Distance Measuring Equipment (DME)
Distance-measuring equipment (DME) has been standardized by the International Civil Aviation Organization (ICAO) as a radio aid for short and medium-distance navigation. It is a secondary type of radar that allows several aircraft to simultaneously measure their distance from a ground reference (DME transponder). The distance is determined by measuring the propagation delay of a radio frequency (RF) pulse that is emitted by the aircraft transmitter and returned at a different frequency by the ground station.
The DME can provide distance to a runway when the DME is collocated with an instrument landing system (ILS) station. En route distance information is provided when a DME is collocated with a very-high-frequency omnidirectional radio range (VOR).
DISTANCE MEASURING PRINCIPLE
DME equipped aircraft transmit encoded interrogating RF pulse pairs on the beacon's receiving channel. The beacon replies with encoded pulse pairs on the airborne equipments receiving channel, which is 63 MHz apart from the beacons channel.
The interval between the interrogation emission and the reply reception provides the aircraft with the real distance information from the ground station; this information displays on the cockpit indicator.
The ground transponder can answer 100 to 200 interrogators at a time; i.e., 2700 to 4800 pulse pairs per second (PPS). It generates random pulse pairs (squitter) to maintain a minimum pulse repetition frequency (PRF) of about 800 whenever the number of decoded interrogations is lower than this range. Older DME ground equipment are typically limited to 100 interrogators at a time (2700 ppps), newer equipment can handle over 200.
The aircrafts receiver receives and decodes the transponders reply. Then it measures the lapse between the interrogation and reply and converts this measurement into electrical output signals. The beacon introduces a fixed delay, called the reply delay, between the reception of each encoded interrogating pulse pair and the transmission of the corresponding reply.
The transponder periodically transmits special identification pulse groups that are interwoven with the reply and squitter pulses; the aircraft decodes these special pulses as Morse tones keyed with the beacon code identification.
The aircrafts receiver uses a stroboscopic technique to recognize the replies to its own interrogations among the many other pulses transmitted by the beacon.
The DME theory of operation is summarized below.
Distance - Measuring Equipment Theory of Operation, Simplified Block Diagram
A DME antenna near an ILS
DME electronics cabinet (415)
Thales DME EQUIPMENT
Thales manufactures the 415 and 435 DMEs. These make up a family of comprehensive and latest-generation DME equipment. The versions share many features, modules, and principles of operation and differ only slightly in power and purpose.
Thales DME FEATURES
The main features of the DME are listed below.