DISTANCE MEASURING EQUIPMENT (DME)
DME equipment is in two parts the equipment on board the aircraft and ground based equipment. The ground base equipment is normally associated with the ground equipment at airports and at VOR beacons. To use the DME the pilot locates the appropriate ground beacon on his chart, (map); he then tunes his DME to the frequency for that particular beacon. The equipment on board the aircraft will then give the pilot the following information: the distance to the beacon; his ground speed, (closing or departing speed); and the time it will take him to reach the beacon or since he left the beacon. This equipment works with Ultra High Frequency, (UHF), radio waves and as such it will not work if there are obstacles in the line of sight, (including the Earths curvature), between the aircraft and the ground equipment. If the aircraft is high enough it will work when it is up to 100 miles from the beacon.