What happens to DME display error as the altitude of the aircraft increases?

As altitude increases, the Distance Measuring Equipment (DME) display error tends to increase due to the geometry of the aircraft's position relative to the DME ground station. At higher altitudes, the aircraft is further away from the surface, which means the slant range (the straight-line distance between the aircraft and the DME station) is greater. This increase in slant range can lead to more pronounced errors in distance calculations, primarily due to the curvature of the earth and the effects of radio signal propagation.

The DME system calculates distance based on the time it takes for a radio signal to travel to the station and back. As altitude increases, especially in mountainous terrain or in areas with varied elevations, the accuracy of this calculation can be affected, leading to an increased potential for error in the DME readout. Thus, as the aircraft climbs, the likelihood of a larger display error becomes a more significant concern, making it crucial for pilots to be aware of these effects when interpreting DME data at higher altitudes.