while stand-alone dme transponders are permitted, dme transponders are usually paired with an azimuth guidance system to provide aircraft with a two-dimensional navigation capability. a common combination is a dme colocated with a vor (vhf omnidirectional range) transmitter in a single ground station. when this occurs, the frequencies of the vor and dme equipment are paired. such a configuration enables an aircraft to determine its azimuth angle and distance from the station. a vortac (vor and tacan) installation provides the same capabilities to civil aircraft but also provides 2-d navigation capabilities to military aircraft.