062 Radio Navigation topic guide
DME and Slant Range Error
DME answers a single question directly: how far is the aircraft from the ground station, measured as a straight line rather than along the ground. The aircraft interrogator transmits paired pulses, the ground transponder replies after a fixed, known delay, and the airborne equipment times the round trip, subtracts that fixed delay, and converts the remainder into distance.
Because the measurement is a straight line between aircraft and station rather than a distance over the ground, the reading is always slightly more than the true ground distance whenever the aircraft is above the station's elevation, and that difference, the slant range error, becomes significant only when the aircraft is both close to and high above the station.
Why the reading is slant range, not ground distance
Picture the aircraft, the station and the height between them as the three sides of a right-angled triangle: the ground distance is one side, the height is the second, and the DME measures the third side, the hypotenuse, directly. At long range the height is a tiny fraction of the hypotenuse, so slant range and ground distance are effectively identical. Close to the station, the height becomes a large fraction of the hypotenuse, or the whole of it, and the two values separate sharply.
This is also why DME-derived groundspeed can mislead close to a station. The equipment differentiates the changing slant range to compute groundspeed, and near overhead passage the slant range briefly stops closing even though the aircraft is still moving fast over the ground, because at that point almost all of the range change is happening in the vertical geometry rather than the horizontal one.
The overhead effect
Directly above a DME station the ground distance is zero, but the DME does not read zero: it reads the aircraft's height above the station converted into nautical miles, because that height is now the entire slant range. At FL360, for example, the aircraft is 36000 ft above mean sea level; dividing by 6076 ft per nautical mile gives approximately 6 NM, so a DME reading of roughly 6 NM at the moment of overhead passage is exactly what the geometry predicts, not an error.
Pilots use this deliberately: a DME reading that stops decreasing and holds close to the aircraft's height expressed in nautical miles is a reliable indication of station passage, and procedures that require crossing a fix at or before a given DME distance are written with this residual overhead value in mind.
UHF and channel pairing
DME operates in the UHF band, well above the VHF frequencies used by VOR and the ILS localiser, but the channels are paired: selecting a VOR or ILS frequency on a receiver that shares a paired DME channel automatically tunes the matching DME frequency as well, so a single frequency selection gives both bearing, or lateral guidance, and distance together without dialling two separate boxes.
That pairing is also why a DME can appear associated with a station even though its physical antenna sits elsewhere, most commonly co-located with the glide path transmitter rather than the localiser, which shifts the origin of the range reading by a small, generally insignificant amount close to the runway.
Worked example
Worked example: DME reading directly overhead a station
Not quite. The correct answer is C.
Correct answer: C
- A. This assumes the DME reads ground distance, which is genuinely zero overhead. The DME instead reads slant range, the straight line to the station, which is not zero when the aircraft is above it.
- B. This reads the flight level's digits directly as nautical miles, skipping the conversion from feet to nautical miles entirely.
- C. Correct. FL360 is 36000 ft; dividing by 6076 ft per NM gives approximately 6 NM, and since the ground distance is zero overhead, that height conversion is the entire slant range reading.
- D. This misplaces a decimal point in the feet-to-nautical-miles conversion, effectively dividing by 600 instead of 6076.
Step by step
- Directly overhead, ground distance is zero, so the entire slant range reading is made up of the aircraft's height above the station.
- Convert FL360 to feet: 36000 ft.
- Convert feet to nautical miles: 36000 divided by 6076 is approximately 6 NM.
- Sanity check: 6 NM is a small, sensible residual reading for a high-altitude overhead passage, consistent with typical DME minimums used to confirm station passage.
Common mistakes
Reading DME distance as ground distance in a close, high pass
The gap between slant range and ground distance grows exactly when it matters most, near the station at altitude, and treating the reading as ground distance overstates how much further there is to run.
Trusting DME-derived groundspeed right at overhead passage
Near the point of closest approach the slant range briefly stops closing even though the aircraft continues at speed, so a DME groundspeed readout can dip misleadingly at exactly the moment a pilot might check it.
Forgetting the feet-to-nautical-miles conversion in an overhead arithmetic question
The examiner is testing the height-to-slant-range relationship specifically, and jumping straight from a flight level to a NM figure without dividing by 6076 produces a confidently wrong order of magnitude.
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Last reviewed July 2026