DME Explained: Slant Range, DME Arcs, Groundspeed Readouts, and How Pilots Use It
- Nathan Hodell
- Sep 18, 2025
- 8 min read
Updated: Aug 5
DME gives pilots one of the most useful pieces of information in navigation: exactly how far they are from a station, updated continuously and accurate to a fraction of a mile. But to use it correctly, you need to understand what it's actually measuring (slant range, not ground distance), how to fly the DME arcs that appear on instrument approaches, and the quirks of the groundspeed and time-to-station readouts that DME provides. Get these details right and DME becomes a powerful tool for both enroute navigation and precise instrument approaches.
This post covers DME in practical depth: the slant range concept and when it matters, flying DME arcs, the groundspeed and time readouts and their limitations, channel pairing with VOR and ILS, identification, and the overall role of DME in navigation.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
What DME Is and How It Works
DME (Distance Measuring Equipment) is a radio navigation system that tells the aircraft its distance from a ground station.
The interrogation/reply process:
The aircraft's DME interrogator transmits a pulse pair to the ground station
The ground station receives it and, after a precise fixed delay, transmits a reply
The aircraft measures the round-trip time
Using the speed of radio waves, it calculates the distance
The distance displays in nautical miles, updated continuously
Why the fixed delay matters:
The ground station's reply has a known, precise delay (typically 50 microseconds)
The aircraft accounts for this delay in its calculation
This ensures accurate distance measurement
The frequency band:
DME operates in the UHF band (962-1213 MHz)
This is different from VOR (VHF)
But DME is paired with VOR/ILS frequencies (more on this below)
Slant Range vs. Ground Distance
The single most important concept about DME: it measures slant range, not ground distance.
The geometry:
DME measures the direct line-of-sight distance from the aircraft to the station — the hypotenuse of a right triangle:
One leg: the aircraft's altitude above the station
Other leg: the horizontal ground distance
Hypotenuse: the DME slant range (what's displayed)
The classic example:
An aircraft directly over a DME station at 6,000 feet:
Ground distance: 0 NM (you're right over it)
DME reading: approximately 1 NM
Why: 6,000 feet ≈ 1 NM, and that's the slant distance (straight down)
DME never reads zero (unless you're at the station's altitude), because there's always the vertical component.
When slant range matters:
Situation | Slant Range Effect |
High altitude, close to station | Significant error |
High altitude, far from station | Negligible |
Low altitude, any distance | Minimal |
On approach, near station | Noticeable |
Worked examples:
Directly over station at 6,000 feet: DME ≈ 1 NM (ground = 0)
10 NM out at 6,000 feet: Slant range ≈ 10.1 NM (ground = 10) — small error
5 NM out at 6,000 feet: Slant range ≈ 5.1 NM — error growing
1 NM out at 6,000 feet: Slant range ≈ 1.4 NM — significant error
50 NM out at 6,000 feet: Slant range ≈ 50.04 NM — negligible
The rule:
The slant range error is significant only when you're both close to the station AND at relatively high altitude. For enroute navigation, the error is negligible.
Flying DME Arcs
DME arcs are a major instrument flying skill that appears on many approaches. They're a curved flight path at a constant DME distance from a station.
What a DME arc is:
A flight path maintaining a constant DME distance
For example, "the 15 DME arc" means staying 15 NM from the station
Used on instrument approaches to transition from the enroute structure to the final approach course
Appears as a curved line on approach charts
How to fly a DME arc:
The technique uses the "turn 10, twist 10" method (or "center the needle, turn 10°"):
Intercept the arc:
Lead the turn before reaching the arc distance
Lead distance depends on groundspeed (roughly 0.5% of groundspeed, or about 0.5-1 NM)
Turn to roughly perpendicular to the station radial
Maintain the arc:
The arc is flown as a series of short straight segments
Keep the station roughly 90° off your wing
When the DME increases past the target, turn slightly toward the station
When the DME decreases past the target, turn slightly away
Use small heading changes (10° at a time)
The "twist" technique:
Set the OBS ahead of your position (e.g., 10° ahead)
When the CDI centers, turn 10° and twist the OBS another 10°
This keeps you on the arc through small increments
Wind correction:
Wind affects the arc
Adjust your turns to compensate
More wind correction on the upwind/downwind portions
Leading the radials:
The approach chart shows radials crossing the arc
These are checkpoints along the arc
Lead your turns at these points
The key skill:
A DME arc is flown as many small straight segments approximating a curve. Keep the station perpendicular to your heading, make small corrections, and the arc stays constant.
The Groundspeed and Time Readouts
Many DME units display groundspeed and time-to-station, but these have important limitations.
Groundspeed readout:
DME calculates groundspeed based on the rate of distance change
BUT it measures the rate of change of slant range
This is accurate only when flying directly toward or away from the station
Off-axis (flying perpendicular), the readout is inaccurate
The directly-toward/away requirement:
DME groundspeed is accurate when tracking directly to/from the station
When flying a DME arc (perpendicular), groundspeed reads near zero (slant range isn't changing much)
When flying off-axis, the readout underreads true groundspeed
Time-to-station readout:
Based on the groundspeed calculation
Same limitations apply
Accurate only when flying directly to the station
The slant range effect also affects this near the station
The practical implications:
Use DME groundspeed only when flying directly to/from the station
On a DME arc, ignore the groundspeed readout (it'll read low)
Time-to-station is most accurate enroute, directly tracking
Channel Pairing with VOR and ILS
DME is paired with VOR and ILS frequencies, simplifying tuning.
