Distance Measuring Equipment (DME)

DME consists of stations on the ground and equipment in the airplane. DME operates on the ultra high frequency (UHF) band of frequencies. When a VOR is fitted with DME equipment, it is referred to as a VOR/DME. Each VHF VOR frequency is paired with a specific UHF DME frequency. This means the pilot only uses the VOR's frequency, and needs not be concerned with the DME's UHF frequency.

Before using DME, identify the DME to be used by listening for its Morse code identifier. DME equipment does not have its own unique identifier. Instead, it uses the identifier of the VOR to which it is attached. The DME will transmit its parent VOR's Morse code identifier once every 30 seconds. The remaining identifiers heard on the frequency are sent from the VOR portion of the navaid. If both the VOR and DME components are operating normally, an uninterrupted series of identifiers will be heard.

DME operates by measuring the time it takes for a radio signal to be relayed from the airplane to the ground based DME equipment and back. The result is displayed in nautical miles to the pilot. Since DME measures the distance between the aircraft and the station, the result is slant range distance and not distance over the ground. This means your altitude is a component of the DME distance. When directly over the station at 6,000 feet, the DME will still indicate 1.0 miles to the station. DME distance error is the greatest at close distances and high altitudes. DME is only accurate when 1 or more nm from the ground facility for each 1,000 feet of altitude.


A TACAN is a ground based navigation aid used for tactical air navigation by military aircraft. Its operation is similar to that of a VOR/DME, but the azimuth transmitted from a TACAN can only be received by military aircraft. Civilian aircraft can receive DME from a TACAN, however.

It is common for a TACAN transmitter to be fitted with equipment which allows it to be used as a VOR or a TACAN. These joint civil/military navigation aids are called VORTACs.

From the civilian users perspective, a VORTAC is identical in use and function to a VOR/DME.

VOR Test Facilities (VOTs)

A VOT broadcasts a test signal to permit pilots to check the accuracy of their aircraft's VOR receiver. VOT locations and frequencies are listed in the airport/facilities directory.

To use a VOT, simply tune its frequency. The VOT does not broadcast a signal usable for navigation. Instead, it broadcasts a test signal which tells your aircraft it is directly north of the VOT station, regardless of actual position. This means that your onboard VOR will always indicate a 0 degrees FROM or 180 degrees TO when the CDI is centered.

Area Navigation (RNAV)

Area navigation systems allow pilots to fly directly to particular points over long distances. Some RNAV systems use signals received from VOR/DME transmitters and a processor to provide the pilot with a direct route to the selected point. Other aircraft use a GNSS or an INS.

An intertial navigation system, or INS, is a self contained unit that keeps track of position by measuring acceleration forces on the device. An INS requires no external inputs for navigation. You simply initialize the unit by informing the system of the starting location, and it feels its way from there. The accuracy of an inertial navigation system degrades over time, as little errors in force measurements continue to add up. To counteract this degradation, most INS systems receive location corrections from another system navigation system, such as a GPS.

GNSS refers to the global navigation satellite system. Three such systems exist. The United States has GPS. Russia operates its version of GPS called GLONASS, and the European's have a GNSS called Galileo.

The global positioning systems consists of 24 satellites, five of which are observable at any time from anywhere on the planet. Four satellites are necessary for a GPS receiver to yield a three dimensional position.

The GPS receiver compares the distances from orbiting satellites to calculate the airplane's position, and the display allows for direct navigation to a specified point.

Without RAIM, the pilot has no assurance of GPS position.