Slant Range in DME

What is Slant Range in DME?

DME (Distance Measuring Equipment) measures the slant range—the true line-of-sight distance between the aircraft and the DME station. This is a three-dimensional measurement, forming the hypotenuse of a right triangle where one side is the aircraft's altitude above the station and the other is the horizontal (plan) distance.

DME Slant Range vs Ground Distance

The DME readout always shows slant range. To find the actual ground distance, you must use the Pythagorean theorem:

• (Slant Range)² = (Altitude)² + (Ground Distance)²

At long distances (where the DME distance in NM is much greater than the aircraft's altitude in thousands of feet), the difference between slant range and ground distance is negligible. However, when the aircraft is close to the station and at high altitude, the error becomes significant.

DME Calculation Example

Suppose an aircraft at 35,000 feet (about 5.76 NM) is 18 NM slant range from a DME. The ground distance is:

• (18)² = (5.76)² + (Ground Distance)²
• Ground Distance = sqrt(324 - 33.14) = sqrt(290.86) = 17.06 NM

Slant Range Effect on DME Readings

• Overhead the beacon: DME reads the aircraft's altitude (in NM), not zero.
• At long range: DME error is minimal; slant range and ground distance are nearly identical.
• At short range/high altitude: DME may overstate the true ground distance.

DME Groundspeed and DME Arc

DME can display groundspeed and time to station, but these are most accurate only when flying directly to or from the beacon at ranges much greater than altitude. When flying a DME arc (a constant-radius turn around the station), the groundspeed readout drops to zero, as the distance to the station isn't changing.

DME Fixing and Charting

When plotting DME ranges on a chart, the radius used is the slant range, centered on the DME station. Two DME ranges create intersecting circles, giving two possible position fixes.