Distance Measuring Equipment (DME) is a crucial tool in aviation for measuring the distance between an aircraft and a ground station. This section will detail its function, how it operates, and some specifics about its frequency and signal characteristics.
How DME Functions
The function of DME is to measure distance. The DME electronically measures the time for a signal to reach a ground station and return. This elapsed time converts to miles on the flight deck.
To know the distance between a plane and a VOR station, the aircraft needs a DME interrogator and ground station. The interrogator transmits pulse pairs to the ground station. After a delay, the station retransmits the pulses at another frequency. The random pulse spacing allows systems to discriminate signals. Similarly, this spacing helps the receiver determine the elapsed time for the transmission-reception cycle.
DME measures slant range between an aircraft and a ground station using radio signals around 1 GHz. Line-of-sight is required. DME-equipped airplanes determine and display distance from land-based transponders by sending and receiving pulse pairs. Ground stations often collocate with VORs or VORTACs.
The carrier wave from the aircraft’s DME modulates with pulse strings. The ground unit receives and returns a signal. The transmit-return time converts into nautical miles for display on the instrument panel, along with speed information. The VOR or VORTAC frequency often couples with the DME frequency to provide a comprehensive navigational aid.
DME operates within the 960-1215 MHz UHF band. The interrogator and transponder both use 3.5 microsecond pulses. Pilots set the onboard DME to match the ground transponder frequency to accurately measure distance.
DME Hold Function
The DME hold function permits the DME to stay channeled to the selected station. This feature is particularly useful when making an approach that lacks a collocated DME, as it allows a nearby VOR/DME to provide distance data.
Don’t forget the hold function, especially during ILS approaches that reference DME distance off a nearby VOR. To use this feature, after tuning the NAV radio to the DME source, pilots must click the "HOLD" button to lock the DME frequency from switching when tuning into a new NAVAID. This prevents unwanted changes in DME frequency—a crucial aspect during an approach.
DME utilizes the 960-1215 MHz UHF band. Interrogators and transponders utilize 3.5 microsecond pulses. Pilots match the DME to the ground transponder frequency to get accurate distance data.