Radio Navigator
Glossary

Slant range

Direct answer

Slant range is the straight-line distance between the aircraft and the DME beacon. Because the aircraft is above the beacon rather than beside it, this always reads slightly more than the distance over the ground.

Also written as: slant range error, DME error.

DME works by timing a radio pulse to the ground station and back, so what it measures is the hypotenuse of a triangle whose other two sides are the ground distance and your height above the station. At normal ranges the difference is negligible. Close in, it dominates.

The practical consequence is that a DME reading never goes to zero. Directly overhead the beacon, the ground distance is nil and the DME reads your height above it, converted to nautical miles. That residual reading is the cue for overhead passage, and it is also why a DME arc flown very close to the beacon behaves badly.

Where this comes up

Sources

  1. ICAO Annex 10 (Aeronautical Telecommunications), Volume I, Radio Navigation Aids. DME interrogation/reply timing, from which slant range follows.
  2. FAA Instrument Flying Handbook (FAA-H-8083-15B), Chapter 9 (Navigation Systems). Slant range error and its behaviour close to the station.

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This article does not constitute flight instruction. Always defer to the guidance of your qualified flight instructor and to current charts and procedures or regulations published by your country's aviation authority.

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