The DME frequency is paired to the co-located VOR or  VORTAC frequency. When the correct frequency is tuned  for the VOR signal, the DME is tuned automatically. Tones  are broadcast for the VOR station identification and then  for the DME. The hold selector on a DME panel keeps the  DME tuned in while the VOR selector is tuned to a different  VOR. In most cases, the UHF of the DME is transmitted  and received via a small blade-type antenna mounted to the  underside of the fuselage centerline.  [Figure 11-121] A traditional DME displays the distance from the DME  transmitter antenna to the aircraft. This is called the slant  distance. It is very accurate. However, since the aircraft is  at altitude, the distance to the DME ground antenna from a  point directly beneath the aircraft is shorter. Some modern  DMEs are equipped to calculate this ground distance and  display it.  [Figure 11-122] Area navigation (RNAV) is a general term used to describe  the navigation from point A to point B without direct over  flight of navigational aids, such as VOR stations or ADF non- directional beacons. It includes VORTAC and VOR/DME  based systems, as well as systems of RNAV based around  LORAN, GPS, INS, and the FMS of transport category aircraft.  However, until recently, the term RNAV was most commonly  used to describe the area navigation or the process of direct  flight from point A to point B using VORTAC and VOR/DME  based references which are discussed in this section. All RNAV systems make use of waypoints. A waypoint is  a designated geographical location or point used for route  definition or progress-reporting purposes. It can be defined  or described by using latitude/longitude grid coordinates or,  in the case of VOR based RNAV, described as a point on a  VOR radial followed by that point’s distance from the VOR  station (i.e., 200/25 means a point 25 nautical miles from the  VOR station on the 200° radial).  Radar capabilities at airports vary. Generally, two types of  radar are used by air traffic control (ATC). The primary radar  transmits directional UHF or SHF radio waves sequentially  A radar beacon transponder, or simply, a transponder,  provides positive identification and location of an aircraft  on the radar screens of ATC. For each aircraft equipped  with an altitude encoder, the transponder also provides the  pressure altitude of the aircraft to be displayed adjacent to the  on-screen blip that represents the aircraft.  [Figure 11-125] RNAV has increased in flexibility with the development of  GPS. Integration of GPS data into a planned VOR RNAV  flight plan is possible as is GPS route planning without the  use of any VOR stations. VOR based RNAV uses the VOR receiver, antenna, and  VOR display equipment, such as the CDI. The computer  in the RNAV unit uses basic geometry and trigonometry  calculations to produce heading, speed, and time readouts  for each waypoint. VOR stations need to be within line-of  sight and operational range from the aircraft for RNAV use.  [Figure 11-124] Figure 11-123  illustrates an RNAV route of flight from  airport A to airport B. The VOR/DME and VORTAC  stations shown are used to create phantom waypoints that are  overflown rather than the actual stations. This allows a more  direct route to be taken. The phantom waypoints are entered  into the RNAV course-line computer (CLC) as a radial and  distance number pair. The computer creates the waypoints  and causes the aircraft’s CDI to operate as though they  are actual VOR stations. A mode switch allows the choice  between standard VOR navigation and RNAV.  Figure 11-122.  Many DME’s only display the slant distance, which  is the actual distance from the aircraft to the DME station. This is  different than the ground distance due to the aircraft being at altitude.  Some DMEs compute the ground distance for display.  Figure 11-121.  A typical aircraft-mounted DME antenna.  11-54 Radar Beacon Transponder Area Navigation (RNAV) Slant distance = 13.0 N.M. Reply pulse Interogation pulse Altitude (approx. 12,000 feet) Actual distance over ground = 12.8 N.M. DME station DME Display N.M. . 0 3 1