Harmon Technologies, Inc.

DME

Distance measuring equipment works by sending a signal to a ground based navigational aid and measuring how long it takes the response to return.  It works much like radar and uses similar radio frequencies.  When responses are received, the DME unit can display the distance from the airplane to the station (in nautical miles) and, if the airplane is flying directly to or from the station, the time to reach the station and groundspeed can also be displayed.

When combined with heading information from the VOR displays, it is very easy to keep a clear picture of the airplanes actual position.

One interesting thing about DME is worth mentioning.  Since DME measures the total distance to the station, it is affected by the aircraft's altitude.  DME actually measures what is called "slant range".  If the airplane is flying at 5,000 feet (a mile up), the DME will indicate a distance of one mile even when the airplane is right over the station.

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