DISTANCE MEASURING EQUIPMENT

Distance measuring equipment (DME) is an ultra high frequency (UHF) navigational aid present with VOR/DMEs and VORTACs. It measures, in nautical miles (NM), the slant range distance of an airplane from a VOR/DME or VORTAC (both hereafter referred to as a VORTAC). Although DME equipment is very popular, not all airplanes are DME equipped. To utilize DME, the pilot should select, tune, and identify a VORTAC, as previously described. The DME receiver, utilizing what is called a "paired frequency" concept, automatically selects and tunes the UHF DME frequency associated with the VHF VORTAC frequency selected by the pilot. This process is entirely transparent to the pilot. After a brief pause, the DME display will show the slant range distance to or from the VORTAC. Slant range distance is the direct distance between the airplane and the VORTAC, and is therefore affected by airplane altitude. (Station passage directly over a VORTAC from an altitude of 6,076 feet above ground level (AGL) would show approximately 1.0 NM on the DME.) DME is a very useful adjunct to VOR navigation. AVOR radial alone merely gives line of position information. With DME, a pilot may precisely locate the airplane on that line (radial).

Most DME receivers also provide ground speed and time-to-station modes of operation. The ground speed is displayed in knots (NM per hour). The time-to-station mode displays the minutes remaining to VORTAC station passage, predicated upon the present groundspeed. Groundspeed and time-to-station information is only accurate when tracking directly to or from a VORTAC. DME receivers typically need a minute or two of stabilized flight directly to or from a VORTAC before displaying accurate groundspeed or time-to-station information.

Some DME installations have a hold feature that permits a DME signal to be retained from one VORTAC while the course indicator displays course deviation information from an ILS or another VORTAC.