-pair interrogation/reply scheme, the fixed 50 µs transponder delay, slant-range measurement, and pairing with VOR/ILS. ↩

See also