How does a DME (Distance Measuring Equipment) work?
The DME instrument in your aircraft sends radio pulses to a transponder located with the navaid you’re using. The transponder sends a pulse back, and by measuring the time between the tranmission of the pulse and the receipt of the reply, the instrument can calculate the distance since it knows how fast radio waves travel in air. DME is accurate to about one tenth of a nautical mile (about 600 feet or 185 meters) and measures the distance direct from the aircraft to the navaid, NOT the ground distance. So if you are 12,000 feet AGL directly above the navaid, your DME will still read 2.0 nm, since you really are 2 nm from the aid.