How pairing works:
Each VOR/ILS frequency is paired with a specific DME channel
When you tune the VOR/ILS frequency, the DME automatically tunes
You don't separately tune the DME frequency
The pairing is built into the system
VOR/DME:
A VOR co-located with DME
Tune the VOR frequency; DME follows
Provides azimuth (VOR) and distance (DME)
ILS/DME:
Many ILS approaches include DME
Tune the ILS frequency; DME follows
DME provides distance along the approach
"Cross 5 DME at 2,000 feet" type fixes
VORTAC:
VOR + TACAN
The TACAN provides the DME function for civilian aircraft
Tune the VOR frequency; DME comes from the TACAN
The practical benefit:
You tune one frequency (the VOR or ILS) and get both navigation and distance. The DME pairing is automatic.
Identifying DME
Like VORs, DME stations must be identified:
The DME identifier:
DME transmits a Morse code identifier
Usually every 30 seconds or so
Often coordinated with the VOR identifier
A higher-pitched tone than the VOR
The combined identification:
At a VOR/DME, you may hear the VOR ident and DME ident
The DME ident confirms the DME is working
If you hear the VOR but not the DME ident, the DME may be out
Why identification matters:
Confirms the DME is operational
Confirms you're receiving the right station
A DME under maintenance removes its identifier
The "DME hold" feature:
Some units have a "hold" function
Locks onto a DME station while you tune the VOR elsewhere
Useful for using DME from one station while navigating with another
The held DME continues to display
DME Limitations
DME has specific limitations to understand:
Line of sight:
DME requires line of sight to the station (UHF)
Terrain and obstacles block the signal
Higher altitude extends range
Same limitation as VOR
Slant range error:
Significant only close and high (as discussed)
Affects approach fixes near the station
Negligible enroute
Station capacity:
A DME station can only handle a limited number of aircraft (typically ~100)
In very busy areas, capacity could theoretically be reached
Rarely an issue in practice
Reception range:
Limited by line of sight and power
Service volumes similar to VOR
Beyond range, no DME information
DME in Modern Navigation
DME's role in the modern system:
With GPS:
GPS provides distance information too
DME remains a backup
DME is independent of GPS (different system)
Valuable when GPS is unavailable
DME/DME RNAV:
Some RNAV systems use multiple DME stations
Triangulating position from DME distances
A backup to GPS-based RNAV
Used by some aircraft for redundancy
The MON consideration:
DME is part of the backup navigation infrastructure
Works with the VOR MON
Provides distance information during GPS outages
Maintained as part of the backup system
Required for some operations:
Above FL240, DME is required if VOR is used for navigation (FAR 91.205)
Many approaches require DME
DME remains operationally important
The FAR 91.205 DME Requirement
A specific regulatory point: DME is required for certain operations.
The rule:
For IFR flight at and above FL240 (24,000 feet)
If VOR navigational equipment is required
DME or a suitable RNAV system is required
Why:
At high altitude, precise position is important
DME provides distance for accurate navigation
Or RNAV (GPS) can substitute
The practical effect:
High-altitude IFR requires DME (or RNAV)
Most aircraft operating at FL240+ have DME or GPS
A consideration for high-altitude operations
Common Misconceptions
"DME shows ground distance.
"No — DME shows slant range (direct distance), which differs from ground distance, especially close and high.
"DME reads zero over the station.
"No — over the station, DME reads approximately your altitude in NM (e.g., ~1 NM at 6,000 feet).
"DME groundspeed is always accurate.
"No — it's accurate only when flying directly to/from the station. On an arc or off-axis, it underreads.
"I tune DME separately.
"Usually not — DME is paired with the VOR/ILS frequency and tunes automatically.
"DME is obsolete with GPS.
"No — DME remains a backup, is required for some operations, and works independently of GPS.
On the Written Test and Checkride
DME appears on tests, especially instrument. The most commonly tested topics:
Slant range vs. ground distance
DME reading over the station (≈ altitude)
When slant range error is significant (close and high)
Channel pairing with VOR/ILS
DME groundspeed limitations
Flying DME arcs
The FL240 DME requirement
Quick Reference
DME Basics:
Distance Measuring Equipment
Measures distance to a ground station
UHF band (962-1213 MHz)
Paired with VOR/ILS frequencies
Slant Range:
DME measures direct (line-of-sight) distance
The hypotenuse: altitude and ground distance are the legs
Over station at 6,000 feet: DME ≈ 1 NM
Error significant only close AND high
Slant Range Examples:
Position | DME (Ground = X) |
Over station, 6,000 ft | ~1 NM (ground 0) |
10 NM, 6,000 ft | ~10.1 NM |
1 NM, 6,000 ft | ~1.4 NM |
50 NM, 6,000 ft | ~50.04 NM |
DME Arcs:
Constant DME distance flight path
Keep station ~90° off the wing
"Turn 10, twist 10" technique
Series of small straight segments
Lead the turn when intercepting
Groundspeed/Time Readouts:
Accurate only flying directly to/from station
On an arc: reads near zero
Off-axis: underreads true groundspeed
Channel Pairing:
Tune VOR/ILS frequency; DME follows automatically
VOR/DME, ILS/DME, VORTAC
No separate DME tuning
Identification:
Morse code identifier (higher pitch than VOR)
Confirms DME operational
DME hold: locks DME while tuning VOR elsewhere
Limitations:
Line of sight (UHF)
Slant range error (close and high)
Station capacity (~100 aircraft)
FAR 91.205:
DME (or RNAV) required at/above FL240 if VOR used
Key Principle:
DME measures slant range, not ground distance. The difference matters only close and high. Master DME arcs and remember groundspeed is accurate only directly to/from the station.
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.