Distance measuring equipment

Navigation · Chapter 44 · DGCA CPL and ATPL

How DME measures slant range with pulse pairs and a 50 µs reply, slant versus ground range, identification with a VOR, accuracy and ground speed.

Remember

A distance from the ground

Distance, not direction • dme (distance measuring equipment) tells you how far you are from a beacon on the ground • Distance alone puts you somewhere on a circle round it
With a VOR beside it • A VOR at the same place adds the radial: a radial and a range give a fix from one station • This is called rho-theta (rho = range, theta = bearing)
Flying an arc • Hold the range steady and you fly a perfect curve round the beacon: a DME arc • DME also gives groundspeed and time to go, and is used for holds, height checks on approach, ATC reports and RNAV

Channels and pairing

You never tune it • Select the VOR or ILS frequency and its paired DME tunes itself: frequency pairing
Ask here, answer there • 962–1 213 MHz (UHF), 1 MHz apart: 252 channels (1–126, each with an X and a Y) • The beacon’s answer is always 63 MHz away from the aircraft’s question
With an ILS • A DME paired with an ILS reads zero at the threshold • It stands elsewhere on the airfield, so it shortens its 50 µs wait to make up the difference